Adaptation of a functional assay for protein S to a Cobas Fara II centrifugal analyzer

An adaptation of the manual Staclot protein S functional assay (American Bioproducts, Parsippany, NJ) for the Cobas FARA II centrifugal analyzer is described. This automated method is based on determining the clotting time by measuring the change in turbidity of the specimen as clotting progresses. The accurate and precise pipetting of the analyzer combined with its enhanced data acquisition and rapid parallel processing features result in a markedly improved procedure compared to the less accurate and reproducible manual method. The assay is linear over the range of protein S concentrations encountered clinically (0% to 150%); has a limit of detection of 3% protein S; within and between day precision (CV) at a level of 50% protein S is 4.7 and 12.6%, respectively; and results agree closely with those obtained on the STA automated hemostasis analyzer (slope = 0.98, intercept 3.92, and r = 0.98). Protein S values obtained with this assay for specimens from 20 normal males and 20 normal females were 115% +/- 22.31% (mean +/- standard deviation [SD]) and 98% +/- 18.2%, respectively. In 14 patients on chronic stable treatment with Coumadin (DuPont Pharmaceuticals, Wilmington, DE) the mean functional protein S activity was 16.2% and the SD 11.7%. The method for determination of protein S activity on the centrifugal analyzer yields results comparable to data reported with a variety of other techniques.