Oral anticoagulants: mechanism of action, clinical effectiveness, and optimal therapeutic range

Thromboembolic diseases of childhood

Over the last decade, pediatric thrombophilia programs have emerged around the world as a new discipline in pediatric hematology. These programs specialize in the diagnosis, prevention and treatment of children with thromboembolic events (TEs) in both the venous and arterial systems. The need for separate pediatric programs has been discussed previously. (J Pediatr Hematol Oncol 1997; 19: 7-22.) The following article will update previous reviews (Hematol Oncol Clin North Am 1998; 12: 1283-1312; Thromb Haemost 1997; 78: 715-725) and will concentrate on three aspects: (1) The risk factors for acquiring TEs; (2) The confirmatory diagnostic tests used in children with TEs; and (3) The different antithrombotic agents used for prevention and treatment. The current knowledge in respect to the above points is only the "tip of the iceberg". Well-designed prospective trials are required to establish the contribution of congenital prothrombotic disorders, appropriate diagnostic strategies, and optimal therapy for children with TEs.

Absorption, distribution, metabolism, and excretion of ximelagatran, an oral direct thrombin inhibitor, in rats, dogs, and humans

The absorption, metabolism, and excretion of the oral direct thrombin inhibitor, ximelagatran, and its active form, melagatran, were separately investigated in rats, dogs, and healthy male human subjects after administration of oral and intravenous (i.v.) single doses. Ximelagatran was rapidly absorbed and metabolized following oral administration, with melagatran as the predominant compound in plasma. Two intermediates (ethyl-melagatran and OH-melagatran) that were subsequently metabolized to melagatran were also identified in plasma and were rapidly eliminated. Melagatran given i.v. had relatively low plasma clearance, small volume of distribution, and short elimination half-life. The oral absorption of melagatran was low and highly variable. It was primarily renally cleared, and the renal clearance agreed well with the glomerular filtration rate. Ximelagatran was extensively metabolized, and only trace amounts were renally excreted. Melagatran was the major compound in urine and feces after administration of ximelagatran. Appreciable quantities of ethyl-melagatran were also recovered in rat, dog, and human feces after oral administration, suggesting reduction of the hydroxyamidine group of ximelagatran in the gastrointestinal tract, as demonstrated when ximelagatran was incubated with feces homogenate. Polar metabolites in urine and feces (all species) accounted for a relatively small fraction of the dose. The bioavailability of melagatran following oral administration of ximelagatran was 5 to 10% in rats, 10 to 50% in dogs, and about 20% in humans, with low between-subject variation. The fraction of ximelagatran absorbed was at least 40 to 70% in all species. First-pass metabolism of ximelagatran with subsequent biliary excretion of the formed metabolites account for the lower bioavailability of melagatran.

Effect of levofloxacin coadministration on the international normalized ratios during warfarin therapy

STUDY OBJECTIVE

To evaluate the effect of levofloxacin coadministration on the international normalized ratio (INR) in patients receiving warfarin therapy.

DESIGN

Prospective analysis.

SETTING

Outpatient clinic at a Veterans Affairs medical center.

PATIENTS

Eighteen adult patients receiving warfarin.

INTERVENTION

On the basis of clinical diagnosis and judgment, levofloxacin was prescribed to the 18 patients for treatment of various types of infection. The INR was measured before and at 2-8-day intervals after the coadministration of levofloxacin therapy, and once after completing therapy. Warfarin dosages were adjusted when necessary.

MEASUREMENTS AND MAIN RESULTS

Warfarin dosages were changed in seven patients as a result of the first nontherapeutic INR values obtained after start of levofloxacin therapy. Owing to a concern regarding noncompliance and the adverse effect of bleeding, warfarin dosage was adjusted in one patient even though his first INR value was in the high end of the therapeutic range (2.98, therapeutic range 2-3). One patient withdrew from the study after the first INR measurement after levofloxacin coadministration. Because of a concern about the possible bleeding complication, warfarin dosage was also adjusted in this patient after obtaining his first INR value. Therefore, only the INR values obtained before and the first INR values obtained after levofloxacin administration were compared to evaluate the effect of levofloxacin on INR determination of warfarin therapy. The INR values obtained before levofloxacin administration did not differ significantly from the first INR values obtained after levofloxacin coadministration (mean +/- SD 2.61 +/- 0.44 vs 2.74 +/- 0.83, 95% confidence interval -0.449-0.196, p=0.419).

CONCLUSION

The INR values measured before and after concomitant levofloxacin therapy were not significantly different. However, the ability to detect a significant difference may be affected by the small number of patients studied. Further studies with a larger sample are required to better determine the effect of levofloxacin coadministration on INR monitoring during warfarin therapy

Evaluation of the clinical and economic impact of a brand name-to-generic warfarin sodium conversion program

BACKGROUND

Substitution of generic warfarin initially was discouraged because of concerns regarding therapeutic failure or toxicity. Although subsequent research with AB-rated (i.e., bioequivalent) warfarin did not confirm initial concerns, the issue is not settled for all clinicians.

OBJECTIVES

We sought to provide additional information regarding the clinical and economic impact of warfarin conversion by analyzing a real-life sample of patients receiving long-term anticoagulation therapy who were switched from brand name to generic warfarin.

METHODS

Patients who had been taking warfarin for at least 180 days and had received uninterrupted oral anticoagulation 90 days before and 90 days after switching to generic warfarin were included. The switch date was based on the first time generic warfarin was dispensed from our pharmacies. The primary end point was the calculated amount of time each patient's international normalized ratio (INR) values were within the patient-specific target INR range in the 90 days before and after the switch. Data regarding adverse events and medical resource utilization were also collected. Pharmacoeconomic analyses were performed.

RESULTS

The analysis included 2299 patients. The overall difference in calculated time INR values were below (22.6% before vs 26.1% after switch, p<0.0001) and within (65.9% before vs 63.3% after switch, p=0.0002) the therapeutic INR range was statistically but not clinically significant. Only 28.0% of patients experienced a change in therapeutic INR control of 10% or less, 33.1% experienced INR control that improved by greater than 10%, and 38.9% experienced INR control that worsened by more than 10%. The difference in total treatment costs associated with brand name and generic warfarin was 3128 dollars/100 patient-years in favor of the generic product. Sensitivity analyses revealed that cost savings associated with warfarin conversion in this health care system were highly dependent on the difference between warfarin costs and cost of treating anticoagulation-related adverse events.

CONCLUSIONS

Most of these patients were successfully switched from brand name to generic warfarin. However, supplemental INR monitoring is warranted when one warfarin product is substituted for another to allow timely detection of those patients who experience significant changes in anticoagulation response.

Oral anticoagulants in patients with coronary artery disease

Oral anticoagulants have been used in patients with vascular disease for over 40 years, yet their role in the secondary prevention of recurrent cardiovascular (CV) events remains controversial. The objectives of this systematic review are to more reliably determine the role of oral anticoagulants with and without antiplatelet therapy in patients with established coronary artery disease (CAD). Randomized trials in which oral anticoagulants were tested in CAD patients who were treated for at least three months were identified, and each trial was classified by the targeted level of intensity of anticoagulation. Data from the trials were combined using the modified Mantel-Haenszel method, and odds ratios were computed. Data from over 20,000 patients indicated that high-intensity oral anticoagulation (international normalized ratio [INR] >2.8) significantly reduced CV complications and increased bleeding compared with controls. Moderate-intensity oral anticoagulation (INR 2 to 3) also reduced CV complications compared with controls. The combination of moderate-intensity oral anticoagulation and aspirin is more effective and equally as safe as aspirin alone. Low-intensity oral anticoagulation (INR <2) in the presence of aspirin does not reduce CV complications and increases bleeding compared with aspirin alone.

Fenofibrate potentiates warfarin effects

OBJECTIVE

To describe 2 patients in whom the initiation of fenofibrate potentiated warfarin's anticoagulant effects.

CASE SUMMARY

A 71-year-old white woman and an 80-year-old white woman with multiple medical conditions were both stabilized on long-term warfarin therapy. During the course of anticoagulation, both patients were prescribed fenofibrate and experienced threefold and twofold increases in international normalized ratio (INR), respectively, requiring total weekly warfarin dosage reductions of 30-40%. Before starting fenofibrate therapy, both patients' coagulation values were within the therapeutic range. When interviewed, patients and caregivers denied bleeding, bruising, changes in diet, alcohol ingestion, nonadherence with therapy, or changes in drug regimen except for the addition of fenofibrate. Upon chart review, evaluation of potentially contributory parameters, such as other changes in drug therapy, thyroid function, liver function, and drug-disease interactions, showed that these parameters remained stable and were ruled noncontributory.

DISCUSSION

The addition of fenofibrate in 2 patients on stable and therapeutic doses of warfarin increased the anticoagulant response to warfarin. A clear temporal relationship with the addition of fenofibrate and the appearance of the interaction was seen. Fenofibrate is highly protein bound, with the potential to displace warfarin from its binding protein, leading to an enhanced hypoprothrombinemic effect. Fenofibrate is also a mild to moderate inhibitor of CYP2C9, the enzyme responsible for warfarin metabolism. The combination of these effects--displacement of warfarin by fenofibrate coupled with decreased metabolism of warfarin--may increase the anticoagulant response to warfarin. Using the Naranjo probability scale, these interactions were designated as probable.

CONCLUSIONS

We suggest serial monitoring of INR and consider an empiric 20% reduction in warfarin dosage when fenofibrate is initiated, with the possibility for a greater warfarin dosage reduction based on INR results.

Coumarin therapy in thrombosis

Oral anticoagulants are the most commonly used agents in the long-term prophylaxis and treatment of arterial and venous thrombotic disorders. As new and expanded indications for their use, such as the prevention of recurrent myocardial infarction or the treatment of systemic embolism in atrial fibrillation, are developed, the use of oral anticoagulants is rising. Also, in North America, oral anticoagulants are used commonly for preventing venous thromboembolism following orthopedic surgery. This article reviews the pharmacology of warfarin sodium, the most commonly used oral anticoagulant in North America, and discusses practical aspects of the use of this agent in thrombotic disorder management.

Diminished anticoagulant effects of warfarin with concomitant mercaptopurine therapy

The drug interaction between mercaptopurine and warfarin is documented, but case reports of the existence or magnitude of this interaction are rare. An increased warfarin dosage was required for a patient receiving 12-week cycles of mercaptopurine for acute promyelocytic leukemia. At the start of mercaptopurine therapy, an upward titration of 25% beyond the warfarin maintenance dosage was required to achieve a therapeutic international normalized ratio. When a cycle of mercaptopurine was completed, a sharp reduction in warfarin dosage was required. The mechanism behind this interaction is unclear. Mercaptopurine may inhibit gastrointestinal absorption of warfarin, or it may induce hepatic enzymes that metabolize the anticoagulant. With dramatic changes in warfarin or anticoagulant requirements, practitioners should be aware of potential thromboembolic sequelae or bleeding complications when these agents are prescribed concomitantly. Frequent monitoring and careful dosage titration are warranted during concomitant administration.

[Oral anticoagulants in the elderly]

PURPOSE

Atrial fibrillation and venous thromboembolism are particularly frequent in the elderly. Whether or not prescribe oral anticoagulant treatment in the elderly is therefore a common question for the physician. Despite the benefits of anticoagulation demonstrated in clinical trials, oral anticoagulant therapy is underused in the elderly.

CURRENT KNOWLEDGE AND KEY POINTS

Indications for oral anticoagulation are discussed specifically in the elderly with a literature review. Only the length of anticoagulation treatment after a venous thromboembolism remained a purpose of discussion regarding the severity of the pathology. The frequency of systemic thromboembolism in nonvalvular atrial fibrillation is increasing with age. Oral anticoagulation reduces this risk. This benefice is to compare with the increasing rate of major bleeding complications in the elderly and in patient who had stroke, hyper-tension, diabetes mellitus or gastrointestinal bleeding. The objective of this article is to focus on the mode to measure oral anticoagulant benefice/risk ratio in the elderly and to propose several ways to minimize the risk for bleeding.

FUTURE PROSPECTS AND PROJECTS

The potential drug side effect severity of oral anticoagulation must lead to find the "reasonable" clinical indications in term of benefice/risk ratio and what measures should be take to increase the safety of oral anticoagulation in the elderly. The comprehensive geriatric evaluation should be considered as a decision-aid tool in long-term oral anticoagulation in the frail elderly. Anticoagulation clinics, informatics'-prescription coupled with dose-adaptation decision-aid adapted to the elderly should be helpful in this research of quality. Finally, prescribers education supports must insist on the early course of therapy that is at higher risk of bleeding.

Total artificial hearts: bridge to transplantation

The CardioWest TAH was created and initially tested at the same time as the Thoratec, Novacor, and HeartMate devices. It was designed as a permanent artificial heart and was the first-ever mechanical circulatory device to be used as destination therapy. Twenty years have passed since that early experience. Pneumatic technology is still current and being developed as in existing or new implantable Thoratec VADs the pneumatic HeartMate, and the Abiomed BVS 5000 pumps. Portable pneumatic drivers have been available since 1982, and in recent times have allowed discharge to home of substantial numbers of patients, thus reducing the length of hospital stays and making mechanical device support less expensive to society and more tolerable to patients. Within months, a portable driver for the CardioWest will be available. The documented benefits of the CardioWest TAH include rescue of: critically ill patients with advanced heart failure; patients with biventricular failure especially those with significant right heart failure, elevated pulmonary vascular resistance, or pulmonary edema; patients with renal or hepatic failure secondary to low cardiac output; patients with massive myocardial damage such as those with post-\infarction VSD or irreversible cardiac graft rejection; patients with mechanical valves or native valve disease; and patients with intractable arrhythmias and heart failure. High device outputs with restoration of normal filling pressures result in high perfusion pressures that have led to dramatic recoveries, convalescence, and return to levels of activity compatible with normal life. The average device output with the CardioWest TAH is higher than any other approved or investigational device. The reason for this resides in design simplicity this device has the shortest and largest inflow pathway. Stroke, in the authors' own series, is rare with a linearized rate of 0.068 events per patient year. If the experiences of La Pitie and the University of Arizona are combined, there has been one stroke in 25 patient years (0.04 events/patient year). Serious infections have been rare (12% of patients). No clinical mediastinitis has occurred. Drivelines have healed in tightly and never caused an "ascending" infection. There has not been a case of device endocarditis. Using a broad definition of bleeding, including takeback reoperation for bleeding, bleeding more than 8 units in the first postoperative 24 hours or 5 units over any other 48-hour period, a 25% to 36% incidence has been documented. No cases of fatal exsanguination have resulted, as there have been with the HeartMate. The incidence of bleeding as an adverse event is about 17% lower than the rate reported for the HeartMate VE LVAD, and it is about the same as that reported for Novacor and for Thoratec. Implantation of this device is relatively easy and often done (with attending help) by the authors' residents. If one follows the guidelines for fitting the device, and takes the recommended advice for implantation, hemostasis is excellent and restoration of immediate cardiac function with high flows is nearly automatic. Use of a neopericardium of 0.1 mm EPTFE at the time of implantation assures atraumatic and relatively quick re-entry for transplantation and prevents the normal inflammatory mediastinal reaction that might be desirable in a destination application. In selected patients the CardioWest TAH is the device of choice for bridge to transplantation. When a portable driver becomes available, out of hospital management of CardioWest TAH patients will be feasible and consideration of use of this device for longer term applications, (e.g., "destination therapy,") will be reasonable. A wearable driver, even smaller than a portable, will improve quality of life and expand the patient population that may be therapeutically served with this system. In short, the CardioWest TAH has come nearly full circle. It was first used as a destination device. It has since been used as a bridge to transplantation in nearly 200 patients as the Jarvik-7/Symbion TAH and, since 1993, in over 225 patients as CardioWest. The results have improved with time. Thromboembolism and infection rates have been competitive with currently available devices. Device reliability and durability have been excellent. Survival rates have been very high in a group of perhaps the sickest patients to be supported with any pulsatile device. Pneumatic technology has improved with portability and miniaturization, and there is reason to believe that it will become even better. Application of modern manufacturing techniques to this very simple device raises the possibility of significant manufacturing cost reduction, in an era of prohibitive cost for other devices. All of this establishes the CardioWest as a valuable device for any program that is seriously interested in end-stage heart disease and a likely device for permanent use in appropriately selected patients.

Measurement of warfarin in plasma by high performance liquid chromatography (HPLC) and its correlation with the international normalized ratio

The measurement of plasma warfarin is required to investigate non-compliance, resistance to anticoagulation, drug metabolism and pharmacokinetic. Methods so far described are based on extraction of warfarin from plasma followed by reversed-phase HPLC. Extraction is the crucial step and may be performed in liquid- or solid-phase. The latter requires the preparation of columns, which makes the procedure variable. We investigated the suitability of the ready-for-use commercial cartridges for sample preparation. The method displayed between-run CV of 11.8%. Recovery was 99%. The coefficients of correlation between warfarin concentration in 50 patients and weekly dosage or INR were 0.55 (p<0.0001) or 0.25 (p=0.079).

Antiplatelet and anticoagulation for patients with prosthetic heart valves

BACKGROUND

Patients with prosthetic heart valves are at increased risk for valve thrombosis and arterial thromboembolism. Oral anticoagulation alone, or the addition of antiplatelet drugs, has been used to minimize this risk. An important issue is the effectiveness and safety of the latter strategy.

OBJECTIVES

To compare the effectiveness and safety of adding antiplatelet therapy to standard oral anticoagulation among patients with prosthetic heart valves.

SEARCH STRATEGY

We searched the Cochrane Central Register of Controlled Trials (Cochrane Library Issue 2, 2003), MEDLINE (January 1966 to August 2002), EMBASE (January 1988 to July 2001) and reference lists of individual reports, review articles, meta-analyses, and consensus statements.

SELECTION CRITERIA

All reports of randomised controlled trials comparing standard dose oral anticoagulation to standard dose oral anticoagulation and antiplatelet therapy in patients with one or more prosthetic heart valves. We included reports published in any language or in abstract form.

DATA COLLECTION AND ANALYSIS

Two reviewers independently performed the search strategy, assessed trials for inclusion criteria, study quality, and extracted data. Adverse effects information was collected from the trials.

MAIN RESULTS

Eleven studies involving 2,428 subjects met the inclusion criteria. Year of publication ranged from 1971 to 2000. Compared with anticoagulation alone, the addition of an antiplatelet agent reduced the risk of thromboembolic events (odds ratio 0.39 (95% confidence interval 0.28 to 0.56; p<0.00001)) and total mortality (odds ratio 0.55 (95% confidence interval 0.40 to 0.77; p=0.0003)). Aspirin and dipyridamole reduced these events similarly. The risk of major bleeding was increased when antiplatelet agents were added to oral anticoagulants (odds ratio 1.66 (95% confidence interval 1.18 to 2.34; p=0.003)). For major bleeding, there was no evidence of heterogeneity between aspirin and dipyridamole and in the comparison of trials performed before and after 1990, around the time when anticoagulation standardization with the international normalized ratio was being implemented.

REVIEWER'S CONCLUSIONS

Adding antiplatelet therapy, either dipyridamole or low-dose aspirin, to oral anticoagulation decreases the risk of systemic embolism or death among patients with prosthetic heart valves. The risk of major bleeding is increased with antiplatelet therapy. These results apply to patients with mechanical prosthetic valves or those with biological valves and indicators of high risk such as atrial fibrillation or prior thromboembolic events. The effectiveness and safety of low dose aspirin (100 mg daily) appears to be similar to higher dose aspirin and dipyridamole.

Warfarin dose adjustments based on CYP2C9 genetic polymorphisms

BACKGROUND

The dose response relationship of warfarin is unpredictable. Polymorphism of the Cytochrome P4502C9 enzyme leads to warfarin hypersensitivity presumably due to decreased metabolism of the S-enantiomer. The purpose of this study was to further characterize the relationship between CYP2C9 genotype and phenotype and to develop a basis for guidelines to interpret CYP2C9 genotype for warfarin dosing.

METHODS AND RESULTS

Patients stabilized on warfarin therapy were recruited from an anticoagulation clinic. Patients were genotyped for CYP2C9*2, CYP2C9*3 and CYP2C9*5 alleles by standard methods of polymerase chain reaction amplification and restriction endonuclease digestion. Phenotype was determined by; dose (mg/kg/d) required to maintain anticoagulation, (INR 2.0-3.0), oral plasma S-warfarin clearance, and the plasma S:R-warfarin ratio. In this cohort, no subjects were found to have the CYP2C9*5 allele. The plasma S-warfarin concentration did not differ with age, dose or CYP2C9 genotype. Both CYP2C9*2 and *3 alleles were associated with lower maintenance dosages, lower total and R-warfarin plasma concentrations, decreased oral clearance of S-warfarin, increased plasma S:R-warfarin ratio and extended S-warfarin elimination half-life. Advancing age was found to decrease Warfarin maintenance dose in subjects with the common active CYP2C9*1/*1 genotype but did not influence dose requirement of subjects with one or more variant CYP2C9 alleles.

CONCLUSIONS

Subjects who have been titrated to a consistent target INR demonstrate comparable plasma S-warfarin concentrations independent of CYP2C9 genotype. The warfarin dose required to maintain a consistent target INR between subjects differs as a function of S-warfarin clearance which is decreased by both CYP2C9*2 and or CYP2C9*3 variant alleles. The variables of CYP2C9 genotype and age can be applied to restrict the dosage range considered for individual patients.

Reliability, validity and ease of use of a portable point-of-care coagulation device in a pharmacist-managed anticoagulation clinic

In a pharmacist-managed anticoagulation clinic, portable point-of-care coagulation devices may facilitate patient monitoring by providing rapid INR measurement. Few studies, however, have validated this type of device.

OBJECTIVE

To evaluate the reliability, validity and ease of use of the CoaguChek S, a new portable coagulation device.

METHODS

A total of 100 patients followed at a pharmacist-managed anticoagulation clinic attended two study visits. INRs were measured using the CoaguChek S and the standard laboratory technique.

RESULTS

Reliability: The test-retest reliability (precision) of the CoaguChek S, estimated by the intraclass correlation coefficient (ICC) and a 95% confidence interval (95% CI), was high (0.98 (0.98-0.99)) and comparable to the standard laboratory technique (0.99 (0.98-0.99)). Interrater reliability was also high (0.97 (0.95-0.98)). Reliability coefficients did not vary with the test-strip lot number nor the CoaguChek S operator.

VALIDITY

When compared with standard laboratory procedure, the ICC (95% CI) was equal to 0.93 (0.91-0.95). The mean difference (95% CI) between INR measured by the laboratory and the CoaguChek S was equal to -0.02 units (-0.06-0.03). The mean absolute and relative absolute differences (95% CI) were equal to 0.24 units (0.21-0.27) and 9% (8%-10%), respectively. Differences tended to increase for INRs greater than 3 units as seen by a mean difference (95% CI) of -0.17 units (-0.35-0.02). This represented a mean absolute difference (95% CI) of 0.44 units (0.33-0.55) and a mean relative absolute difference of 12% (9%-15%). Concordance between therapeutic decisions based on CoaguChek S and laboratory results was high (Kappa = 0.68). In 34 cases (18%), the therapeutic decision would have been different. However, in 15 of these discordant observations, the difference between the CoaguCheck S and laboratory INR was <or=0.25 units. Ease of use: In 3% of cases, no INR could be measured by the CoaguChek S. The percentage of extra finger pricks and extra test-strips were equal to 25.8% and 23.7%, respectively.

CONCLUSIONS

When used by health professionals in a pharmacist-managed anticoagulation clinic, the CoaguChek S is reliable, valid and easy to use. However, its validity tends to decrease as the INR increases, possibly due to the low sensitivity of the thromboplastin. If the CoaguChek S INR is supratherapeutic, we would therefore recommend confirming the results with a standard laboratory measurement.

Effect of soy milk on warfarin efficacy

OBJECTIVE

To describe the effect of soy milk on the international normalized ratio (INR) in a patient treated with warfarin.

CASE SUMMARY

A 70-year-old white man who was stable on warfarin therapy developed subtherapeutic INR values after ingesting soy protein in the form of soy milk. The subtherapeutic INR values could not be explained by factors known to reduce the INR such as noncompliance, new medications, other alternative therapies, increased physical activity, changes in medication storage, or increased consumption of vitamin K. INR values returned to therapeutic concentrations within 2 weeks after discontinuation of the soy milk. Repeated coagulation test results during the next 2 months remained within the normal range.

DISCUSSION

Impaired warfarin metabolism may be ascribed to various pharmacodynamic and pathologic factors. Subtherapeutic INR values may also be the result of concomitant administration of metabolic inducers resulting in drug-drug, drug-herb, or drug-food interactions. An objective causality assessment in this case revealed that INR decline in a warfarin-treated patient as a result of soy milk ingestion was in the range of possible to probable. Speculative mechanisms of soy milk-induced INR decline such as changes in warfarin absorption or metabolism resulting from alterations in the P-glycoprotein efflux system or organic anion-transporting polypeptides are discussed.

CONCLUSIONS

The temporal relationship between the start of soy milk and subtherapeutic INR values in 1 patient suggests that soy milk may have possibly or probably caused a decline in INR. Soy protein food-drug interactions with warfarin may be more common than the literature suggests, and further studies are needed to determine the exact mechanism. Healthcare professionals should be alerted to the potential implications of this food-drug interaction.

Reliability of point-of-care prothrombin time testing in a community clinic: a randomized crossover comparison with hospital laboratory testing

The success in achieving therapeutic international normalized ratio (INR) targets in the control of warfarin using a whole-blood point-of-care testing (POCT) monitor (CoaguChek) in a community clinic was compared with hospital laboratory coagulometer prothrombin time (PT) testing in a randomized crossover study. Forty-six patients were randomized into two groups. At each visit, capillary blood was taken for the POCT monitor and venous blood for the laboratory coagulometer. In Group 1, for 6 months, dosage was based on the CoaguChek and for the second 6 months on the coagulometer. In the second group, the order was reversed. Dosages were determined using the dawn ac computer programme. Success was assessed by the percentage of time patients were maintained within the INR targets. Agreement between laboratory and monitor INR, and patient satisfaction were also assessed. Results with the POCT monitor compared well with the hospital coagulometer. Time in INR target range between the groups was similar, with 60.9% on the POCT monitor and 59.3% with the laboratory coagulometer in Group 1 and in Group 2, respectively, 64.3% and 63.4% with no significant difference in mean INR. An INR above 4.0 gave some discrepant results. International Sensitivity Index calibrations of the two test systems indicated that the INRs were dependable. Patient questionnaires showed greater satisfaction with community POCT monitoring.

Differences in warfarin dosing decisions based on international normalized ratio measurements with two point-of-care testing devices and a reference laboratory measurement

STUDY OBJECTIVES

To assess the accuracy of warfarin dosing decisions and the degree of numeric bias between two point-of-care devices using a local reference laboratory's international normalized ratio (INR) as the standard measure, and to determine the relationship between dosing decisions and INR values obtained with the point-of-care devices.

DESIGN

Prospective study.

SETTING

Outpatient anticoagulation clinic.

SUBJECTS

Two hundred two patients taking oral warfarin and 10 control subjects.

INTERVENTIONS

For the two point-of-care devices, AvoSure and ProTime, the finger-stick method was used to collect capillary blood samples in each subject. At the same visit, one venous blood sample was collected from each subject for the laboratory analysis.

MEASUREMENTS AND MAIN RESULTS

Dosing agreement was assessed as the proportion of agreement between each device and the laboratory in terms of maintenance dosage adjustments (increase, decrease, or no change). The level of agreement between each device and the laboratory was evaluated by dosing agreement analysis, bias analysis, and concordance coefficient analysis. In the dosing agreement analysis, 78% of INR values from the AvoSure device would have resulted in the same dosing decision as that with the laboratory INR values compared with 66% from the ProTime device (p < 0.001). The mean bias for the ProTime device (0.5 +/- 0.4 INR units) was significantly higher (p = 0.005) than that for the AvoSure device (0.4 +/- 0.5 INR units). The ProTime device overestimated low INR values to a greater extent than did the AvoSure device. Concordance between the laboratory measurement and each device was similar (rho(c) = 0.82 for ProTime, rho(c) = 0.76 for AvoSure).

CONCLUSIONS

Assessing dosing decisions yielded distinct, useful clinical information. The AvoSure device is associated with less systematic bias and a higher degree of clinical agreement with our reference laboratory measurement than those of the ProTime device.

Prothrombotic States that Predispose to Stroke

Hereditary prothrombotic states of clinical importance include factor V Leiden, the prothrombin 20210A mutation, deficiencies of protein C, protein S, or antithrombin, sickle cell disease, and hyperhomocysteinemia. Major acquired prothrombotic states include cancer, myeloproliferative disorders, the antiphospholipid syndrome, and heparin-induced thrombocytopenia. Because most of the hereditary prothrombic states are not established risk factors for arterial thrombosis, routine laboratory testing in most patients with ischemic stroke should be limited to complete blood count, lupus anticoagulant, anticardiolipin antibodies, and plasma total homocysteine. Additional testing for factor V Leiden, prothrombin 20210A, antithrombin, protein C, and protein S may be indicated for patients under the age of 50 or those with paradoxical cerebral embolism. The treatment of acute ischemic stroke in patients with prothrombotic states is similar to that in patients without an identifiable prothrombotic condition, and may include antiplatelet agents, anticoagulants, or thrombolytic therapy in patients who otherwise meet eligibility criteria. The potential benefit of chronic anticoagulation therapy for the primary or secondary prevention of stroke in patients with prothrombotic states has not been addressed in controlled clinical trials. Specific therapeutic approaches for the prevention of stroke are established for patients with sickle cell disease, myeloproliferative disorders, and heparin-induced thrombocytopenia, and are under investigation for hyperhomocysteinemia and the antiphospholipid syndrome.

Long-term Anticoagulation Therapy in Prevention of Stroke

Long-term anticoagulation continues to be used and investigated as a means to prevent new or recurrent stroke. The best-established indications for long-term anticoagulation are cardiac abnormalities capable of producing intracardiac thrombi that may embolize into the brain or systemic circulation. The firmest cardiac indications are mechanical prosthetic heart valves, mitral stenosis with atrial fibrillation, and atrial fibrillation with additional features placing them at increased risk for stroke. These and other "major" cardiac potential sources of emboli should be considered as anticoagulation candidates unless the estimated risk of bleeding is prohibitive. A number of noncardiac potential causes of stroke are generally managed with long-term anticoagulation. These include arterial dissections, cerebral venous sinus thrombosis, the antiphospholipid antibody syndrome, and congenital and acquired coagulopathies. Recent randomized studies do not support the use of long-term anticoagulation for the prevention of recurrent stroke in patients with noncardioembolic stroke that is not due to the previously outlined disorders. Whether long-term anticoagulation is beneficial in the specific population of patients with major documented intracranial atherosclerotic stenosis is currently under investigation.

Which approach to anticoagulation management is best? Illustration of an interactive mathematical model to support informed decision making

BACKGROUND

Among patients with atrial fibrillation or mechanical heart valves, determining the best approach to oral anticoagulation largely depends on comparing the costs of anticoagulation management with the costs of events (thromboembolism and bleeding) averted. The Anticoagulation Management Event/Cost Model (ACME) is an interactive mathematical model intended to help clarify these trade-offs.

METHODS

The ACME is a series of linked, nested spreadsheets. At the least detailed level, the user specifies the percentage of patients falling into various management strategies (no anticoagulation, usual physician care, anticoagulation service, patient self-testing/self-management), and the ACME estimates event rates and costs. At more detailed levels the ACME performs a series of weighted average calculations combining, for example, utilization times unit price. Cost categories are divided into event-related and management-related costs (costs of management, testing, and medication).

RESULTS

Regardless of how anticoagulation is subsequently managed, perhaps the greatest benefit is obtained by moving patients who are not currently receiving anticoagulation onto warfarin. Additional benefits can be obtained by eliminating outliers (extremely high or extremely low anticoagulation levels). If changing to a more intensive approach also serves to reduce the tendency for physicians to prescribe anticoagulate below the optimal range, additional savings can be anticipated. The cost calculation typically involves a trade-off between increased up-front costs of anticoagulation management versus greater down-line savings associated with a decreased number of events. To assess the quality of anticoagulation within a given organization, it is critical to know the distribution of clotting levels for the population under anticoagulation.

CONCLUSIONS

Interactive mathematical models, if sufficiently well documented, can be helpful in clarifying decisions regarding costs and benefits of various methods of anticoagulation.

Patient-specific factors predictive of warfarin dosage requirements

OBJECTIVE

To identify patient-specific factors predictive of maintenance warfarin dosage requirements >5 mg/d.

METHODS

One hundred forty-six adults taking warfarin were identified from a community hospital and an outpatient anticoagulation clinic. Patient demographics and data on warfarin doses, laboratory results, and medication use were obtained by abstracting patient records. Estimates of vitamin K intake were obtained using a questionnaire and structured interview. Multiple logistic regression was used to identify patient characteristics independently predictive of warfarin maintenance requirements >5 mg/d. An assessment tool for estimating an individual patient's likelihood of requiring warfarin maintenance doses >5 mg/d was derived from the logistic regression model and was assessed in both the study cohort and a separate historical validation cohort of 125 patients.

RESULTS

Five factors were independently associated with warfarin requirements >5 mg/d: age <55 years, male gender, African American ethnicity, vitamin K intake >400 micro g/d, and body weight >or=91 kg. The assessment tool derived from these factors correctly classified semiquantitative warfarin requirements as non-high-dose in 84 of 93 study cohort patients and 71 of 78 validation cohort patients, and correctly classified requirements as high-dose in 10 of 13 study cohort patients and 11 of 15 validation cohort patients.

CONCLUSIONS

African American ethnicity is a newly identified predictor of warfarin requirements >5 mg/d and is independent of dietary vitamin K intake. An assessment tool incorporating this and other predictors can estimate a patient's likelihood of requiring such dosages.

The nature and frequency of potential warfarin drug interactions that increase the risk of bleeding in patients with atrial fibrillation

PURPOSE

To determine the frequency with which atrial fibrillation (AF) patients receiving warfarin are prescribed interacting drugs that could increase bleeding risks.

METHODS

We retrospectively examined medical records for 704 Medicare beneficiaries > or = 65 years of age discharged from Kansas hospitals with AF. We identified all patients receiving warfarin and examined discharge prescriptions for drugs that could increase bleeding risk either by increasing the international normalized ratio (INR) or directly inhibiting hemostasis.

RESULTS

Of 256 patients discharged on warfarin, 138 (54%) were prescribed another medication that could increase bleeding risk. Among these patients, 106 (41%) were discharged with a total of 150 prescriptions for drugs that could interact with warfarin to increase the INR. Antibiotics accounted for 67% of these prescriptions. Fifty-three patients (21%) received 56 prescriptions for drugs which could inhibit hemostasis. These were primarily antiplatelet drugs with 61% of the prescriptions for aspirin. Patients with coronary artery disease were more likely than others to be prescribed warfarin plus antiplatelet agents (OR = 2.80; p = 0.04). More than one interacting drug was prescribed for 20% of the patients.

CONCLUSIONS

AF patients discharged on warfarin were frequently prescribed concomitant medications that increase bleeding risks. These patients should be closely monitored and counseled to watch for signs of bleeding.

How to evaluate the influence of blood collection systems on the international sensitivity index. Protocol applied to two new evacuated tubes and eight coagulometer/thromboplastin combinations

Previous reports indicate that the international sensitivity index (ISI) of coagulometer/thromboplastin combinations (measuring systems) may be influenced by blood collection systems. We applied a new protocol to assess the extent with which two new blood collection systems (evaluation tubes) influence the ISI of eight widely used measuring systems. The evaluation tubes were made of plastic and had draw volumes of 1.8 and 2.7 ml; citrate concentration was 0.109 M. Well-established collection tubes (from the same manufacturer)-which were made of siliconized glass with draw volume and citrate concentration of 4.5 ml and 0.105 M, respectively-served as control. Plasmas from 20 healthy subjects and 60 patients on oral anticoagulants, collected with the evaluation and control tubes, were tested for prothrombin time (PT) with the eight measuring systems. Plasmas collected with the control tubes were also tested with an international standard for thromboplastin. Results were used to calculate the ISIs of the measuring systems. These were then used to calculate the differences of the crossover international normalized ratio (INR) (i.e., the INR obtained with PT ratios of plasmas collected into the evaluation tubes and the ISI determined with plasmas collected into the control tubes). Overall, the differences were small but measurable, especially for the 2.7-ml draw evaluation tubes and with some of the measuring systems (mean difference, </=5.50%; mean relative deviation (MRD), </=5.82%). Although statistically significant, these differences are not clinically relevant as they are unlikely to affect decision making in drug prescription. In conclusion, the study shows that the evaluation tubes compare well with the control tubes from the same manufacturer. The protocol described here should be adopted for the evaluation of other blood collection systems.

Factor Xa inhibition in the prevention of venous thromboembolism and treatment of patients with venous thromboembolism

Venous thromboembolism (VTE) is a life-threatening complication following orthopedic surgery. Selective factor Xa inhibition is a new antithrombotic approach designed to avoid difficulties associated with heparins and other current anticoagulants. Several antifactor Xa compounds are in early investigation, but fondaparinux (Arixtra; NV Organon, Oss, The Netherlands; Sanofi-Synthelabo, Paris, France) is the first and most advanced compound in the development of a new class of synthetic antithrombotic agents--the selective factor Xa inhibitors. Fondaparinux has a highly favorable pharmacokinetic profile; four large phase 3 trials comparing subcutaneous fondaparinux 2.5 mg once daily with the low molecular weight heparin (LMWH) enoxaparin in doses approved by regulatory bodies showed that fondaparinux reduced the overall risk of VTE in major orthopedic surgery by > 50% without increasing clinically relevant bleeding. Fondaparinux also appears to be a very promising candidate for the treatment of patients with existing VTE.

Managing anticoagulation in patients with atrial fibrillation

Patients with nonvalvular atrial fibrillation have an increased risk of cerebral thromboembolism. Oral anticoagulation therapy, however, provides primary and secondary stroke prevention in these patients. Here, update your knowledge of warfarin initiation, titration, monitoring, and adjustment.

New Oral Anticoagulants

The risk of bleeding, narrow therapeutic index, and need for routine monitoring make oral anticoagulation with warfarin a less than ideal oral anticoagulant. Intravenous therapies such as heparin are not orally bioavailable and have a narrow therapeutic index and high risk of bleeding. New oral antico agulants currently under investigation may have more ideal characteristics for long-term administration. The mechanism of thrombogenesis, limitations of current therapy, and new investigational oral anticoagulants are discussed.

Treatment of venous thromboembolism

The combination of unfractionated heparin or low molecular weight heparin and oral anticoagulants is currently the treatment of choice for most patients with venous thromboembolism. Oral anticoagulants are started at the same time and heparin is discontinued after at least 5 days when the levels of the International Normalized Ratio reach the therapeutic range between 2.0 and 3.0. Low molecular weight heparin has potential advantages over heparin and is administered in subcutaneous weight-adjusted fixed doses without need for monitoring. This has made the home treatment of a large proportion of patients possible. Randomized clinical trials and several subsequent reports from clinical practice have demonstrated the efficacy and safety of this approach. The results of currently ongoing trials aimed to assess the efficacy and safety of newer compounds for the initial treatment of venous thromboembolism are expected. Oral direct thrombin inhibitors or selective factor-Xa inhibitors have the potential to become the treatment of choice in the next decade. The optimal duration of the secondary prophylaxis with oral anticoagulants is still a matter of debate. The rate of recurrence has been shown to be elevated, particularly in patients with idiopathic venous thromboembolism. A 3-month therapy is therefore currently recommended when a transient risk factor is identified, life-long treatment is recommended for patients with a second episode of venous thromboembolism. The presence of active cancer or a thrombophilic state may require long-term anticoagulation, although not all the congenital hypercoagulable states seem to carry the same level of risk. In all other cases, 6 months are recommended, but a long-term monitoring of the patients is advisable. The use of more aggressive strategies such as thrombolysis is limited to patients presenting with massive pulmonary embolism or signs of right ventricular dysfunction.

Fondaparinux, a synthetic pentasaccharide: the first in a new class of antithrombotic agents - the selective factor Xa inhibitors

Despite currently available antithrombotic therapies, venous thromboembolism (VTE) remains a major cause of morbidity and mortality. Fondaparinux sodium (pentasaccharide), the first in a new class of antithrombotic agents developed for the prevention and treatment of VTE, inhibits thrombin generation by selectively inhibiting factor Xa. Fondaparinux exhibits complete bioavailability by the subcutaneous route and is rapidly absorbed, reaching its maximum concentration approximately 2 h post dosing. It has a terminal half-life of 13 to 21 h, permitting once-daily dosing. Fondaparinux's reproducible linear pharmacokinetic profile exhibits minimal intrasubject and intersubject variability, suggesting that individual dose adjustments will not be required for the vast majority of the population and that there will be no need for routine hemostatic monitoring. At therapeutic concentrations (<2 mg/L), fondaparinux exhibits >94% binding to its target protein, antithrombin. Within this same concentration range there is no specific binding by fondaparinux to plasma proteins commonly involved in drug binding, indicating a low potential for drug-drug interactions by protein displacement. Unlike antithrombotic agents prepared from animal extracts (heparins), fondaparinux is chemically synthesized; this leads to batch-to-batch consistency and the absence of potential risk of contamination problems. In recently completed phase III clinical trials in VTE prevention in major orthopedic surgery, fondaparinux showed significant superiority over the low-molecular-weight heparin enoxaparin, providing an overall >50% (P < 0.001) reduction in VTE risk without increasing clinically important bleeding. Additional clinical data support its potential benefits in other venous and arterial thrombotic disorders. In view of these collective findings, fondaparinux is expected to play a major role in the prevention and treatment of venous and arterial thrombotic disease.

Improving the safety profile of warfarin

Since warfarin remains the predominate drug administered for long-term anticoagulation, optimizing therapy for maximum antithrombotic effect with minimal bleeding risk continues to challenge clinicians. Genetic differences exist that affect an individual's response to warfarin. The clinician can use genetic information in the future, along with current approaches to improved monitoring and dose schedules, to maintain even more successfully the appropriate therapeutic range for anticoagulation. The international normalized ratio (INR) is a good indicator of warfarin effect and addressing a high INR result often prevents bleeding complications. However, even with a therapeutic INR, patients are still at risk for bleeding. This review will discuss optimizing warfarin dosing, reducing an elevated INR, and managing the anticoagulated patient who has a bleeding event.

Warfarin versus aspirin in the secondary prevention of stroke: the WARSS study

The role of anticoagulation in the secondary prevention of noncardioembolic stroke has long been an area of debate. Previous evidence has shown that anticoagulation is unsafe at an International Normalized Ratio between 3.0 and 4.5. Results of the recently published Warfarin-Aspirin Recurrent Stroke Study (WARSS) suggest that there is no difference between warfarin and aspirin in the prevention of recurrent ischemic stroke or death or in the rate of major hemorrhage. Differences in the therapeutic interventions used may have had an effect on the differences in endpoints achieved as compared with previous studies. Results of ongoing trials are anticipated to further clarify the role of anticoagulation in the secondary prevention of stroke.

Improving the quality of anticoagulation of patients with atrial fibrillation in managed care organizations: results of the managing anticoagulation services trial

Randomized trials have indicated that well-managed anticoagulation with warfarin could prevent more than half of the strokes related to atrial fibrillation. However, many patients with atrial fibrillation who are eligible for this therapy either do not receive it or are not maintained within an optimal prothrombin time-international normalized ratio (INR) range. We sought to determine whether an anticoagulation service within a managed care organization would be a feasible alternative for providing anticoagulation care. We performed a multi-site randomized trial in six large managed care organizations in the United States. Subjects were aged 65 years or older and had nonvalvular atrial fibrillation. At each site, physician practices were divided into two geographically defined practice clusters; each site was randomly assigned to have one intervention and one control cluster. The intervention cluster received an anticoagulation service that satisfied specifications for high-quality anticoagulation care and was coordinated through the managed care organization. Control clusters continued with their usual provider-based care. We measured the proportion of time that warfarin-treated patients in each of the clusters (intervention and control) were in the target range for the INR at baseline, and again during a follow-up period. Five of the six selected sites succeeded at developing an anticoagulation service. Patients in the intervention and control clusters had similar demographic characteristics, contraindications to warfarin, and risk factors for stroke. Among patients (n = 144 in the intervention clusters; n = 118 in the control clusters) for whom data were available during the baseline and follow-up periods, the changes in percentages of time in the target range were similar for those in the intervention clusters (baseline: 47.7%; follow-up: 55.6%) and in the control clusters (baseline: 49.1%; follow-up: 52.3%; intervention effect: 5%; 95% confidence interval: -5% to 14%; P = 0.32). Although it was feasible in a managed care organization to implement anticoagulation services that were tailored to local circumstances, provision of this service did not improve anticoagulation care compared with usual care. The effect of the anticoagulation service was limited by the utilization of the service, the degree to which the referring physician supports strict adherence to recommended target ranges for the INR, and the ability of the anticoagulation service to identify and to respond to out-of-range values promptly.

Treatment of venous thromboembolism

Venous thromboembolism (VTE), comprised of pulmonary embolism (PE) and deep vein thrombosis (DVT), is a disease entity with a significant morbidity and mortality. Anticoagulation, initially with intravenous heparin and followed with long term warfarin treatment is the traditional therapy for VTE. Low molecular weight heparin, (LMWH) has a greater bioavailability than unfractionated heparin and can be administered subcutaneously. LMWH has resulted in shorter hospitalizations, reduced inicidents of major bleeding complications, and has moved the treatment of VTE for selected patients to the out-patient setting. Thrombolytic therapy has been recommended in patients with life threatening PE such as those with right ventricular dysfunction or hypotension. There are advances in the technology for clot removal with catheter embolectomy and clot fragmentation. Inferior vena cava filters can be place percutaneously in patients who are at high risk for VTE or those in whom anticoagulation is contraindicated. Since VTE is often asymptomatic, prevention is the most effective means to reduce morbidity and mortality.

[Oral anticoagulant treatment: practical aspects and significance of anticoagulant clinics]

Vitamin K antagonists (VKA) decrease the synthesis of the active forms of four coagulation factors (factors II, VII, IX, X) and three inhibitors (proteins C, S, Z). There are VKA having a short half life (Sintrom, Pindione) and VKA having a long half life (Apegmone, Previscan, Coumadine). The treatment is monitored by the INR which in the majority of the indications must range between two and three. The first INR is usually performed 36 to 72 h after starting the treatment. There are a number of drug interactions. The rate of major bleedings range from 1.1 to 4.9 for 100 patient-year according to the published studies. Since around 600,000 patients are treated by VKA in our country, the absolute number of serious bleeding is high (> or = 17,000 per year). Anticoagulant clinics are structures aimed to instruct the patient and to advise the general practitioner to monitor the treatment, using computer assisted methods. It has been reported that these structures reduce the incidence of bleeding and of thrombotic events by 3 to 4 times.

Deep Venous Thrombosis and Pulmonary Embolism

Venous thromboembolic disease, including deep venous thrombosis (DVT) and pulmonary embolism (PE), is an under-diagnosed and under-appreciated medical problem that results in significant patient morbidity and mortality. Inadequate venous thromboprophylaxis in surgical as well as medically ill patients results in DVT and PE that negatively impact patient outcomes and increase health-care costs. A high index of clinical suspicion combined with an evidence-based use of diagnostic tests helps identify patients with acute thrombosis. Failure to accurately and promptly diagnose and treat DVT and PE can result in excess morbidity and mortality due to postthrombotic syndrome, pulmonary hypertension, and recurrent thrombosis. Conversely, unnecessary anticoagulation provides risk in the absence of any tangible benefit. The immediate commencement of parenteral anticoagulant therapy with intravenous unfractionated heparin or a subcutaneous low molecular weight heparin (LMWH) upon presentation with DVT or PE (often even before objective diagnosis confirmation) is necessary to minimize propagation, embolization, and recurrence rates. We favor weight-based LMWH therapy in most of our patients with DVT because of the ability to treat exclusively or primarily in the outpatient setting. We still admit patients with PE for a minimum duration of 2 days for close observation. Subsequent conversion to oral anticoagulation with warfarin (target INR of 2.0 to 3.0 in most patients) should include an overlap with parenteral therapy of at least 4 to 5 days and until a stable target INR has been achieved. A minimum of 3 to 6 months of anticoagulation is recommended following a first episode of idiopathic DVT and any PE. A shorter course of therapy may be sufficient following a situational (eg, after surgery and postpartum) or calf DVT. Long-term, and at times lifelong, therapy should be considered in patients with thrombosis in the setting of a persistent acquired or inherited hypercoagulable state. Thrombolytic therapy probably should be reserved for young patients with iliofemoral DVT, any patient with a threatened limb due to impending venous limb gangrene, and those with PE who have objective evidence of cardiopulmonary compromise. Unfavorable risk-to-benefit and cost-to-benefit ratios make more extensive use of thrombolytics undesirable. The prevention of the postthrombotic syndrome with fitted, graduated compression garments and age- and gender-appropriate cancer screening are indicated in all patients with DVT in an attempt to minimize morbidity and mortality. Hypercoagulable state testing is indicated when the results of individual tests will significantly impact the choice of anticoagulant, intensity of therapy, therapeutic monitoring, family screening, family planning, prognosis determination, and most of all, duration of therapy.

Accuracy and precision of the CoaguChek S versus laboratory INRs in a clinic

BACKGROUND

The CoaguChek S is the next-generation coagulation monitor for measuring the international normalized ratio (INR) that replaces the CoaguChek device. Studies are lacking comparing the CoaguChek S with local laboratory INR assessment to ensure its accuracy and precision for monitoring patients on anticoagulation.

OBJECTIVE

To evaluate accuracy, precision, and technical ease-of-use of the CoaguChek S compared with laboratory measurements.

METHODS

Accuracy was evaluated in 101 patients by parallel assessment of INRs (CoaguChek S and laboratory); precision was evaluated in 31 patients using duplicate INRs from CoaguChek S and laboratory and from liquid quality controls. Accuracy was determined using orthogonal regression, Bland-Altman plot, and clinical applicability (INRs discrepant in categorization of INR goal and resulting in different therapeutic decisions). Precision was examined by comparing mean difference +/- SD between repeated INRs from CoaguChek S and laboratory, coefficient of variation (CV), and coefficient of repeatability (CR). The influence of low and elevated INRs on accuracy and precision was also examined. To assess ease-of-use of the monitor, the number of technical errors was recorded.

RESULTS

The CoaguChek S significantly correlated to laboratory measurement (r = 0.93); 16.7% of INRs resulted in discrepant categorization and 24.5% would have required a different therapeutic plan. The CV and CR compared well between CoaguChek S and laboratory (6% vs. 4.9%; 0.455 vs. 0.346, respectively). When subgroups of INR values <4.0 and <3.0 were evaluated, the precision improved with both methods. Precision, based on liquid quality controls, was good (CV 4.6% = low-level; 3.3% = high-level). The CoaguChek S was found to have an error rate of 1.8%.

CONCLUSIONS

The CoaguChek S is an accurate and precise alternative to laboratory assessment of the INR at values <4.0; it is an efficient device with a low likelihood of errors during testing.

Questions and Answers from the F.I.X. • Insulin Drips • Warfarin Protocol • Glucosamine Chondroitin Adverse Reaction • Proton-Pump Inhibitor Review

The Formulary Information Exchange (The F.I.X.) is an online drug information service available to subscribers to The Formulary Monograph Service at http://theformulary.com , or you may log on through drugfacts.com . In this column, we present samples of recent dialog on The F.I.X. If you would like more information on The Formulary Monograph Service or The F.I.X., please call 800-322-4349.

Evaluation and management of acute ischemic stroke

Stroke continues to be a major cause of adult mortality and disability. After numerous clinical trials and hundreds of millions of dollars spent on research, only two drugs are effective in treating patients with acute stroke. Recombinant tissue-plasminogen activator improves the chance of an excellent outcome in treated patients by 30%. Danaparoid sodium improves the chance of a very favorable outcome in treated patients with stroke due to large artery atherosclerosis. Although acute treatments are limited, our understanding of stroke pathogenesis and the importance of preventing poststroke complications has improved patient outcome significantly.

Increased sensitivity to warfarin in elderly Hispanics

It has been suggested that aging enhances the pharmacologic effect of warfarin, but there is little information about the effects of warfarin in aging minority populations. The authors examine the response of an aging Hispanic population to warfarin. Charts in their anticoagulation clinic were retrospectively examined for the following information: age, sex, weight, duration of anticoagulant therapy, number of medical problems, number of medications, number of minor or major bleeding episodes, prothrombin time, warfarin dose, and international normalized ratio (INR). The dose-adjusted prothrombin time ratio (PTR) and dose-adjusted INR were calculated by dividing the PTR and INR by the mean warfarin dose. Four groups were compared by age: < 50, 50 to 59, 60 to 69, and > or = 70. A total of 243 charts were reviewed: 113 female, 130 male; 90% were Hispanic. The most common indication for anticoagulation was atrial fibrillation. Elderly patients had more medical problems (3.1 vs. 2.4) and took more medications (3.4 vs. 2.4) than younger patients. The dose-adjusted PTR and dose-adjusted INR increased with aging (0.59 vs. 0.38 and 0.85 vs. 0.59, p < .05 ANOVA). In a multiple linear regression analysis, only age remained significantly associated with the anticoagulant effect. These results are consistent with previous reports on the effect of warfarin in aging patients and extend these data to the Hispanic population.

Possible effect of refrigeration of warfarin on the international normalized ratio

A 43-year-old African-American woman taking warfarin for prevention of ischemic stroke experienced fluctuating international normalized ratio (INR) values over 8.5 months; no cause could be identified. After reading a pharmacy information sheet that accompanied a warfarin refill, she reported that she had been refrigerating her warfarin because her other drugs had been "sticking together." She then was instructed to store her warfarin at room temperature. During the 8.5 months she had been refrigerating her warfarin, 80% of her INR values had been outside her goal range versus 37.5% during 9 months of storage at room temperature. A MEDLINE search and communication with the drug's manufacturer provided no information regarding storage of warfarin outside the temperature range of 59-86 degrees F and resultant changes in potency of the drug. Because of potential fluctuation in anticoagulation control, patients should be reminded to store their warfarin at room temperature.

Selection of anticoagulants or antiplatelet-aggregating agents for prevention of stroke

Stroke is one of leading causes of mortality and morbidity in the United States. Stroke prevention includes treatment of the stroke risk factors and long-term use of antithrombotic agents. Various agents have been studied for stroke prevention and other trials are ongoing. The aim of this article is to provide an overview of the recent guidelines, recommendations, and clinical trial results using antithrombotic therapy for stroke prevention.

Clinical practice guidelines. Managing oral anticoagulant therapy

Clinical practice guidelines (CPG) provide recommendations based on the summary of large volumes of literature and expert opinion. They are not absolute care directives, but are intended to facilitate individualized decision making related to patient care situations. Each month this column reviews a CPG of direct relevance to clinical practice. Readers are urged to contact the author, Mary Jo Goolsby (mjgoolsby@aanp.org) with requests for reviews of specific clinical guidelines or submissions of manuscripts addressing major guidelines that readers have found helpful in their practice. The Clinical Practice Guideline column is designed to inform practitioners of the wide range of accessible and current recommendations on a myriad of clinical topics. The guideline reviewed this month is actually one part of a large report stemming from the Sixth American College of Chest Physicians (ACCP) Consensus Conference on Antithrombotic Therapy. The management of oral anticoagulant therapy is but one portion of the overall report, complemented and supported by other components, including a preceding chapter that summarizes the mechanisms of action, clinical effectiveness, and therapeutic range of oral agents. This CPG focuses on following issues: (a) initiating and maintaining dosage, (b) managing non-therapeutic dosages, (c) managing the agents during invasive procedures, and (d) recognizing and responding to adverse events. Special situations are discussed, including management of patients who are elderly or pregnant, and comparisons of the management in different models of care. There is in-depth discussion of the literature, accompanied by a summary of recommendations.

Drugs in pregnancy. Anticoagulants

Pregnancy is a period of heightened coagulability and enhanced risk for thrombotic complications. Thromboembolism is the leading cause of maternal mortality. Anticoagulants are very useful during pregnancy for the acute treatment of venous thromboembolism and for the prevention of recurrent venous thromboembolism. They may also be beneficial in patients with thrombophilias, particularly among women who have experienced adverse pregnancy outcomes such as recurrent pregnancy loss. Anticoagulation is essential but problematic in the management of pregnant women with mechanical heart valve prostheses. When utilizing these medications among pregnant women the potential benefits must be balanced against the possibility of maternal haemorrhagic complications, adverse effects on the pregnancy or toxic effects on the fetus. This chapter summarizes current knowledge about the anticoagulant agents, their potential toxicities and their therapeutic role in pregnant women with various indications for anticoagulant therapy.

Difficulties in anticoagulation management during coadministration of warfarin and rifampin

The clinical significance of rifampin's induction of warfarin metabolism is well documented, but no published studies or case reports have quantified this interaction with respect to the international normalized ratio (INR). A patient receiving concomitant rifampin and warfarin to treat a mycobacterial infection and intraventricular thrombus, respectively, underwent routine INR testing at a pharmacist-managed anticoagulation clinic to assess his anticoagulation regimen. A 233% increase in warfarin dosage over 4 months proved insufficient to attain a therapeutic INR during long-term rifampin therapy More aggressive titration of the warfarin dosage was needed. In addition, a gradual 70% reduction in warfarin dosage over 4-5 weeks was necessary to maintain a therapeutic INR after rifampin discontinuation, demonstrating the clinically significant offset of this drug interaction. Extensive changes in warfarin dosage are required to attain and maintain a therapeutic INR during the initiation, maintenance, and discontinuation of rifampin.

Perioperative management of the chronically anticoagulated patient

Common indications for chronic anticoagulation include mechanical prosthetic heart valve, non-rheumatic atrial fibrillation, and venous thromboembolism. Perioperative management of the chronically anticoagulated patient is a complex medical problem, and includes the following issues: urgency of surgery, risk of thromboembolism in the absence of anticoagulation, bleeding risk, consequences of bleeding, ability to control bleeding physically, and duration of bleeding risk after the procedure. Most patients can be managed safely by stopping oral anticoagulants 4-5 days before surgery and restarting anticoagulation after the procedure at the patient's usual daily dose. In general, dental procedures and cataract extraction can be performed without interrupting anticoagulation. Most other procedures can be safely performed with an INR < or = 1.4. For patients with double-wing prosthetic valves (e.g., St. Jude, Carbomedics) in the aortic position, uncomplicated atrial fibrillation, or a remote (>3 months) history of venous thromboembolism, oral anticoagulants can be stopped 4-5 days before surgery and restarted at the usual daily dose immediately after surgery. For other patients at higher risk of thrombosis, "bridging therapy" with outpatient low molecular weight heparin is safe and effective. For urgent procedures, a small dose of oral vitamin K usually will reduce the INR within 24-36 hours to a level sufficient for surgery and avoids exposure to transfused blood products.