Ecarin-Based Methods for Measuring Thrombin Inhibitors

Ecarin is a venom from the saw-scaled viper, Echis carinatus, which catalyzes prothrombin into meizothrombin. This venom is used in several hemostasis laboratory assays, including ecarin clotting time (ECT) and ecarin chromogenic assays (ECA). The use of these ecarin-based assays was first implemented as a tool for monitoring the infusion of a direct thrombin inhibitor, hirudin. Subsequently, this method has been more recently employed for measuring either the pharmacodynamic or pharmacokinetic properties of the oral direct thrombin inhibitor, dabigatran. In this chapter, the procedure for performing manual ECT and automated and manual ECA for measuring thrombin inhibitors is described.

Assays to Monitor Bivalirudin

Bivalirudin (Angiomax, Angiox) is a parenteral direct thrombin inhibitor (DTI) that is used for patients with heparin-induced thrombocytopenia (HIT), where heparin cannot be used due to the risk of thrombosis. Bivalirudin is also licensed for use in cardiology procedures (e.g., percutaneous transluminal coronary angioplasty; PTCA). Bivalirudin is a synthetic analogue of hirudin found in the saliva of the medicinal leech and has a relatively short half-life of ~25 min. Several assays can be used to monitor bivalirudin; these include the activated partial thromboplastin time (APTT), activated clotting time (ACT), ecarin clotting time (ECT), an ecarin-based chromogenic assay, thrombin time (TT), the dilute TT, and the prothrombinase-induced clotting time (PiCT). Drug concentrations can also be measured using liquid chromatography tandem mass spectrometry (LC/MS) and clotting or chromogenic-based assays with specific drug calibrators and controls.

Performance of various laboratory assays in the measurement of dabigatran in patients receiving therapeutic doses: a prospective study based on peak and trough plasma levels

OBJECTIVES

To study dabigatran etexilate, a new oral anticoagulant that functions as a direct thrombin inhibitor.

METHODS

This study evaluates four methods, one of which is performed in three different laboratories, and compares results against dabigatran levels measured by Boehringer-Ingelheim (Ingelheim, Germany) using mass spectrometry.

RESULTS

Although routine monitoring is not required, measurement of plasma concentrations may be necessary in certain clinical situations. Routine coagulation assays such as the prothrombin time, activated partial thromboplastin time, and thrombin time do not reliably determine levels of dabigatran anticoagulation. Alternative assays, when calibrated with a dabigatran standard, such as the modified dilute thrombin time, ecarin clotting time, and ecarin chromogenic assay, may be appropriate, although a comparison of these methods using samples from patients taking dabigatran has not been performed.

CONCLUSIONS

Although results using all methods in this study demonstrate adequate correlation, measured dabigatran levels varied in a statistically significant manner, even when the same method was used by different laboratories. The clinical significance of this variation in dabigatran concentrations is uncertain.