Thrombotic disease is a significant cause of mortality and morbidity, with an estimated lifetime risk of greater than 10% in Western populations. One of the most common hereditary thrombophilias is the factor V Leiden mutation, which is identified with a screening assay for activated protein C (APC) resistance and confirmed by DNA analysis. In this study, we compared the commercially available Pefakit (Pentapharm, Basel, Switzerland) and Cryocheck (Precision BioLogic, Dartmouth, Canada) assays, 2 recently developed Russell viper venom (RVV)-based screening tests, with the activated partial thromboplastin time (aPTT)-based screening test currently used in our hospital's clinical laboratory. We found that the aPTT-based assay for resistance to APC had a sensitivity of 100%, a specificity of 70%, and a positive predictive value (PPV) of 70%, whereas both of the RVV-based assays exhibited high sensitivity, specificity, and PPV at 100%. In addition, we found that these new functional assays are more cost-effective relative to the screening algorithm previously used in our clinical laboratory and could potentially eliminate the need for DNA analysis, although further study is required.