title: Assay of protein S in systemic lupus erythematosus.
abstract: Free protein S:Ag and protein S activity determined by clotting assay are often regarded as synonymous; however, in the study group of patients, abnormal protein S activity (PS:Act) results correlated poorly with abnormal free (PS:Ag(r2 = 0.164). Significantly none of the patients in whom lupus anticoagulant and low free protein S:Ag were detected 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 and thus give falsely high results. As reduced levels of free protein S:Ag and/or protein S activity are considered risk factors of thrombosis in systemic lupus erythematosus, these findings question the suitability of the protein S activity assay for patients who may have phospholipid antibodies.
authors.affiliation: Haemophilia Centre, St Thomas' Hospital, Guy's and St Thomas' NHS Trust, London, UK.
authors.forename: A S
authors.initials: AS
authors.lastname: Lawrie
authors.forename: M E
authors.initials: ME
authors.lastname: Lloyd
authors.forename: F
authors.initials: F
authors.lastname: Mohamed
authors.forename: S
authors.initials: S
authors.lastname: Irons
authors.forename: G R
authors.initials: GR
authors.lastname: Hughes
authors.forename: G F
authors.initials: GF
authors.lastname: Savidge
mesh_terms: D017153:Antibodies, Anticardiolipin / Q000032:analysis; D000990:Antithrombin III / Q000032:analysis; D005260:Female; D006801:Humans; D016682:Lupus Coagulation Inhibitor / Q000032:analysis; D008180:Lupus Erythematosus, Systemic / Q000097:blood*; D008297:Male; D011486:Protein C / Q000032:analysis; D017293:Protein S / Q000032:analysis*
chemical_list: D017153:Antibodies, Anticardiolipin; D016682:Lupus Coagulation Inhibitor; D011486:Protein C; D017293:Protein S; D000990:Antithrombin III