Two spectrophotometric assays for prothrombin have been developed and compared with a one stage coagulant and an immunological assay. One of these assays (called the XAPC assay) uses a combination of factor Xa, phospholipid, Ca2+ and factor V as activator of prothrombin, and measures only normal prothrombin. The second (the ECAR assay) uses Echis carinatus venom as activator. This assay measures both normal prothrombin and PIVKA II (protein induced by vitamin K antagonists/absence). Combination of the results obtained by the XAPC and ECAR assays provides rapid and reliable information on the degree of “subcarboxylation” of prothrombin (oral anticoagulation, vitamin K deficiency).
For patients on long term anticoagulant treatment the prothrombin time (Thrombotest) shows better correlation with the ratio prothrombin/prothrombin plus PIVKA II (XAPC/ ECAR) than with the factor II concentration. For patients starting the anticoagulant treatment there is no correlation between the Thrombotest time and the XAPC/ECAR ratio.
It seems doubtful that (a) spectrophotometric factor II assay(s) will be as useful as the prothrombin time in the control of oral anticoagulation.
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