Chromogenic Factor X Assay for Monitoring Warfarin Anticoagulation in a Child With a Prosthetic Mitral Valve

Mechanical mitral valve replacement in infants and young children is associated with substantial morbidity and mortality. Lifelong anticoagulation is required, with all the accompanying challenges of maintaining levels in infants and children whose dietary input continually changes. Even with careful control of all aspects that can perturb the coagulation cascade, these patients have a substantial lifelong risk of thrombotic and hemorrhagic complications that can also affect the durability of the valve. Anticoagulation is usually achieved utilizing warfarin with the degree of anticoagulation measured via the international normalized ratio (INR). Unfortunately, in some cases, the INR can be falsely elevated and lead to inappropriate reassurance. We describe a 4-year-old patient with complex congenital heart disease palliated via a single ventricular pathway with a mechanical atrioventricular valve replacement. The patient experienced acute valvular thrombosis while receiving warfarin with INR at target levels. Chromogenic factor X (CFX) levels were discordant with INR measurements, suggesting a subtherapeutic level of anticoagulation despite maintaining the standard INR target. Therefore, CFX levels were used to interpret INR measurements and guide an individualized approach to anticoagulation. We propose a new role of CFX: to verify and guide warfarin anticoagulation in high-risk pediatric patients including those undergoing mechanical mitral valve replacement.

Evaluation of Bivalirudin’s Effect on International Normalized Ratio to Determine an Appropriate Strategy for Transitioning to Warfarin

Background: Direct thrombin inhibitors are recommended in confirmed or suspected heparin-induced thrombocytopenia. False elevation of the international normalized ratio (INR) occurs with these agents making bridging to warfarin challenging. There is limited data regarding bivalirudin’s effect on INR. Objective: To evaluate bivalirudin’s effect on the INR and determine a strategy for transitioning to warfarin. Methods: This was a retrospective observational study. Included patients were >18 years old receiving primary bridging therapy with overlapping bivalirudin and warfarin for at least 72 hours. Patients with administration of alternate anticoagulants during the transition interval or active major bleeding within 48 hours prior to bivalirudin initiation were excluded. The primary endpoint was to determine the effect on INR at first therapeutic activated partial thromboplastin time after bivalirudin initiation and prior to warfarin initiation. Secondary endpoints included change in INR 12 and 24 hours after bivalirudin initiation, change in INR 4 hours after bivalirudin cessation, and incidence of major bleeding or new thrombotic events. Results: Thirty-four patients met study criteria. For the primary endpoint, the change in INR at first therapeutic activated partial thromboplastin time was 0.37 (range = 0.28-0.48), which occurred at 8.4 hours (range = 4.6-14.2; n = 14). INR increased at 12 and 24 hours by a median of 0.55 and 0.5 from baseline, respectively. Median change in INR 4 to 8 hours post-bivalirudin cessation was −0.48. Conclusion: Targeting an INR > 2.5 when bridging to warfarin will account for this false elevation and maintain an INR above 2.0 on bivalirudin discontinuation.

Minimal Prolongation of Prothrombin Time with Extended Exposure to Argatroban

In the setting of acute heparin-induced thrombocytopenia (HIT), argatroban is one of the initial anticoagulants of choice, which is eventually bridged to warfarin over a period of 5 or more days. Argatroban prolongs prothrombin time (PT) and increases international normalized ratio (INR). However, the effects of prolonged argatroban exposure on the PT and INR are not known. We describe an unusual case of prolonged argatroban treatment in a patient with heparin-induced thrombocytopenia with thrombosis syndrome (HITTS) resulting in a minimal elevation of the INR. The patient received a total of 58 days of argatroban and was resistant to warfarin therapy, requiring a 13-day bridge to achieve a therapeutic INR of 2.0 to 3.0. Ultimately, argatroban was successfully transitioned to warfarin therapy when the INR was 2.7 on both agents, producing the confirmatory true INR of 2.4. Argatroban and warfarin cotherapy did not increase the INR beyond 4.0 after prolonged argatroban exposure. Clinicians should consider this unusual response in other cases of prolonged argatroban use, and monitor INR carefully during warfarin and argatroban cotherapy. The use of other methods to monitor anticoagulant therapy, such as chromogenic factor X assay (CFX), may be helpful in this setting.

VKORC1 and CYP2C9 genotypes are predictors of warfarin-related outcomes in children

BACKGROUND

Despite substantial evidence supporting a pharmacogenetic approach to warfarin therapy in adults, evidence on the importance of genetics in warfarin therapy in children is limited, particularly for clinical outcomes. We assessed the contribution of CYP2C9/VKORC1/CYP4F2 genotypes and variation in other genes involved in vitamin K and coagulation pathways to warfarin dose and related clinical outcomes in children.

PROCEDURE

Clinical and genetic data for 93 children (age ≤ 18 years) who received warfarin therapy were obtained. DNA was genotyped for 93 selected single nucleotide polymorphisms using a custom assay.

RESULTS

With a median age of 4.8 years, our cohort included more young children than most previous studies. Overall, 76.3% of dose variability was explained by weight, indication, VKORC1-1639G/A and CYP2C9 *2/*3, with genotypes accounting for 21.1% of variability. There was a strong correlation (R(2) = 0.68; P < 0.001) between actual and predicted warfarin dose using a pediatric genotype-based dosing model. VKORC1 genotype had a significant impact on time to therapeutic international normalized ratio (INR) (P = 0.047) and time to over-anticoagulation (INR > 4; P = 0.024) during the initiation of therapy. CYP2C9*3 carriers were also at increased risk of major bleeding while receiving warfarin (adjusted OR = 11.28). An additional variant in CYP2C9 (rs7089580) was significantly associated with warfarin dose (P = 0.020) in a multivariate clinical and genetic model.

CONCLUSIONS

This study confirms the importance of VKORC1/CYP2C9 genotypes for warfarin dosing in a young pediatric cohort and demonstrates an impact of genetic factors on clinical outcomes in children. Furthermore, we identified an additional variant in CYP2C9 of potential relevance for warfarin dosing in children.

Diagnosis and management of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a prothrombotic disorder caused by antibodies to platelet factor 4/heparin (PF4/H) complexes. It presents with declining platelet counts 5 to 14 days after heparin administration and results in a predisposition to arterial and venous thrombosis. Establishing the diagnosis of HIT can be extremely challenging. It is essential to conduct a thorough clinical evaluation in addition to laboratory testing to confirm the presence of PF4/H antibodies. Multiple clinical algorithms have been developed to aid the clinician in predicting the likelihood of HIT. Once HIT is recognized, an alternative anticoagulant should be initiated to prevent further complications.

High-dose argatroban for treatment of heparin-induced thrombocytopenia with thrombosis: a case report and review of laboratory considerations

PURPOSE

A case report describing high-dose argatroban for the treatment of heparin-induced thrombocytopenia (HIT) with thrombosis and associated considerations in interpreting laboratory monitoring data are presented.

SUMMARY

A 51-year-old woman with an extensive history of coronary artery disease arrived at the emergency department with complaints of chest pain. The patient was admitted, and coronary artery bypass graft surgery was ultimately performed. The patient had a baseline platelet count of 177,000 cells/μL. During hospitalization, the patient received heparin, and her platelet count dropped to 12,000 cells/μL 13 days after the initiation of heparin. The patient developed swelling around a peripherally inserted central catheter and later developed deep vein thrombosis. An argatroban infusion of 2 μg/kg/min was initiated, with a target activated partial thromboplastin time (aPTT) of 40-80 seconds. After 5 days of therapy, the patient had increased swelling in her right arm and an aPTT of 56 seconds. Her goal aPTT was subsequently increased. Six days later, the patient developed a left-lower-extremity DVT despite aPTTs within the goal range. A new aPTT target of >75 seconds was set. The infusion rate was increased to 15.5 μg/kg/min to attain the target aPTT. Results of an in vitro test led to an alternative interpretation of aPTT and International Normalized Ratio values that aided in the monitoring of argatroban during the high-dose infusion.

CONCLUSION

A patient with HIT with thrombosis was successfully treated with unusually high dosages of argatroban and may have had serum argatroban concentrations exceeding what has commonly been thought to be the therapeutic range.

Heparin induced thrombocytopenia and re-thrombosis associated with warfarin and fondaparinux in a child

An 11-year-old female developed heparin induced thrombocytopenia (HIT) with thrombosis during therapy for lower extremity deep vein thrombosis and pulmonary embolism. Transition from bivalirudin, a direct thrombin inhibitor (DTI), to warfarin resulted in extensive re-thrombosis, and fondaparinux therapy similarly failed. She was then treated with argatroban, and transitioned successfully to warfarin after 9 weeks. The risk of re-thrombosis was ultimately reduced by allowing time for the thrombogenic potential to abate. The argatroban/warfarin transition was monitored with chromogenic factor X levels. This case highlights several difficult problems in pediatric thrombosis.

Reducing harm associated with anticoagulation: practical considerations of argatroban therapy in heparin-induced thrombocytopenia

Argatroban is a hepatically metabolized, direct thrombin inhibitor used for prophylaxis or treatment of thrombosis in heparin-induced thrombocytopenia (HIT) and for patients with or at risk of HIT undergoing percutaneous coronary intervention (PCI). The objective of this review is to summarize practical considerations of argatroban therapy in HIT. The US FDA-recommended argatroban dose in HIT is 2 microg/kg/min (reduced in patients with hepatic impairment and in paediatric patients), adjusted to achieve activated partial thromboplastin times (aPTTs) 1.5-3 times baseline (not >100 seconds). Contemporary experiences indicate that reduced doses are also needed in patients with conditions associated with hepatic hypoperfusion, e.g. heart failure, yet are unnecessary for renal dysfunction, adult age, sex, race/ethnicity or obesity. Argatroban 0.5-1.2 microg/kg/min typically supports therapeutic aPTTs. The FDA-recommended dose during PCI is 25 microg/kg/min (350 microg/kg initial bolus), adjusted to achieve activated clotting times (ACTs) of 300-450 sec. For PCI, argatroban has not been investigated in hepatically impaired patients; dose adjustment is unnecessary for adult age, sex, race/ethnicity or obesity, and lesser doses may be adequate with concurrent glycoprotein IIb/IIIa inhibition. Argatroban prolongs the International Normalized Ratio, and published approaches for monitoring the argatroban-to-warfarin transition should be followed. Major bleeding with argatroban is 0-10% in the non-interventional setting and 0-5.8% periprocedurally. Argatroban has no specific antidote, and if excessive anticoagulation occurs, argatroban infusion should be stopped or reduced. Improved familiarity of healthcare professionals with argatroban therapy in HIT, including in special populations and during PCI, may facilitate reduction of harm associated with HIT (e.g. fewer thromboses) or its treatment (e.g. fewer argatroban medication errors).

Review: Recent Advances in Argatroban-Warfarin Transition in Patients With Heparin-induced Thrombocytopenia

Heparin-induced thrombocytopenia is a devastating, life-threatening, immune-mediated complication of therapy with unfractionated heparin, and less frequently, with low molecular weight heparin. Direct thrombin inhibitors are now standard therapy for the prevention of thrombosis in heparin-induced thrombocytopenia. Argatroban, a small synthetic molecule that inhibits thrombin at its active site, is increasingly used as the direct thrombin inhibitors of choice. Transition to longer term oral anticoagulation needs to be instituted after the platelet count has risen, because of the persistent risk of thrombosis. Although guidelines available in the literature outline the management of heparin-induced thrombocytopenia, they are not presented in a concise and comprehensive manner easily followed by physicians. This article reviews current recommendations, relevant studies, and clinical management trials carried out on patients with heparin-induced thrombocytopenia and provides updated, detailed guidelines for treatment of heparin-induced thrombocytopenia with emphasis on a key part of the management, the argatroban—warfarin transition.

Transition From Argatroban to Oral Anticoagulation With Phenprocoumon or Acenocoumarol: Effect on Coagulation Factor Testing

Treatment with the thrombin inhibitor argatroban is often followed by vitamin K-antagonist treatment. In this study, the behavior of coagulation factors measured under these treatment regimens is shown. Healthy subjects received infusions of 1.0, 2.0, or 3.0 µg/kg/hr argatroban before and during phenprocoumon or acenocoumarol dosing. Quantitation of factors II, VII, IX, and X by clot-based assays resulted in dose dependent, approximately 20%, lower than expected values in the presence of argatroban. On the contrary, values for the inhibitors, protein C and protein S, were higher. Cotherapy exaggerated the effect by vitamin K-antagonist alone. However, testing by immunologic and chromogenic assays did not show any effect by argatroban. Coupled with a lack of bleeding in the subjects, these data suggests that argatroban does not affect coagulation proteins and that the observations are only an assay artifact. Assay interferences must be considered when measuring coagulation proteins in patients receiving thrombin inhibitors.

HEMATOLOGY: ISSUES IN THE DIALYSIS PATIENT: When Heparin Causes Thrombosis: Significance, Recognition, and Management of Heparin-Induced Thrombocytopenia in Dialysis Patients

Heparin-induced thrombocytopenia (HIT) is characterized by thrombocytopenia and paradoxical hypercoagulability. HIT occurs when an antibody ("HIT antibody") produced against the complex of heparin and platelet factor 4 (PF4) causes systemic platelet consumption and activation. Nephrologists encounter HIT in the care of end-stage renal disease (ESRD) patients because heparin is a routine anticoagulant in hemodialysis. The incidence of HIT in ESRD appears to be lower than in other clinical settings. However, HIT is equally life threatening in ESRD patients and therefore demands the same prompt recognition and aggressive treatment. Diagnosing HIT requires the detection of HIT antibodies. A functional assay (e.g., [(14)C] serotonin release assay) relies on the patient's HIT antibodies to activate donor platelets at pharmacologic heparin concentrations. The more common antigen assay (e.g., enzyme-linked immunosorbent assay [ELISA]) detects the binding of the patient's HIT antibodies to antigens (e.g., heparin-PF4 complex) in a microtiter well and does not involve platelets. The moment HIT is suspected, heparin should be stopped and an alternative anticoagulant initiated immediately, even before the result of a serologic test becomes available. The advent of several new anticoagulants in the last decade, especially argatroban and bivalirudin, has expanded treatment options for HIT in dialysis patients. This review discusses the epidemiology, pathogenesis, clinical features, diagnosis, and treatment of HIT, with special emphasis on concepts relevant to the care of dialysis patients.

Transitioning from argatroban to warfarin in heparin-induced thrombocytopenia: an analysis of outcomes in patients with elevated international normalized ratio (INR)

BACKGROUND

Heparin-induced thrombocytopenia (HIT) can lead to catastrophic thromboembolic complications and requires treatment with an alternative, rapidly active anticoagulant, such as a direct thrombin inhibitor (DTI), either to prevent or treat these complications. Switching to oral warfarin after initial treatment with a DTI is necessary in most patients. Most references related to warfarin suggest that an increased risk for bleeding will occur with elevated international normalized ratios (INRs) > 4.6. In patients receiving argatroban, it is not uncommon to achieve an INR > 4 during this transition. Because the clinical outcomes in patients achieving an INR > 4 during combined argatroban/warfarin therapies for HIT are not well described, we evaluated the clinical outcomes of 111 patients with this phenomenon.

METHODS

We identified patients from the prospective studies of argatroban anticoagulation, Argatroban-911 and Argatroban-915. Data collected from these studies included death from all causes, amputation, new thrombosis, major bleeding, INR values, argatroban doses, aPTT values, platelet counts, and duration of therapy.

RESULTS

Patients were on argatroban monotherapy for a median of 2.8 (0.1-8.1) days, and on cotherapy for a median of 3.7 (0.9-12.8) days. The median platelet count was 70.9 (18-325) x 10(9)/L at the time of HIT diagnosis and increased to 94 (30-324) x 10(9)/L by the time warfarin was initiated. At a median argatroban dose of 1.4 (0.2-2.0) mcg/kg/min, the maximum INR ranged from 4.1 to 21.2 (median 6.4, n = 111) and the corresponding aPTT ranged from 48.1 to 105 (median 71, n = 93) seconds. After argatroban cessation, the first recorded INR within 4 to 24 hours ranged from 1.5 to 12.5 (median 2.9, n = 58). Adverse clinical outcomes occurred in 9 (8.1%) patients during cotherapy and in 12 (10.8%) patients after argatroban anticoagulation was discontinued. Adverse clinical outcomes included 7 cases of new thrombosis, 3 amputations, 12 deaths and 1 major bleed. Eleven of 12 (91.7%) patients died due to causes other than thrombosis, and most deaths (83%) occurred following cotherapy. Five (4.5%) patients developed new thrombosis during argatroban/warfarin cotherapy despite an INR > 4. In contrast only 1 (0.9%) patient experienced major bleeding.

CONCLUSION

In patients receiving argatroban/warfarin cotherapy and with an elevated INR > 4, the risk for thrombosis exceeds the risk of bleeding. Traditional paradigms concerning elevated INRs and warfarin may need to be redesigned for the patient population on cotherapy with direct thrombin inhibitors.Abbreviated Abstract. The clinical outcomes of 111 patients with INRs > 4 while on combined argatroban (dose < or = 2 mcg/kg/min) and warfarin were evaluated. Adverse clinical outcomes (7 new thrombosis, 3 amputations, 12 deaths and 1 major bleed) occurred in 21 patients. Eleven deaths were due to causes other than thrombosis. Five patients developed new thrombosis while only 1 had major bleeding. The risk for thrombosis exceeds the risk of bleeding in patients with HIT despite an INR > 4.

Transitioning from argatroban to warfarin therapy in patients with heparin-induced thrombocytopenia

Argatroban, a direct thrombin inhibitor used for thromboprophylaxis or treatment in heparin-induced thrombocytopenia (HIT), is routinely monitored using the activated partial thromboplastin time (aPTT) yet also prolongs the international normalized ratio (INR). Peritransitional INRs, aPTTs, anticoagulant dosing patterns, and outcomes were evaluated in 165 HIT patients who were transitioned, without guidelines, from argatroban to warfarin therapy. Argatroban (median doses: 1.5-2.0 mcg/kg/min) and warfarin (median dose: 5 mg initially with 3.8 mg/day thereafter) overlapped a median 4 days. Median (5-95th percentile) aPTTs (in seconds) and INRs, respectively, were 59.8 (38.8-82.9) and 3.2 (1.7-7.0) during argatroban monotherapy, 68.6 (44.5-104) and 5.3 (2.4-16) maximally during cotherapy, 59.9 (38.7-92.2) and 4.0 (2.2-11.6) immediately before argatroban cessation during cotherapy, and 36.0 (25.6-60.2) and 2.3 (1.3-7.3) within a median 10-12 hours after argatroban cessation. Major bleeding occurred in 1 (0.6%) patient pretransitionally and no patient during or after cotherapy. Eighteen (10.9%) patients experienced 19 peritransitional adverse outcomes (one death, two amputations, 16 new thromboses); these patients had more severe HIT than event-free patients (median baseline platelet count, 39 vs. 83 x 10(9)/L). Of 108 patients with post-transitional INR data, 43 achieved a therapeutic INR (prospectively defined as 1.9-3.5), 34 were subtherapeutic, and 31 were supratherapeutic, with no across-group trend in new thrombosis. Hence in the absence of guidelines, physicians transfer patients from argatroban to warfarin therapy with acceptably low complication rates in HIT, without systematically over- or under-dosing warfarin. Furthermore, INRs greater than 5 commonly occur in HIT patients during argatroban monotherapy and argatroban/warfarin cotherapy, without major bleeding.

Comparison of Two Different Ecarin Clotting Time Methods

BACKGROUND

Ecarin Clotting Time (ECT) assay specifically determines the inhibition of meizothrombin by direct thrombin inhibitors (DTI). Blood coagulation factor levels lowered by vitamin K antagonists (VKA) may prolong ECT. Concomitant treatment of VKA with DTI may influence differently the two published ECT methods.

METHODS

Lepirudin (100-3,000 ng/ml), argatroban (300--3,000 ng/ml) and melagatran (30--1000 ng/ml) were added to normal plasma (NP; n=12) samples and to plasma of patients on stable vitamin K antagonist therapy with warfarin (VKAP; n=12). ECT assays were performed according to [5] (method 1) and according to [6] (method 2). Data were subjected to multifactorial variance analysis.

RESULTS

Normal ranges were 35.5+/-2.8 s in NP versus 31.8+/-1.2 s in VKAP with method 1 (p< 0.001) and 44.3+/-3.9 s in NP vs. 51.4+/-8.3 s in VKAP with method 2 (p< 0.004). Besides the inhibitors (p<0.0001), the method used (p<0.0001) and the group (NP vs. VKAP, p=0.003) had an influence on the ECT. Inhibitors (p< 0.02) or method used (p< 0.03) and the group (NP vs. VKAP, p=0.0001) influenced also the ECT ratio.

DISCUSSION

Both ECT methods are suitable for monitoring different DTIs over a large linear range with both methods during concomitant treatments with vitamin K antagonists. The ECT ratio improves but not abolishes the differences between the methods. Additive effects of vitamin K antagonists on ECT methods have to be taken into consideration in clinical routine.

Alternative parenteral anticoagulation with argatroban, a direct thrombin inhibitor

Argatroban, a direct thrombin inhibitor, effectively inhibits free and clot-bound thrombin without the need of a cofactor and exerts dose-dependent anticoagulant effects that are rapidly active and rapidly reversible (elimination half-life: 39-51 min). Argatroban provides predictable parenteral anticoagulation and is well tolerated with an acceptably low bleeding risk in a variety of clinical settings, including heparin-induced thrombocytopenia, acute ischemic stroke, percutaneous coronary intervention and hemodialysis. This review will discuss the clinical pharmacology and utility of argatroban; in particular, clinical trial experiences will be discussed in patients with, or at risk of, heparin-induced thrombocytopenia (where heparins must be avoided) including those requiring hemodialysis or percutaneous coronary intervention, and in patients with acute ischemic stroke (where heparins are not generally recommended).

Effects of pentasaccharide (fondaparinux) and direct thrombin inhibitors on coagulation testing

CONTEXT

Direct thrombin inhibitors (DTIs) and fondaparinux represent a new class of anticoagulants. The effects of DTIs on activated partial thromboplastin time and prothrombin time measurements have been reported previously, but there are limited data on the impact of these anticoagulants on other coagulation tests.

OBJECTIVE

To determine the effects of fondaparinux and 3 DTIs (argatroban, bivalirudin, and lepirudin) on miscellaneous coagulation tests.

DESIGN

Bivalirudin, lepirudin, argatroban, and fondaparinux were added to pooled normal plasma and tested for fibrinogen, antithrombin (thrombin and Xa substrate methods), plasminogen, protein C (clot and chromogenic methods), protein S, von Willebrand factor, D-dimer, lupus anticoagulant testing (dilute Russell viper venom test [DRVVT] with ratio), and factors II, IX, and X activities.

RESULTS

We found no drug interference on antithrombin, plasminogen, chromogenic protein C, von Willebrand factor, or D-dimer results. All DTIs falsely decreased fibrinogen values, while falsely increasing protein C and protein S levels. All DTIs prolonged the DRVVT, and only argatroban yielded DRVVT ratios less than 1.2. Lepirudin demonstrated no effect on factor II activity, and only argatroban demonstrated decreased factor X activity. All DTI samples demonstrated a linear, dose-dependent, false decrease of factor IX activity.

CONCLUSIONS

Using in vitro methods, we demonstrated DTI effects on numerous clot-based assays, but we found no interference with latex agglutination, chromogenic, or platelet aggregation methods. Fondaparinux only affected measurement of protein S activity. Caution must be used when interpreting coagulation test results on patients receiving these drugs.

Diagnosis and management of heparin-induced thrombocytopenia

Heparin use is ubiquitous, wherein 1 to 5% of patients exposed to standard unfractionated heparin develop thrombocytopenia due to antibodies to a complex of heparin and platelet factor 4. Classic features include onset of thrombocytopenia after 5 to 10 days of ongoing heparin exposure, a 50% fall in the platelet count from baseline, resolution of the thrombocytopenia 5 to 10 days after cessation of heparin and a high risk of thrombosis noted in 30 to 75% of patients with heparin-induced thrombocytopenia (HIT) in terms of every-other-day platelet-count monitoring in patients on standard unfractionated heparin. And those patients developing thrombocytopenia necessitate an accurate, readily accessible diagnostic test for HIT. Diagnosis has been recently facilitated by the development of an enzyme-linked immunosorbent assay (ELISA) test for the heparin-P4 antibody complex, although this test carries a relatively low specificity. Widespread use of the ELISA demonstrates a relatively high prevalence of the antibody in patients exposed to heparin in certain settings, such as cardiopulmonary bypass, wherein a quarter of patients have a positive ELISA of unclear significance. Once HIT is diagnosed, the high risk of thrombosis necessitates empiric anticoagulation with an antithrombin such as argatroban or lepirudin, or the heparinoid danaparoid. Additional agents under further study include the antithrombin bivalirudin and the pentasaccharide fondaparinux. Future issues in HIT include increasing awareness for HIT, improving the specificity of HIT testing and the development of new anticoagulants for HIT that will enable out-patient management.

Effect of Lepirudin on the International Normalized Ratio

BACKGROUND:

Many patients receiving direct thrombin inhibitor (DTI) therapy require transition to warfarin. This transition may be complicated by DTI-induced elevations in the international normalized ratio (INR). While the effect of argatroban on the INR has been characterized, data assessing the effect of lepirudin on the INR are limited.

OBJECTIVE:

To evaluate the effect of lepirudin on the INR.

METHODS:

Patients receiving lepirudin therapy between January 2000 and May 2001 were identified using the pharmacy database, and a retrospective chart review was conducted. Patients were included for analysis if they had paired activated partial thromboplastin time (aPTT) and INR data while receiving lepirudin monotherapy.

RESULTS:

Fifty-three paired aPTT and INR data points from 8 patients receiving lepirudin monotherapy were collected. The Organon MDA 180 instrument was used for aPTT and prothrombin time (PT) determination. Organon MDA Platelin L reagent was used for the aPTT and Organon Simplastin L reagent was used for the PT. The international sensitivity index (ISI) of the Simplastin L thromboplastin was 2.0. The mean ± SD lepirudin dose was 0.05 ± 0.04 mg/kg/h. Linear regression was used to identify the INRs that correspond to a therapeutic aPTT value of 45–75 seconds (1.5–2.5 times mean laboratory normal of 30 sec). The correlation between aPTT and INR was 0.77. An aPTT of 45–75 seconds with lepirudin correlated to an INR of 1.6–3.2.

CONCLUSIONS:

Based on laboratory results, when using a thromboplastin with an ISI of 2, lepirudin appears to elevate the INR in the absence of warfarin.

Managing Complications of Anticoagulant Therapy

Understanding the frequency, risk factors, and management of anticoagulant-induced adverse events will assist clinicians in optimizing patient outcomes. The most frequent adverse event of all anticoagulants is major bleeding. Risk factors for major bleeding have been identified with the heparin compounds, the direct thrombin inhibitors (DTIs), fondaparinux, and warfarin therapy. Understanding these risk factors can help prevent bleeding events. For cases of clinically significant bleeding, reversal agents exist primarily for heparin and warfarin. Although less common, nonbleeding adverse events of anticoagulant therapy can also be life threatening. The heparin compounds are associated with the development of heparin-induced thrombocytopenia (HIT) and osteoporosis. HIT can result in life-threatening thrombosis and is usually managed with a DTI. Nonbleeding adverse events with warfarin therapy include skin reactions and the development of venous limb gangrene. Appropriate initiation of warfarin therapy may decrease the risk of venous limb gangrene.

Warfarin Initiation and Monitoring with Clotting Factors II, VII, and X

OBJECTIVE

To report a case of a patient with antiphospholipid antibody syndrome and multiple thromboses who developed heparin-induced thrombocytopenia (HIT) and subsequent international normalized ratio (INR) prolongation possibly due to antiphospholipid antibodies.

CASE SUMMARY

A 56-year-old white woman with a history of antiphospholipid antibody syndrome and thrombosis taking chronic warfarin was admitted for gastrointestinal concerns and found to have an INR >14. Warfarin was discontinued, vitamin K was administered, and a heparin infusion was initiated. Over the next 2 days, thrombocytopenia, hypotension, tachycardia, hyponatremia, and progressive abdominal pain developed. Upon transfer to a tertiary care center, HIT was diagnosed, and a lepirudin infusion was initiated. Subsequently, a sudden elevation of the INR occurred (>14) with low prothrombin (factor II) activity. After INR values declined to 2–3, warfarin was reinitiated with dosing adjusted using factor X and II activity levels. Clotting factors II and X activities were measured to monitor long-term warfarin therapy, with no evidence of complications after 7 months.

DISCUSSION

Typically, the INR is used to assess the intensity of anticoagulation. The INR value represents the reduction of clotting factors II, VII, and X. In rare circumstances, an independent inhibitor or interfering substance can interfere with the process of measuring the INR. In such situations, an alternative approach can be direct measurement of clotting factor concentrations.

CONCLUSIONS

Factor II and/or factor X activity levels provided an alternative means for measuring the anticoagulant effects of warfarin in the presence of a significant inhibitor (antiphospholipid antibodies) that biased the INR measurements.

Low-Molecular-Weight Heparin–Induced Thrombocytopenia in a Child

OBJECTIVE

To report a case of probable acute venous thrombosis caused by heparin-induced thrombocytopenia (HIT) in a pediatric patient with a normal platelet count after prolonged enoxaparin therapy.

CASE SUMMARY

An 11-year-old African American female with Crohn's disease developed extensive vena cava thrombosis. Her deep vein thrombosis (DVT) was treated with intravenous unfractionated heparin followed by extended outpatient warfarin therapy. Four months later, the warfarin was stopped and subcutaneous enoxaparin 1.5 mg/kg once daily was substituted prior to an elective colonoscopy. She was readmitted 6 weeks later with acute DVT with a platelet count of 233 × 10 3 /mm 3 , significantly lower than the count of 550–700 × 10 3 /mm 3 5 months previously and the count of 433 × 10 3 /mm 3 3 months earlier. An enzyme-linked immunosorbent assay for heparin-platelet factor 4 antibodies was strongly positive and a d-dimer was elevated at 2.9 mg/L (normal <1.5). She was treated with lepirudin followed by warfarin when repeat d-dimer on day 3 was normal. An ultrasound at that time showed no clot extension, and the platelet count had risen to >300 × 10 3 /mm 3 . Over the next 4 months, there was no further thrombosis.

DISCUSSION

HIT appears to be rare in the pediatric population, and only a few cases treated with a direct thrombin inhibitor have been reported. This is the first case report to our knowledge of a pediatric patient developing HIT secondary to enoxaparin. An interesting feature of this case is the development of HIT in the face of a normal platelet count, which is rare but has been reported in adults.

CONCLUSIONS

Pediatric patients receiving low-molecular-weight heparin are still at risk for developing HIT. Treatment of HIT should involve the initial use of a direct thrombin inhibitor to manage thrombosis until the platelet count returns to higher values. Once the platelet count returns, warfarin can be used for long-term thrombosis management.

Argatroban for heparin-induced thrombocytopenia in hepato-renal failure and CVVHD

OBJECTIVE

To report a case of significant hepatic and renal failure with the use of argatroban in a patient with heparin-induced thrombocytopenia (HIT) requiring continuous veno-veno hemodialysis (CVVHD).

CASE SUMMARY

A 37-year-old Hispanic man with diabetes mellitus, ethanol abuse, and recent crank use developed a venous thrombosis and possible pulmonary embolism, followed by acute liver and renal failure requiring continuous veno-venous hemofiltration. After several days of heparin therapy, the dialyzer circuit thrombosed and the platelet count decreased to 82 x 10(3)/mm(3). Antibodies for HIT by enzyme-linked immunosorbent assay were positive. Heparin was discontinued, and an argatroban infusion was initiated. After 39 hours on argatroban, the infusion was stopped when minor bleeding was observed with a concurrent activated partial thromboplastin time (aPTT) of 100 seconds. The dialyzer circuit did not clot during the argatroban infusion and did not apparently influence dose-related aPTT measurements. Several days were required for the aPTT to decrease after stopping the argatroban infusion. Fresh frozen plasma had no apparent effect on aPTT values.

DISCUSSION

Several different agents are currently available to provide effective anticoagulation in patients who have immune- mediated HIT. Argatroban is hepatically cleared and may be the preferred direct thrombin inhibitor in the presence of significant renal impairment, but conversely has prolonged effects in hepatic failure. Limited data are available regarding use of direct thrombin inhibitors in patients who have severe hepatic impairment with concurrent renal failure requiring CVVHD. Scant data are available regarding the effect of hemodialysis on the elimination of argatroban.

CONCLUSIONS

The optimal anticoagulant for use in a patient with HIT and concurrent hepatic and renal failure is unclear. No direct thrombin inhibitor has a proven advantage; therefore, selection may depend on clinical factors. Dosing in these patients requires very careful management.

Pharmacotherapy of heparin- induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a life-and-limb threatening condition that is associated with the development of antibodies that activate platelets and the coagulation system in the presence of unfractionated heparin or low molecular weight heparin. The binding of antibody to heparin-PF-4 complexes can activate platelets, leading to an acute, often catastrophic, thrombotic diathesis. The most common laboratory finding is the development of thrombocytopenia 5 or more days after beginning heparin treatment, which occur in up to 1 - 5% of patients exposed to heparin, depending on type of heparin and indication for anticoagulation. The onset of thrombocytopenia can be immediate or delayed for several weeks after the exposure to heparin. Approximately 50 - 60% of patients who develop HIT manifest acute venous or arterial thrombosis and a significant percentage of these patients die or develop vascular gangrene of a limb that requires amputation. Given the severe sequelae associated with HIT, recognition and immediate medical management is essential. Treatment of a patient with HIT is complex, as there are several different anticoagulants now available which have been shown to be useful. Optimal management depends on each patient's individual clinical manifestations, as well as the need for ongoing anticoagulation therapy. No single agent or treatment approach can be considered to be 'standard practice' as very few clinical trials have been completed, compare different treatment options. The use of warfarin alone in a patient with HIT, must be avoided in order to avoid the possibility of further activating coagulation, which may hasten the development of venous limb gangrene. There are several different tests available that detect HIT antibodies and each has different sensitivity and specificity for HIT. In this review we discuss the epidemiology and natural history of HIT, risk factors associated with the development of HIT and the clinical and laboratory tests that aid in the diagnosis and treatment. Special emphasis is given to addressing the management of HIT in special populations, particularly patients with renal or liver disease, acute coronary syndromes, pregnancy, paediatrics and patients who require cardiopulmonary bypass surgery.

Effects of lepirudin, argatroban and melagatran and additional influence of phenprocoumon on ecarin clotting time

INTRODUCTION

Direct thrombin inhibitors (DTI) prolong the ecarin clotting time (ECT). Oral anticoagulants (OA) decrease prothrombin levels and thus interact with actions of DTIs on the ECT method during concomitant therapy.

MATERIALS AND METHODS

Actions of lepirudin, argatroban and melagatran on ECT were investigated in normal plasma (NP) and in plasma of patients (n=23 each) on stable therapy with phenprocoumon (OACP). Individual line characteristics were tested statistically.

RESULTS

Control ECT in OACP was prolonged compared to NP (50.1+/-0.9 vs. 45.7+/-0.8 s; p<0.001). Lepirudin prolonged the ECT linearly. Argatroban and melagatran delivered biphasic dose-response curves. OA showed additive effects on the ECT of lepirudin but not of argatroban and melagatran. Both in NP and OACP, the first and second slopes of melagatran were steeper compared to argatroban (primary analysis; p<0.001). When using the same drug, slopes in OACP were steeper than in NP (secondary analysis; p<0.001). At similar molar concentrations, the crossing points of both slopes were significantly higher with melagatran (323.1+/-11.0 s in NP and 333.2+/-8.2 s in OACP) than with argatroban (219.6+/-14.7 and 248.4+/-15.2 s) corresponding to ratios of 7.1+/-0.2 and 6.7+/-0.2 (melagatran) vs. 4.8+/-0.3 and 4.9+/-03 with argatroban (p<0.0001).

DISCUSSION

The patterns of interactions between vitamin K antagonists and DTI effects are different for bivalent (increase of slope without affecting linearity) and monovalent inhibitors (slight increase or alteration of nonlinear slopes), but there are also differences between the two monovalent inhibitors on thrombin inhibition as determined by ECT.

Argatroban use during pediatric interventional cardiac catheterization

Argatroban is a synthetic direct thrombin inhibitor that does not interact with or induce heparin-dependent antibodies. It is approved for use in adults for prevention and treatment of thrombosis associated with heparin-induced thrombocytopenia (HIT). It has been administered safely in adults with HIT during coronary interventions. There are no reports of argatroban use for anticoagulation in pediatric patients. The present case describes the use of argatroban during coil embolization of a Fontan fenestration in a child with a history of HIT. The patient received a single bolus dose of 150 microg/kg of argatroban at the onset of the intervention. The fenestration was successfully occluded with a detachable coil. The activated clotting time (ACT) was > 200 sec throughout the procedure. The ACT returned to baseline 72 min after the bolus. No complications occurred. This case demonstrates the safe and successful use of argatroban during a transcatheter intervention in a pediatric patient with a history of HIT. The use of argatroban is promising for anticoagulation in children who require an alternative to heparin.

Treatment of heparin-induced thrombocytopenia

OBJECTIVE

To describe heparin-induced thrombocytopenia (HIT or HIT-2), an immune-mediated adverse reaction to heparin or low-molecular-weight heparin. Available treatment options and considerations in developing a therapy approach are discussed.

DATA SOURCES

A search of the National Library of Medicine (1992-June 2001) was done to identify pertinent literature. Additional references were reviewed from selected articles.

STUDY SELECTION

Articles related to laboratory recognition and treatment options of HIT, including the use of agents in selected clinical conditions, were reviewed and included.

CONCLUSIONS

HIT is a rare but potentially severe adverse reaction to heparin that was, until recently, poorly understood and had limited treatment options. Recent advances describing the recognition and clinical manifestations of immune-mediated HIT, including recently available antithrombotic treatment options, have dramatically changed outcomes for patients having this syndrome.

Drug-induced and drug-dependent immune thrombocytopenias

Thrombocytopenia is a frequent comorbid condition in many in hospital patients. In some patients, drugs are the cause of low platelet counts. While cytotoxic effects of anti-tumor therapy are the most frequent cause, immune mechanisms should also be considered. This review addresses thrombocytopenias in four groups. Heparin-dependent thrombocytopenia (HIT), by far the most frequent drug-induced immune-mediated type of thrombocytopenia, has a unique pathogenesis and clinical consequences. HIT is a clinicopathological syndrome in which antibodies mostly directed against a multimolecular complex of platelet factor 4 and heparin cause paradoxical thromboembolic complications. The mechanisms through which heparin can enhance thrombin generation are discussed and treatment alternatives for affected patients are presented in detail. It is of primary importance to recognize these patients as early as possible and to substitute heparin with a compatible anticoagulatory drug, such as hirudin, danaparoid or argatroban. Patients seem to benefit from therapeutic doses of alternative treatment rather than from low-dose prophylactic doses. With the increasing use of glycoprotein (GP) IIb/IIIa inhibitors in patients with acute coronary syndromes, thrombocytopenias are increasingly recognized as an adverse effect of these drugs. Up to 4% of treated patients are affected. Most important, pseudothrombocytopenia, a laboratory artefact, is as frequent as real drug-induced thrombocytopenia and must be excluded before changes in treatment are considered. The pathogenesis of these thrombocytopenias is still debated; an immune mechanism involving preformed antibodies is likely. However, since these antibodies are also detectable in a high percentage of normal controls and of patients not developing thrombocytopenia, their impact is still unclear. Patients with real thrombocytopenia are at an increased risk of bleeding; treatment consists of cessation of the GP IIb/IIIa inhibitor and platelet transfusions in cases of severe hemorrhage. Classic immune thrombocytopenia can be induced by some drugs, e.g. gold, which trigger anti-platelet antibodies indistinguishable from platelet autoantibodies found in autoimmune thrombocytopenia. Drug-induced and drug-dependent immune thrombocytopenia is induced by antibodies recognizing an epitope on platelet GP formed after binding of a drug to a platelet glycoprotein. Still unresolved is whether antibody binding is the consequence of a conformational change of the antigen, the antibody, or both. These antibodies typically react with monomorphic epitopes on platelet GP, but only in the presence of the drug or a metabolite. Although several platelet GP have been identified as antibody target (GPIb/IX, GPV, GP IIb/IIIa), antibodies in an individual patient are highly specific for a single GP. Clinically, these patients present with very low platelet counts and acute, sometimes severe, hemorrhage. Treatment is restricted to withdrawal of the drug and symptomatic treatment of bleeding.

Argatroban: a direct thrombin inhibitor for heparin-induced thrombocytopenia and other clinical applications

Argatroban, a direct thrombin inhibitor derived from arginine, is an effective anticoagulant indicated for prophylaxis or treatment of thrombosis in patients with heparin-induced thrombocytopenia (HIT). Argatroban has been used as an alternative anticoagulant in patients with HIT in various clinical conditions including interventional cardiovascular procedures that require anticoagulation. Satisfactory clinical outcomes with acceptable complications have been reported in these patients. Whether argatroban offers additional clinical advantage over conventional heparin therapy in patients without HIT remains unclear. Argatroban has been evaluated as an alternative anticoagulant to replace heparin in various clinical studies, especially in patients with coronary artery disease or cerebral vascular disease. To date, it remains unclear if argatroban is more effective than heparin, although the agent seems to cause less bleeding complications. This article reviews the pharmacology of argatroban and its clinical application beyond the management of HIT, with particular emphasis on interventional cardiology procedure, acute myocardial infarction, unstable angina pectoris, cerebral thrombosis or ischemic stroke, peripheral obstructive arterial disease, and extracorporeal circulation.

Argatroban for prevention and treatment of thromboembolism in heparin-induced thrombocytopenia

OBJECTIVE

To renew the pharmacology, pharmacokinetics, efficacy adverse events, and cost of argatroban in the prevention and treatment of thromboembolism in patients with heparin-induced thrombocytopenia (HIT).

DATA SOURCES

A MEDLINE search (1980 to August 2000) of English-language literature was conducted using the search term argatroban to identify pertinent case reports, clinical trials, abstracts, and review articles. Additional reports were identified from the reference lists compiled in the literature reviewed, as well as from the manufacturer.

DATA SYNTHESIS

Argatroban is a synthetic direct thrombin inhibitor indicated for parenteral use in the prevention and treatment of thromboembolism in patients with HIT. Its elimination half-life is approximately 40-50 minutes, and it is primarily eliminated by hepatic metabolism and biliary secretion. Compared with historical controls, argatroban-treated patients with HIT or HIT with thrombosis (HITTS) experienced lower rates of the composite end point of death, amputation, and new thrombosis. Dosing is initiated at 2 microg/kg/min and adjusted to maintain the activated partial thromboplastin time at 1.5-3 times the patient's baseline. In Japan, argatroban is approved for use in acute ischemic stroke and chronic peripheral occlusive disease. It has also been used as an alternative to unfractionated heparin (UFH) in patients with a history of HIT or HITTS undergoing percutaneous coronary intervention and other procedures. Additionally, argatroban has been compared with UFH in patients with acute myocardial infarction who were receiving thrombolytic therapy. Hemorrhage is the primary adverse event associated with argatroban. Argatroban increases the prothrombin time, making assessment of the intensity of warfarin therapy during concurrent administration more complex.

CONCLUSIONS

The use of argatroban in patients with HIT and HITTS is associated with improvement in clinical outcomes compared with historical controls. Argatroban offers several practical advantages over other available agents with respect to dosing, monitoring, reversibility of effect with discontinuation of the drug, and cost.