The interference of factor VIII excess on the detection of lupus anticoagulants by the activated partial thromboplastin time.

SUMM. Antiphospholipids antibodies (AAT) were investigated in plasma of 62 patients with systemic lupus erythematosus, and studied for the possible interference of the excess in factor VIIIc-Von Willebrand factor (VWF-VIIIC) complex in the detection of lupus anticoagulants (LA) by the activated partial thromboplastin time (APTT). We used four commercial reagents and noted that each one exhibited a different level of sensitivity to LA; the most sensitive one allowed the detection of 16 LA positive plasmas while the less sensitive reagent detected only 6 positives. The Elisa test for IgG and IgM AAP detection was positive in 25 out of 62 plasmas (40%). Comparaison of the Von Willebrand factor antigen level and the APTT values showed a significant negative correlation with 2 reagents (r = -0,369, p < 0,01 and r = -0,272, p < 0,05 respectively) that were also the less sensitive to LA. The interference of an excess in VWF-VIIIc complex was further studied by the addition of purified factor VIII in 3 LA positive plasmas. Our results suggests that an excess of factor VIIIc could lead to false negative LA, using certain APTT reagents. We conclude that a more accurate LA detection require: sensitive reagents, a pool of normal plasmas selected with normal factor VIII level, as well as repeated testing of blood sampling withdrawn away of an inflammatory process to avoid false negative LA detection.