Radioimmunoassay of Factor V in Human Plasma and Platelets

Homogeneous, single-chain human factor V was used to develop a double antibody competition radioimmunoassay to measure factor V concentrations in plasma and platelets. Standard curves were constructed that allow for the detection of as little as 20 ng factor V/ml of plasma. Normal factor V concentrations range from 4 to 14 μg/ml of plasma with an average value of 7.0 ± 2.0 μg/ml (n = 64). No correlation was observed between antigen levels and age or sex. The radioimmunoassay data are consistent with factor V clotting assays, providing freshly drawn plasma is used in the bioassay. Radioimmunoassay of washed platelets indicate that 0.63-1.93 μg of factor V is present per 2.5 × 108 platelets (4612-14128 molecules of factor V per platelet). When normalized to individual hematocrits and platelet count, the data indicated that platelets contribute approximately 18%–25% of the factor V found in whole blood. In addition, two individuals with functionally deficient factor V were examined and found to be deficient in both antigen and activity.