Sensitive enzyme immunoassay for the measurement of platelet factor 4 in blood plasma

A sandwich enzyme immunoassay method for the measurement of platelet factor 4 (PF4) was developed with the use of polystyrene balls with immobilized antibody F(ab')2 fragments and the same antibody Fab' fragments labeled with beta-D-galactosidase from E. coli. The measurable range was 30 pg to 3 ng of PF4 per tube. Within-run and between-run coefficients of variation were less than 10%. The results obtained with the enzyme immunoassay correlated well with those of a radioimmunoassay (r = 0.952, slope = 0.954, gamma-intercept = 2.43 ng/ml). Platelets contained large amounts of PF4 (7.21 +/- 1.97 ng/10(6) cells or 2.51 +/- 1.13 ng/mg protein), whereas the PF4 levels in red blood cells and lymphocytes were negligible, confirming the specific localization of PF4 in platelets. The applicability of the immunoassay method was tested to determine the in vitro release of PF4 during preparation and storage of platelet concentrates.