A modified prothrombin time to measure the anticoagulant activity of dabigatran

We evaluated the suitability of a diluted prothrombin time (dPT) for the measurement of dabigatran activity in plasma. Three different thromboplastin preparations were used: Recombiplastin (Instrumentation Laboratory), Thromborel (Siemens) and Simplastin (OrganonTeknika), which were diluted in 10 mM CaCl2 to give a clotting time in normal plasma of 35-40 s (dilutions ranging from 1:50 to 1:200). Pooled human plasma was spiked with dabigatran at concentrations ranging from 0.03 to 1 μM (20-700 ng/ml). With all three thromboplastins, the dPT displayed a linear response over the entire range of dabigatran concentrations (R2 >0.99). The concentration of drug doubling the dPT was 0.23 μM for Recombiplastin, 0.29 μM for Thromborel, and 0.33 μM for Simplastin. The intra- and inter-assay coefficients of variation were 2.2% and 8.4%, respectively. The inaccuracy of dPT (Recombiplastin), as calculated by testing 81 plasma samples from different donors spiked with randomly generated amounts of dabigatran (0.05-1 μM, corresponding to 35-700 ng/ml), was 11.7% as compared to 16.5% of the Hemoclot, the gold standard clotting test for dabigatran assay. The percentage of dabigatran-containing samples giving a dPT in the normal reference range (% misinterpretation) was 3.7% as compared to 2.5% for Hemoclot. In conclusion, the dPT is a simple and inexpensive test displaying linearity, good responsiveness and reasonable accuracy for the measurement of clinically relevant concentrations of dabigatran. It might be associated to more specific anti-IIa assays in patients under dabigatran treatment to obtain a better picture of the “global” degree of anticoagulation, particularly during anticoagulation reversal with non-specific antidotes (e.g. PCC) or prior to surgical or invasive interventions.