Summary
A collaborative study on the measurement of protein S (PS) antigen (total and free)
in a freeze-dried ampouled test plasma by assay against local house standard plasmas
was carried out in eleven laboratories. Potency estimates of total PS showed good
agreement between laboratories with a geometric coefficient of variation (gcv) of
5.9% and an overall combined potency of 0.84 units per ml. Potency estimates of free
PS antigen in the test sample were associated with increased variability between laboratories
resulting from the polyethylene glycol (PEG) precipitation step which is used to separate
free PS from PS bound to the C4b binding protein. Free PS in the test could be expressed
relative to either total PS in the house standards (e. g. 0.28 units per ml) or relative
to free PS in the house standards following PEG precipitation (e. g. 0.71 units free
PS per ml).
