The study allowed us to define a uniform approach to LA testing and evaluation for laboratories employing the same methods.

Laboratory assessment of Lupus anticoagulant (LAC) is very challenging because of inter and intralaboratory variability, which makes it difficult to standardize and harmonize results expression. Five hospital laboratories in North-eastern Italy shared their efforts and their experience in a cross-laboratory study, conducting the diagnostic process as homogeneously as possible and providing a better interpretation for LAC positivity. Hundred normal samples from healthy subjects (20 from each center) were processed to confirm negative upper limits and calculate positivity cutoffs of LAC integrated assays, that is dilute Russell's viper venom time (dRVVT) and silica clotting time (SCT). Moreover, 311 samples previously diagnosed by the laboratories as positive for LAC were analyzed to characterize different positivity levels for each assay. As far as the analysis of healthy subjects is concerned, negative upper limits are set at 1.17 and 1.19 for dRVVT and SCT screen ratio, respectively. Positivity cutoffs are set at 1.20 for dRVVT and 1.23 for SCT, expressed as Test Ratio calculated on screen and confirm integrated tests. Positive results for each integrated assay are subsequently divided into three subgroups: weak, moderate and strong; the results obtained are presented as a score proposal that can provide LAC interpretation. The combined use of both dRVVT and SCT assays and the definition of different positivity levels may lead to clearer, more objective LAC reporting. An interpretative table for LAC-proposed score provides LAC-positive results and it is now adopted by all centers involved in the study.