Abstract
Abstract 23
To date, all published studies assessing the effects of the new oral anticoagulants dabigatran (dabi) and rivaroxaban (riva) on coagulation assays have used normal pooled plasma spiked with known concentrations of anticoagulant and therefore do not reflect the variability expected from patients taking these medications.
We collected citrated venous blood samples from patients taking either dabi (n=43) or riva (n=10) for management of either atrial fibrillation or acute venous thrombosis. All subjects were required to have taken at least 5 doses of drug in order to reflect steady-state conditions and informed consent was obtained. The interval between last dose and phlebotomy ranged from 1 – 18 hours. Platelet-poor plasma was prepared and using the STA-R.. coagulometer (Diagnostica Stago), the following assays were performed: PT/INR using Neoplastin Plus.. (NP) and Innovin.. (IN), aPTT (Actin.. FS), thrombin time (TT), clot-based protein S (PS) activity, free PS antigen, chromogenic protein C (PC) activity, clot-based antithrombin activity, Clauss fibrinogen, dilute Russell's viper venom time (DRVVT), and factor VIII (FVIII) activity at several dilutions (1:10 to 1:160). Hemoclot.. dilute thrombin time and STA.. -Rotachrom.. anti-Xa assays were performed for quantitation of dabi and riva drug levels, respectively, using Aniara commercial plasma calibrators.
Median INR levels for patients on dabi were 1.1 (IN) and 1.2 (NP) and for riva were 1.15 (IN) and 1.3 (NP). The proportion of subjects with INR levels above 1.3 for dabi were 15% (IN) and 28% (NP) and for riva were 0% (IN) and 40% (NP). Median aPTT levels were 42s dabi and 32.5s riva. Although 73% of patients on dabi had an elevated aPTT (>38s), 60% of these were only mildly elevated (39–45s). Only one subject on riva had an elevated aPTT. The TT was extremely sensitive to the presence of dabi, as 100% of treated subjects had an elevated TT (>20s) and 73% were above the linearity cutoff (>100s). All subjects on riva had normal TT. As shown in Figure 1A, the clot-based PS assay was affected markedly by dabi, with patients showing artificially high PS activity with increasing drug levels, but no such effect was observed for riva (Figure 1B). For both drugs, the PS free antigen and chromogenic PC assays were unaffected. Figure 1C shows dabi also caused false elevations in antithrombin activity, though it did not affect the Clauss fibrinogen assay, which uses a much higher concentration of thrombin. Riva had no discernable effect on the antithrombin and fibrinogen assays (data not shown). Figure 1D shows that increasing levels of both anticoagulants caused artefactual decreases in FVIII activity. Increasing dilutions of patient plasma caused progressive elevation of FVIII activity, with median activity levels nearly doubling between 1:10 and 1:160 dilutions for both dabi (0.65 to 1.28) and riva (0.81 to 1.41). Finally, the DRVVT assay was prolonged (>20% elevated versus normal plasma) for 86% of dabi and 60% of riva patient plasmas. No differences were noted for any of these results between dabi 110 mg (n=14) and 150 mg (n=29) dosing.
For the assay platforms assessed in this study, the majority of patients taking dabi and riva had near-normal INR and aPTT levels. This is discrepant with results from studies using spiked plasma samples. Clinicians should be cautioned that normal INR and aPTT results do not rule out the presence of therapeutic anticoagulant levels and the results should not be used to guide clinical decisions. Clotting-based thrombophilia testing should be avoided for patients taking these medications due to assay interferences, particularly with dabigatran.
Effects of dabi and riva on various standard coagulation assays; dabi (A) and riva (B) effects on PS activity, PS free antigen, and PC activity; (C) dabi effects on antithrombin and fibrinogen assays; (D) dabi and riva effects on FVIII activity assay (1:40 dilution).
Effects of dabi and riva on various standard coagulation assays; dabi (A) and riva (B) effects on PS activity, PS free antigen, and PC activity; (C) dabi effects on antithrombin and fibrinogen assays; (D) dabi and riva effects on FVIII activity assay (1:40 dilution).
Teal:Bayer: Consultancy, Honoraria; Boehringer-Ingelheim: Consultancy, Honoraria. Lee:Bayer: Honoraria; Boehringer-Ingelheim: Honoraria.
