New guidelines for lupus anticoagulant: sensitivity and specificity of cut‐off values calculated with plasmas from healthy controls in mixing and confirmatory tests

Abstract

The updated guidelines for lupus anticoagulant (LA) diagnosis indicate locally calculate the cut-off values of the index of circulating anticoagulant (ICA) and the clotting time in seconds (s) for mixing studies and % of correction (%C) for confirmatory tests. We assess sensitivity (SEN) and specificity (SPC) of the cut-off values obtained as the 99th percentile from 60 plasmas of healthy individuals. We analysed 647 plasmas from patients studied in the last 3 years, and results were revaluated according to the new criteria and cut-off values. Four hundred and three had LA, and 75 of them were under oral anticoagulants (OA). We performed three screening tests: activated partial thromboplastin time (APTT), diluted Russell viper venom time (dRVVT) and dilute prothrombin time (dPT), and previous diagnosis was carried out using our home-made cut-off calculated by receiver operating characteristics curves. We reanalysed the mixing and confirmatory data of APTT/dRVVT, the tests selected in the new guidelines. To evaluate SPC, 244 plasmas (160 OA and 84 congenital deficient patients) were studied. Considering mixing studies, the cut-off values demonstrate that SEN of ICA-APTT was 94% and of clotting time in second (s) 83%, with an SPC of 77% and 84%, respectively. For ICA-dRVVT, SEN was 72% and for clotting time in second (s) 77%, with SPC of 98% and 84%, respectively. The cut-off values for %C for confirmatory APTT show good SEN 82% and high SPC 96%; for confirmatory dRVVT lower SEN 77%, but a SPC of 100%. The combination of mixing and confirmatory tests interpreted according to the new guidelines can clearly differentiate LA from other coagulopathies.