The new oral anticoagulants and the future of haemostasis laboratory testing

Abstract

The tests currently employed within most haemostasis laboratories to monitor anticoagulant therapy largely comprise the prothrombin time (PT)/ International Normalised Ratio (INR) and the activated partial thromboplastin time (APTT). These are respectively used to monitor Vitamin K antagonists (VKAs) such as warfarin, and unfractionated heparin. Additional tests that laboratories may also employ for assessing or monitoring unfractionated heparin include thrombin time (TT) and the anti-Xa assay, which can also be used to monitor low molecular weight heparin. Several new anti-thrombotic agents have recently emerged, or are in the final process of clinical evaluation. These novel drugs that include Dabigatran etexilate and Rivaroxaban would not theoretically require monitoring; however, testing is useful in specific situations. The tests currently used to monitor VKAs and heparin are typically either too sensitive or too insensitive to the new drugs to be used as 'typically performed in laboratories', and may thus require some methodological adjustments to increase or decrease their sensitivity. Alternately, different tests may be better employed in these assessments. Whatever the case, laboratories may soon be performing a reduced or possibly increased number of tests, the same kind of tests but perhaps differently, or conceivably different assay panels. Specific laboratory guidance on the choice of the appropriate test to be ordered according to the drug being administered, as well as on appropriate interpretation of test results, will also be necessary. The current report reviews the current state of play and provides a glimpse to the possible future of the coagulation laboratory.

Figure 1.

A simplified representation of the classical secondary pathway of coagulation as represented by the common in-vitro coagulation tests, namely the prothrombin time (PT), the activated partial thromboplastin time (APTT) and the thrombin time (TT).

Figure 2.

Control points for classical anticoagulants, and representation of monitoring by routine coagulation assays. A. Control points for vitamin K antagonists (VKAs) are factors (F) II, VII, IX and X. Thus, any assay that acts as a measure of the activity of one or more of these factors can potentially act as a tool to assess drug effects. The prothrombin time (PT) adjusted to an International Normalised Ratio (INR) is determined to offer the best practical solution in this regard. B & C. Control points for heparin, which exerts its effects via antithrombin (AT). Low molecular weight heparins (LMWH; e.g., Clexane and Fragmin) express primarily anti-Xa activity. Unfractionated heparin (UH) expresses primarily anti-thrombin (anti-IIa) activity. To a lesser extent, LMWH also expresses some anti-thrombin activity and UH also expresses some anti-Xa activity. The assays judged to best measure the relevant activity and drug effects in this system are the APTT and anti-Xa assays. The APTT is sensitive to UH and is thus used to monitor UH therapy. The APTT is relatively insensitive to LMWH, and thus this can only be monitored by the anti-Xa assay. The TT is also sensitive to UH, but not LMWH.

Figure 3.

Control points for new agents directed against thrombin (FIIa) and factor (F) Xa, and representation of testing by specific coagulation assays. These agents do not require antithrombin (AT) in order to exert their anticoagulant effects. A. Similar to LMWH, Rivaroxaban has anti-Xa activity and can therefore be monitored by an anti-Xa assay. Note, however, that the standard assay used to monitor LMWH is too sensitive to Rivaroxaban, and needs to be modified. Similar to UH, Dabigatran has anti-IIa activity, and thus the TT is sensitive to Dabigatran. Note, however, that the standard TT assay is too sensitive to Dabigatran, and needs to be modified. B. A representation of the ecarin clotting assay, likely to emerge as a useful test for the assessment of anti-thrombin (FIIa) agents such as Dabigatran. The assay is sensitive to the effects of such agents, is simple to perform, able to be adapted to and automated on most routine laboratory coagulation instruments, and also likely to be capable of standardisation. A chromogenic version is also available. C. A representation of the (dilute) Russell Viper Venom Time (dRVVT) assay, currently under-investigated for any potential utility in assessing the effects of Dabigatran and Rivaroxaban, but which may emerge as a useful test for such purpose.

Figure 4.

Comparative sensitivities of PT, APTT and dRVVT assays to Dabigatran and Rivaroxaban using data derived from the West-mead laboratory. Reagents were respectively: Thromborel S (Siemens, Sydney Australia), TriniClot APTT HS (Tcoag, Bray, Ireland), STA^®-Staclot^® DRVV Confirm reagent (Stago, Sydney Australia). Dabigatran and Rivaroxaban plasma standards as tested were from Hyphen BioMed (purchased from Haematex, Sydney Australia). All testing was performed on a STA-R Evolution instrument (Stago, Sydney Australia). A. Dabigatran concentration vs. clotting times. B. Rivaroxaban concentration vs. clotting times. C. Dabigatran concentration vs. clotting ratio (clotting time/baseline clotting time). D. Rivaroxaban concentration vs. clotting ratio (clotting time/baseline clotting time). Note the comparative dRVVT findings in each case. In particular, the dRVVT was sensitive to both anticoagulants and clotting ratios for dRVVT were reasonably linear and similar for Dabigatran and Rivaroxaban over the tested concentrations.