MIXCON-LA: a precise, sensitive and specific aPTT-based assay for detection of lupus anticoagulant

Lupus anticoagulants (LA) are associated with an increased risk of thrombosis and laboratory detection is of major importance. Multiple tests are available for screening and confirmation, but they differ in sensitivity and specificity, frequently lacking the ability to discriminate between the presence of LA, heparin, and oral anticoagulants. Based on the test-principle of the Lupus Ratio-test, an automated, sensitive APTT-based assay, using mixtures of a lupussensitive and a lupusinsensitive APTT-reagent with normal plasma for detection of lupus anticoagulants was developed. Ninety-nine healthy volunteers, ten patients treated with unfractionated heparin intravenously, 19 patients taking stable oral anticoagulation, five patients with hemophilia A, and 15 patients with antiphospholipid-antibody-syndrome (APS) were investigated. In all patients, two APTTs were performed, one with each reagent, on 1:1 mixtures of test plasma and normal plasma (MIXCON-LA assay). The ratio between the two clotting times was divided by the corresponding ratio for the normal plasma. This final lupus ratio (LR) was used for evaluation. The within-series imprecision and the between-series imprecision were excellent with coefficients of variation between 1.5% and 1.9%. The mean +/- 2 SD of the LR of the 99 healthy volunteers was used as reference range (LR: 0.95-1.07). All patients treated either with heparin or oral anticoagulants remained negative in the MLXCON-LA assay (specificity, 100%), while one of five patients with hemophilia A, in whom a factor VIII-inhibitor developed, showed a false-positive result. In 13 of 15 patients with APS, an increased ratio was observed (sensitivity, 87%). This assay system allows precise, specific, and sensitive detection of lupus anticoagulants.