Mixing studies in lupus anticoagulant testing are required at least in some type of samples

According to the ISTH guidelines for lupus anticoagulant (LAC) testing, the second step in the three-step procedure (screening, mixing, and confirmation) is the mixing test, which improves the discrimination between the presence of an inhibitor and coagulation factor deficiencies such as those occurring in patients receiving vitamin K antagonists (VKAs).

From a retrospective analysis of dilute Russell viper venom (dRVVT) results, we evaluated the impact of the mixing test result on the interpretation of LAC positivity.

We interpreted the dRVVT clotting times with and without taking into account the results of the mixing test in a patient population with prolonged screening test (n = 267) with special attention to the patients receiving VKAs.

The number of samples classified as 'LAC positive' differed substantially depending on the method of interpretation; 170 and 235 of 267 samples were classified as LAC positive with the three- and two-step procedure, respectively. Discrepancy between the two-step (without mixing step) and the three-step procedure was due to not including a mixing test and was more pronounced in the VKA patient population. Screen/confirm ratios carried out on a 1:1 mix of patient and normal pooled plasma (NPP) gave a lower incidence of 59 of 267. We advise continuing to perform mixing test to avoid false-positives. In patients with discrepant results between the two- and three-step dRVVT interpretation, mainly observed in VKA-treated patients, we advise retesting of the patients preferable beyond the period of anticoagulant therapy and additional testing for anti-beta2GPI and/or anti-cardiolipin antibodies.