A novel clotting assay for quantitation of plasma prothrombin (Factor II) using echis multisquamatus venom

Measuring plasma prothrombin activity seems useful for evaluating thromboti c risk and managing oral anticoagulant therapy as an adjunct to the interna tional normalized ratio. Therefore, we designed a new plasma prothrombin as say based on the ability of Echis multisquamatus venom to activate prothrom bin with only calcium as a cofactor In this assay, I part of undiluted citr ated plasma is added to 5 parts of a venom reagent and the clotting time is measured. The assay's advantages are that dilution of the test plasma is r equired only when prothrombin activity exceeds 100%, a single standard curv e can be used over months for a given batch of stock reagent, and barium-ad sorbed plasma is used for dilution of rest plasma and construction of the s tandard curve, thus eliminating the need for prothrombin-deficient plasma. However one should be aware of the following: (I) test samples must contain at least 200 mg/dL fibrinogen; and (2) when prothrombin concentrations wer e below 50%, the venom-based assay often gave values up to 10% higher than the thromboplastin-based assay. Values obtained in 262 plasma samples reste d with the venom-based assay and with a thromboplastin-based prothrombin as say correlated well (r(2) = 0.93).