Clinical usefulness of the dilute Russell viper venom time test for patients taking warfarin.

Warfarin use often causes false-positive results in the dilute Russell viper venom time test (DRVVT). Thus, three sets of guidelines-those presented by the International Society on Haemostasis and Thrombosis (ISTH), the British Committee for Standards in Haematology (BCSH), and the Clinical and Laboratory Standards Institute (CLSI)-are advocated. We evaluated the clinical usefulness of the testing methods recommended in these three guidelines using laboratory samples. Of the 242 samples from patients using warfarin, 38 were positive for lupus anticoagulant (LA). After adding normal pooled plasma (NPP) as recommended in the ISTH, BCSH, and CLSI guidelines, the number of samples testing positive for LA decreased to 13, 18, and 19, respectively. The number of samples with inconsistent results between the activated partial thromboplastin time and mixing test, and the DRVVT following the ISTH, BCSH, and CLSI guidelines were four of 205 (1.9%), 15 of 242 (6.2%), and 17 of 242 (7.0%), respectively. In patients with an international normalized ratio (INR) ≥3.0, 11 of 37 (29.7%) and 12 of 37 (32.4%) samples showed inconsistent results according to the BCSH and CLSI guidelines, respectively. The accuracy of the DRVVT result may thus decrease in markedly anticoagulated patients.