Clinical significance of the mixing test in laboratory diagnoses of lupus anticoagulant

Abstract

The mixing test is used to determine the presence of inhibitors in laboratory diagnoses of lupus anticoagulant. Updated international guidelines state that an integrated lupus anticoagulant test system does not require the mixing test; an appraisal of the mixing tests in integrated lupus anticoagulant test systems is, therefore, required. We investigated the clinical relevance of mixing tests by using the best cutoff value of the mixing test through thrombotic risk analysis. A retrospective analysis was performed on 525 specimens with positive screening tests by using two integrated lupus anticoagulant tests: diluted Russell's Viper venom (dRVVT) and silica clotting time. The diagnostic performance of two interpretation formulas (percentage correction, Rosner index) was assessed, and the thrombotic risk of a subgroup based on the mixing results was investigated. Finally, the thrombotic risk of lupus anticoagulant positivity based on the integrated lupus anticoagulant test system procedures was assessed for the appraisal of mixing test exclusion in integrated lupus anticoagulant test systems. The best cutoff values of mixing test interpretation methods based on dRVVT were as follows: 60.1% for percentage correction and 15.7 for Rosner index. There was no substantial difference in the thrombotic risk between percentage correction and the Rosner index. The mixing-positive group showed a higher lupus anticoagulant titer and higher thrombotic risk than the mixing-negative group. However, even the mixing-negative group carried a significant risk of thrombosis. Finally, lupus anticoagulant positivity determined by the updated two-step procedure (screening and confirmation tests) showed higher thrombotic risk than that determined by the traditional three-step procedure (screening, mixing, and confirmation tests). Although a positive mixing result can predict a high risk of thrombosis, negative mixing results are also associated with a substantial thrombotic risk. The updated two-step procedure without the mixing test is relevant for lupus anticoagulant detection in the context of thrombotic risk estimation.