The simultaneous measurement of total and free protein S by ELISA.

An ELISA for the simultaneous measurement of total and free protein S is described. Polyethylene glycol precipitation was used to remove the C4b -binding protein/protein S complex from plasma. This allows free protein S to be measured. The total protein S was measured directly on plasma samples that had been diluted to dissociate the C4b binding protein/protein S complex. The concentration of total and free protein S found in normal subjects (n = 24) was 92.9 (SD = 10) u/dl and 40.3 (SD = 8.8) u/dl respectively. The within assay coefficient of variation was 5.1%. The between assay coefficient of variation was 5.7%. Warfarin therapy caused a reduction in the level of both total and free protein S but this reduction correlated poorly with the INR value.