Imprecision of prothrombin time monitoring of oral anticoagulation. A survey of hospital laboratories

Results of a Survey of Hospital Coagulation Laboratories in the United States, 2001

Context.—Coagulation and bleeding problems are associated with substantial morbidity and mortality, and inappropriate testing practices may lead to bleeding or thrombotic complications.

Objective.—To evaluate practices reported by hospital coagulation laboratories in the United States and to determine if the number of beds in a hospital was associated with different practices.

Design.—From a sampling frame of institutions listed in the 1999 directory of the American Hospital Association, stratified into hospitals with 200 or more beds (“large hospitals”) and those with fewer than 200 beds (“small hospitals”), we randomly selected 425 large hospitals (sampling rate, 25.6%) and 375 small hospitals (sampling rate, 8.8%) and sent a survey to them between June and October 2001. Of these, 321 large hospitals (75.5%) and 311 small hospitals (82.9%) responded.

Results.—An estimated 97.1% of respondents reported performing some coagulation laboratory tests. Of these, 71.6% reported using 3.2% sodium citrate as the specimen anticoagulant to determine prothrombin time (81.3% of large vs 67.7% of small hospitals, P < .001). Of the same respondents, 45.3% reported selecting thromboplastins insensitive to heparin in the therapeutic range when measuring prothrombin time (59.4% of large vs 39.8% of small hospitals, P < .001), and 58.8% reported having a therapeutic range for heparin (72.9% of large vs 53.2% of small hospitals, P < .001). An estimated 96.3% of respondents assayed specimens for activated partial thromboplastin time within 4 hours after phlebotomy, and 89.4% of respondents centrifuged specimens within 1 hour of collection. An estimated 12.1% reported monitoring low-molecular-weight heparin therapy, and to do so, 79% used an assay for activated partial thromboplastin time (58% of large vs 96% of small hospitals, P = .001), whereas 38% used an antifactor Xa assay (65% of large vs 18% of small hospitals, P = .001).

Conclusions.—Substantial variability in certain laboratory practices was evident. Where significant differences existed between the hospital groups, usually large hospitals adhered to accepted practice guidelines to a greater extent. Some reported practices are not consistent with current recommendations, showing a need to understand the reasons for noncompliance so that better adherence to accepted standards of laboratory practice can be promoted.

Point-of-care prothrombin time measurement for professional and patient self-testing use. A multicenter clinical experience. Oral Anticoagulation Monitoring Study Group

We enrolled 386 subjects in a multicenter study of a point-of-care (POC) prothrombin time (PT) testing device. POC tests were performed by health care professionals using venous and finger-stick specimens and by patients using finger-stick specimens. Venous blood also was analyzed in the local hospital laboratory and a national reference laboratory. Accurate POC results were obtained by professionals using both types of specimens. Patients' results were equivalent to those of professionals. The identification of the patient's therapeutic status based on the International Normalized Ratio (INR) was equivalent for POC and local hospital laboratory PT results; 75% of local laboratory results and 77% of POC results were within 0.4 INR of reference laboratory results, while 93% of either system (POC or local laboratory) were within 0.7 INR. Patients overwhelmingly reported satisfaction with the self-test, including the finger stick and device operation. The INR from the POC device is clinically equivalent to the laboratory INR for assessment of anticoagulation status and management decisions in professional and self-testing environments. Patients can learn to perform accurate PT testing, and POC PT testing is feasible in patients' homes.

Monitoring oral anticoagulant treatment with the TAS near-patient test system: comparison with conventional thromboplastins

BACKGROUND

A number of instruments have been developed for determination of prothrombin time (PT) and International Normalised Ratio (INR) at locations not limited to central laboratories.

AIM

To evaluate one such portable instrument, the Thrombolytic Assessment System (TAS), which can be used in a near-patient setting.

METHODS

Samples from 20 normal subjects and 48 patients treated with warfarin for venous thromboembolic disease were studied. The warfarin group was divided into: initiation phase (n = 10), combined warfarin and heparin (n = 10), stabilised therapy (n = 20), and over anticoagulated patients (n = 8). PTs and INRs were determined in each group using three conventional thromboplastins (Diagen Activated, Manchester Reagent, and Instrumentation Laboratory) and two TAS techniques (whole blood or plasma). An independent International Sensitivity Index (ISI) calibration of the TAS system was performed.

RESULTS

Calculated ISIs for the TAS were 1.028 and 0.984 for plasma and whole blood analysis, respectively, compared with manufacturer's values of 0.98 and 0.97. INR results with TAS (whole blood) were 11% less than those obtained with Diagen Activated (p < 0.01) and 16% less than those obtained with Instrumentation Laboratory (p < 0.001) when manufacturers' mean normal PT and ISI were used for TAS INRs. TAS (whole blood) results were similar to TAS plasma or Manchester Reagent results. The use of a locally determined mean normal prothrombin time (MNPT) improved agreement between TAS and the other reagents, abolishing the significant difference between INRs determined with TAS (whole blood) and Diagen Activated techniques.

CONCLUSION

The TAS system can be used with whole blood or plasma and produces similar INRs to those obtained with Diagen Activated or Manchester Reagent using manufacturer's ISI and a locally determined MNPT. Results were lower with TAS or Manchester Reagent compared with those obtained with Instrumentation Laboratory thromboplastin.

Primary stroke prevention in nonvalvular atrial fibrillation: implementing the clinical trial findings

OBJECTIVE

To review the clinical trials evaluating warfarin for primary stroke prophylaxis in nonvalvular atrial fibrillation (NVAF), to discuss the relative benefits and risks of warfarin versus aspirin therapy, and to review the clinical practice guidelines and identify potential barriers to their implementation in clinical practice.

DATA SOURCES

A MEDLINE literature search was performed to identify clinical trials of antithrombotic therapy for NVAF, clinical practice guidelines, studies evaluating physician practices and attitudes, cost-effectiveness studies, and pertinent review articles. Key search terms included atrial fibrillation, stroke, antithrombotic, warfarin, aspirin, and cost-effectiveness.

DATA EXTRACTION

Prospective, randomized clinical trials were selected for analysis. Clinical practice guidelines from recognized panels of experts were reviewed. Comprehensive review articles were selected.

DATA SYNTHESIS

NVAF is a common arrhythmia that is associated with a substantial risk for stroke. Seven prospective, randomized, clinical trials have conclusively demonstrated the efficacy of warfarin for stroke prevention. The greatest benefits are achieved in older patients and those with comorbidities that increase their risk for stroke. The potential benefits of preventing a devastating stroke, however, must be weighed against the potential for bleeding complications. Warfarin has been shown to be cost-effective in high-risk patients, provided the rate of complications is minimized. Nonetheless, many physicians remain hesitant to implement warfarin therapy in older, high-risk patients. The clinical data on aspirin are less consistent than those observed with warfarin. Aspirin appears to be most effective in younger individuals or those considered to be at low risk for stroke.

CONCLUSIONS

In patients with NVAF, the personal, social, and economic consequences of stroke are often devastating. Clinical trials have provided definitive proof that the risks of stroke can be significantly reduced through the use of appropriate antithrombotic therapy. Despite this evidence and the recommendations of a number of clinical practice guidelines, variations in care exist that continue to place patients at risk. Additional outcomes research is needed to evaluate the impact of the clinical trial findings and practice guidelines on clinical practice and to develop methods for overcoming barriers to implementation.

Using the international normalized ratio to standardize prothrombin time

The international normalized ratio, or INR, was introduced in 1983 by the World Health Organization, or WHO, Committee on Biological Standards to more accurately assess patients receiving anticoagulation therapy. The INR mandates the universal standardization of prothrombin time. This article describes the method used to calculate INR, as well as its clinical relevance to the practice of dentistry.

The anticoagulant, antithrombotic and haemorrhagic effect of long-term warfarin on experimental venous and arterial thrombosis in the rat

The amount of thrombus formed in one hour, on standard platinum wire in aorta and vena cava of 48 control and 96 warfarin treated rats of both sexes, was measured. Warfarin was given in varying doses (0.1-0.18 mg/kg) for ten consecutive days before operation to enable the coagulation factors to stabilize. Factors II, VII and X levels as well as prothrombin time (PT) values were obtained for each animal. Factor levels and PT ratio were related to reduction in arterial and venous thrombus weight. The relation between reduction in thrombus weight and depletion in coagulation factor levels was best for factor II and fairly good for factor X at all levels of anticoagulation. All animals which were haemorrhagic had factor II levels of below 15%, with a mean of 8%. At low warfarin dosage a significant reduction of thrombus (> 30%) went undetected by any test, but was least frequently undetected by factor II and most frequently undetected by PT. All rats with diminished thrombus had at least one factor depleted. Haemorrhagic animals were found with any PT ratio in excess of unity. Factor II reflected antithrombotic and haemorrhagic effects of warfarin much better than factors X, VII, or PT.

Management of anticoagulation therapy with the International Normalized Ratio

Nurse practitioners have both the education and the counseling skills to evaluate and manage patients receiving warfarin therapy. The benefits of such treatment should be weighed against potential risk before initiation of therapy. Patients receiving warfarin need to be closely monitored because of the narrow therapeutic range of the drug. The INR system provides the most consistent results of dose response to warfarin, but has not been adopted by all laboratories. Patients should be counseled on the need for accurate laboratory testing. Testing should be done at the same laboratory, and by one that uses the INR system. Until a standardized and sensitive thromboplastin is produced by recombinant technology, the INR system is the most practical and clinically useful method of monitoring anticoagulation therapy.

Use of INR to assess degree of anticoagulation in patients who have dental procedures

Dental professionals frequently treat patients who are receiving anticoagulation therapy. Proper treatment may require adjustment of the anticoagulant dose usually on the basis of the patient's current prothrombin time. This test has been shown to be less accurate than previously thought. The international normalized ratio is another method that attempts to standardize the degree of anticoagulation and to improve reproducibility of results. This system is slowly being implemented in laboratories in the United States. Practitioners who treat patients taking anticoagulants need to be aware of this system in order to make appropriate management decisions.

INR reporting in Canadian medical laboratories. Thrombosis Interest Group of Canada

A written survey of all licensed medical laboratories in Canada performing coagulation testing was performed to investigate the level of knowledge and overall usage of the INR system for reporting prothrombin time results in medical laboratories. There was an overall response rate of 857 of 1,228 laboratories surveyed. Fifty-seven percent of responding laboratories utilized some format of INR reporting. The ISI of the individual thromboplastin utilized was known by 89% of laboratories. The ISI of the thromboplastin utilized was known to be specific for the particular reagent/instrument combination in 44% of cases. Fifty-five percent of client physicians preferred PT results to be reported in seconds while 42% desired an INR format. The situation in Canada is similar to the United States in that further education regarding the INR system for PT reporting is required by both medical laboratories and physicians.

Effect of thromboplastin and coagulometer interaction on the precision of the International Normalised Ratio

AIMS

To examine the magnitude of thromboplastin and coagulometer interactions on the precision of International Normalised Ratio (INR) values when the manufacturers' recommended instrument specific International Sensitivity Index (ISI) values are adopted for the INR calculation.

METHODS

The variability of INR values obtained from four automated phototopical coagulometers frequently used in North American laboratories was studied. When used with five commercial thromboplastins of moderate to high sensitivity (ISI values 0.92-1.97), 20 prothrombin time results were generated for each of a population of 98 patients on established warfarin treatment.

RESULTS

The mean INR values of the patients ranged from 2.05 to 2.81, depending on which reagent/coagulometer combination was used. Within patient variation increased as the median INR value increased. The mean coefficient of variation of within patient INR values was 10%; the mean coefficient of variation of the prothrombin time results in seconds and prothrombin time ratio were 21 and 18%, respectively.

CONCLUSIONS

There was considerable bias in the estimated ISI values of the thromboplastins compared with the manufacturers' instrument specific ISI value. Despite this apparent imperfection, our study clearly showed that the INR is preferable to other prothrombin time reporting formats for assessing the degree of anticoagulation for patients on warfarin treatment.

The use and limitations of the INR system

This study was undertaken to answer some practical questions physicians and medical directors of clinical laboratories face when they contemplate replacing their high international sensitivity index (ISI) thromboplastins with low ISI ones and using international normalized ratio (INR) in place of prothrombin time ratio (PTR) for monitoring warfarin therapy. To the question of whether low-ISI thromboplastins would produce a prolonged PT on normal patients, the answer is probably no. To the question of the extent of normalization of disparate PTs, determined by high and low ISI thromboplastins, of patients on oral anticoagulants upon the conversion of PTR to INR, the answer is a mixed one. For those whose PTs were 14-20 sec, conversion of PTR to INR would markedly, but not completely, normalize the PTR values. In other words, there would be a lessening of disparity of the PTR after the conversion. For patients whose PTs were > 20 sec, conversion of PTR to INR could even widen the disparity seen with the PTR. Finally, when PTs were assayed on different coagulation devices with the same reagent, highly congenial results were obtained.

Monitoring warfarin therapy with the international normalized ratio

OBJECTIVE

To report a case of suboptimal warfarin monitoring.

CASE SUMMARY

A patient with a history of rheumatic heart disease and a mechanical mitral valve was admitted to the local hospital complaining of left-sided weakness. At the time, she was receiving warfarin 5 mg/d. Upon admission her prothrombin time (PT) was 15 s. An initial computed tomography (CT) scan of the head was negative. On the basis of the initial findings, it was unclear whether the symptoms were caused by a cerebrovascular accident (CVA). The patient was transferred to the University Medical Center for a more thorough evaluation. The diagnosis of CVA was confirmed by a repeat CT scan seven days after the event. On the basis of the information obtained from the local hospital, it was determined that the initial PT of 15 s converted to an International Normalized Ratio (INR) of 1.5, which is below the recommended range for patients with mechanical heart valves. Prior to discharge, the warfarin dosage was increased to obtain an INR in the recommended range of 2.5-3.5.

DISCUSSION

This case illustrates the problems that exist with the current system of PT reporting and the potential advantages of using the INR. Variations in the sensitivity of the thromboplastin reagents used to perform the PT may result in misinterpretation of the level of anticoagulation and errors in warfarin dosage adjustments. The potential for suboptimal anticoagulation is greatly increased in patients, such as the one reported here, who are having PTs performed by multiple laboratories.

CONCLUSIONS

To maximize efficacy and minimized the risk of bleeding complications, warfarin therapy must be individualized and closely monitored. Standardization of PT monitoring through the use of the INR would significantly reduce the potential for suboptimal anticoagulation associated with the traditional system of reporting.

Laboratory monitoring of warfarin therapy in Utah

Accurate laboratory monitoring of oral anticoagulation has been emphasized as an important factor in providing safe and effective therapy for patients with thromboembolism. However, recent reports indicate that coagulation laboratories may not be providing optimal clinical information to clinicians who treat these patients. We surveyed all hospital coagulation laboratories in Utah to determine their format for reporting prothrombin time results in patients receiving oral anticoagulants. We found that less than 50% of laboratories used the reliable reporting format, i.e., the International Normalized Ratio (INR), and that many of the laboratories using the INR format may be reporting incorrect values. Our survey also found a significant lack of interest by physicians in requesting that their laboratories adopt reliable reporting methods. These results indicate a substantial lack of understanding by laboratories and clinicians of the importance of using reliable methods to monitor oral anticoagulation. Significant educational efforts will be required to correct this problem.

Effect of laboratory variation in the prothrombin-time ratio on the results of oral anticoagulant therapy

BACKGROUND

Patients receiving long-term anticoagulant therapy may be subject to unnecessary risks of bleeding or thromboembolism because of variability in the commercial thromboplastins used to determine prothrombin time and consequent uncertainty about the actual intensity of anticoagulation.

METHODS

We explored the effect of this uncertainty on the benefits and risks of anticoagulation in patients with prosthetic heart valves, using models of thromboembolic and hemorrhagic complications as a function of the intensity of anticoagulation, with quality-adjusted life expectancy and average variable costs used to describe outcomes.

RESULTS

Anticoagulation provides a striking benefit for patients whose treatment is conducted within the recommended range of the international normalized ratio (INR)--i.e., 2.5 to 3.5--but if uncertainty about the laboratory results causes the intensity of anticoagulation to fall outside this range, the gain becomes smaller. Uncertainty about the true intensity of anticoagulation may reduce the potential gain in life expectancy, adjusted for quality of life, by more than half and may increase the ratio of costs to effectiveness to almost five times the optimal value. Variability in the intensity of anticoagulation is even greater if older recommendations advocating a higher level of anticoagulation are followed.

CONCLUSIONS

Uncertainty about the sensitivities of the commercially available thromboplastins used in the United States can have important clinical and economic effects. This problem could be eliminated if clinical laboratories uniformly reported the intensity of anticoagulation as the INR, by adjusting prothrombin-time ratios for variability in thromboplastins.