The Taipan snake venom time can be used to detect lupus anticoagulant in patients treated by rivaroxaban

Sir, Identification of lupus anticoagulants (LA) by laboratory assays is critical for the diagnosis of antiphospholipid syndrome (APS). However, antibody heterogeneity, variations in interpretation strategies, and the absence of a gold standard explain that there is no ideal diagnostic strategy. According to the guidelines for lupus anticoagulant detection [1], and recommendations of the British Committee for Standards in Haematology (BCSH) [2], the choice of tests is limited to dilute Russel’s viper venom time (dRVVT) and a sensitive activated partial thromboplastin time in nonanticoagulated patients. Assays based on snake venoms such as textarin and ecarin were not recommended by the International Society of Thrombosis and Haemostasis (ISTH) [1] because no standardized commercial assay was available. However, taipan and ecarin venoms have been available for a few years from a single manufacturer for use in LA assays [3]. Anticoagulant treatment by direct oral anti-Xa anticoagulants such as rivaroxaban may compromise the biological detection of LA when FXa-dependent assays such as dRVVT are used, with frequent false-positive results [4, 5]. Combination of Taipan snake venom time (TSVT) with ecarin clotting time (ECT) that are direct prothrombin activators unaffected by FXa inhibition has therefore been proposed by some investigators [6, 7] and the BCSH [2] for the detection of LA in orally anticoagulated patients with vitamin K antagonists or rivaroxaban. The aim of our study was to further evaluate this proposition. Blood samples from 30 healthy volunteers were collected to generate reference-level intervals. Plasma samples from 30 patients with a personal history of thromboembolic disease and treated by vitamin K antagonist were also studied. Of these 30 patients, 15 presented APS and the presence of LA had already been demonstrated according to the ISTH subcommittee recommendations [1] (Group LA+). In the other 15 patients, testing for LA was consistently negative and APS was initially ruled out as no anticardiolipin antibodies (aCL) and no anti-b2-glycoprotein I antibodies (antib2GPI) were detected (Group LA ). Fourteen additional plasma samples from patients treated by rivaroxaban in our hospital for venous thromboembolic disease were also studied. Plasma samples from healthy donors (n = 10) and from symptomatic VTE (i.e. with venous thrombotic events) patients with negative lupus anticoagulant (n = 15) were spiked with rivaroxaban solution in order to obtain final concentrations of 50, 100, and 250 ng/ mL, which are frequently encountered in clinical practice. Specific anti-Xa activity was measured on each sample by a chromogenic-based assay (STA-liquid anti-Xa ; Stago, Asni eres, France). Taipan snake venom time and ECT were performed using a semiautomatic coagulometer (Star4 ; Stago), Taipan snake venom, Echis snake venom, diluted Bell and Alton platelet substitute (Diagnostic reagent; Reagent Ltd, Oxon, UK), and CaCl2, as previously described [6]. The dRVVT screen ratio and dRVVT screen/confirm ratio were calculated according to the manufacturer’s instructions (Staclot DRVVT ; Stago) with a positive cutoff value of 1.2. Normalized ratios for TSVT and ECT were obtained by dividing the patient’s test result (in seconds) by the mean of the reference interval (RI) (in seconds) specific to the two tests performed. RI values were defined by the mean times measured with 30 plasma samples from healthy subjects, that is, 47.85 s (range 44.00–51.6) and 22.82 s (range 21.5–24.7) for TSVT and ECT, respectively. Importantly, the distribution of all values obtained with plasma samples from healthy donors (TSVT, ECT, and ratios) was always Gaussian [8]. The normal TSVT ratio was therefore defined as lower than 1.08 (mean TSVT ratio + 2 SD), in agreement with the cutoff value provided by manufacturer, that is, 1.12. The ECT was measured when the TSVT ratio was higher than 1.08. Then, the correction percentage of the TSVT ratio with ECT ratio was calculated as follows: [(TSVT ratio ECT