Studies on immunological assay of vitamin K dependent factors

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Abstract
Summary. We describe and compare five assay systems for Protein C (PC) in human plasma; a functional assay for PC activity, Laurell electroimmunoassay with EDTA or calcium (EDTA-Laurell or Ca-Laurell), radioimmunoassay (RIA) and immunoradiometric assay (IRMA). The lower limit of sensitivity of PC in normal reference plasma was 2 × 10−3 units/ml by RIA and 1 × 10−4 units/ml by IRMA, but the ratio of maximum binding to blank binding was superior in RIA. In normal plasma (= 20), there was no significant discrepancy of PC levels between RIA, Laurell (EDTA− and Ca−) and functional assay. In normal serum, PC antigen (PC:AG) measured by Ca-Laurell showed a high value (1.37 0.24 units/ml) compared to the other assays. PC:AG of a purified sample increased its level by Ca-Laurell method during activation by thrombin. Activated PC (PCa) has decreased affinity for anti-PC rabbit IgG, similarly to decarboxy (warfarinized) PC. In warfarin-treated individuals (= 12) who were anticoagulated orally for more than 3 weeks, functional activity was lower (0.27 0.12 units/ml) than that measured by RIA (0.64 0.12 units/ml) or EDTA-Laurell (0.62 0.06 units/ml), whereas PC:AG measured by CA-Laurell had a normal value of 0.96 0.40 units/ml. After stopping administration, the activity showed a gradual increase to the normal at 2 weeks, while PC:AG increased more rapidly to normalize in 1 week.