FACTOR-XII DETERMINATIONS IN THE PRESENCE AND ABSENCE OF PHOSPHOLIPIDANTIBODIES

Factor XII (FXII) levels were determined in plasma samples from 29 nor mal donors, 10 patients with inherited FXII deficiency (all lupus anti coagulant [LA] negative) and 67 LA positive patients, using clotting ( FXIIct), chromogenic substrate (FXIIcs) and immunochemical (FXIIag) as says. Excellent correlations were obtained in the three DUI assays wit h the LA negative samples and between the FXIIcs and FXIIag assays in the LA positive samples. Correlations between both the FXIIcs and FXIl ag with FXIIct in the LA positive patients were poor. Of 67 LA positiv e samples studied, 25 (37.3%) showed lower values in the FXIIct assay; 13 (19.4%) of these patients were pseudo FXII deficient with values o f FXII below the lower limit of normal. These results indicate that a diagnosis of FXII deficiency can be made inappropriately in the presen ce of phospholipid antibodies and that such a diagnosis should not be made by FXIIct assay alone.