ASSAY OF PROTEIN-S IN SYSTEMIC LUPUS-ERYTHEMATOSUS

Free protein S:Ag and protein S activity determined by clotting assay are often regarded as synonymous; however, in the study group of patie nts, abnormal protein S activity (PS:Act) results correlated poorly wi th abnormal free PS:Ag (r(2) = 0.164). Significantly none of the patie nts in whom lupus anticoagulant and low free protein S:Ag were detecte d were found to have low PS:Act. These results could not be explained by activated protein C resistance, suggesting that lupus anticoagulant may cause prolongation of the clotting time within the PS:Act assay a nd thus give falsely high results. As reduced levels of free protein S :Ag and/or protein S activity are considered risk factors of thrombosi s in systemic lupus erythematosus, these findings question the suitabi lity of the protein S activity assay for patients who may have phospho lipid antibodies.