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

Introduction: 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. Methods: 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. Results: 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%. Conclusion: The combination of mixing and confirmatory tests interpreted according to the new guidelines can clearly differentiate LA from other coagulopathies.