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A comparison between two activated protein C resistance methods as routine diagnostic tests for factor V Leiden mutation

The most common commercially available test measuring activated protein C (APC) resistance relies on the the anticoagulant response to added APC in an activated partial thromboplastin time (APTT) based method. Another method is a Russell Viper venom time (RVVT) based system. To improve the specificity for factor V Leiden of the APTT based method, pre‐dilution of test plasma in FV‐deficient plasma has recently been recommended. In this study we tested the relative suitabilities of the APTT‐based system, the RVVT‐based system and their corresponding assays modified by pre‐dilution in FV‐deficient plasma, for screening asymptomatic subjects, a group of thrombophilic patients (in particular those with low APC ratios), patients on oral anticoagulants, and patients with lupus anticoagulant (LAC). We found the RVVT‐based assay to be superior to the APTT‐based method in the separation of normals from those with FV Leiden mutation both in asymptomatic subjects and in the thrombophilic patient group. Both modified assays demonstrated a sensitivity and specificity of 100% for FV Leiden, as verified by genotyping in asymptomatic subjects, thrombophilic patients and patients on oral anticoagulants, with the modified RVVT‐based assay giving better separation between normals and FV Leiden. Inhibition of phospholipid‐dependent coagulation by LAC antibodies rendered the APTT‐based system less suitable than the phospholipid‐rich RVVT‐based one, and as nine of the 20 LAC‐positive patients were on warfarin, we showed only the modified RVVT assay to be a reliable predictor of factor V Leiden in this patient group.