Interpretation of normal plasma mixing studies in the laboratory diagnosis of lupus anticoagulants.

INTRODUCTION

Mixing studies are part of the laboratory diagnosis of lupus anticoagulants (LA). They are used to determine the evidence of an inhibitor by demonstrating persistence of an abnormal clotting time by mixing patient plasma with normal plasma. Since there is no standardised interpretation of results of mixing studies, percent correction formulas are proposed. The sensitivity of mixing studies strongly depends on the interpretation of the results.

MATERIALS AND METHODS

A retrospective analysis was performed on 361 samples, 75 LA-positive and 286 LA-negative samples. For all the LA-positive samples and for 181/286 LA-negative samples mixing tests on one or more screening tests were performed. A percent correction formula and the Rosner index were calculated for all mixing tests on aPTT and dPT.

RESULTS

The <70% correction formula for the mixing tests on aPTT showed the highest sensitivity (95%). The Rosner index had a sensitivity (93%) comparable with the <70% correction formula. dPT is shown to be less sensitive in the detection for LA and, even when the screening test is prolonged, interpretation of the mixing test by the percent correction formula misclassifies many samples. Rosner index in the interpretation of mixing tests for dPT is more sensitive than the percent correction formula, 49% and 57%, respectively.

CONCLUSIONS

This study demonstrates that the Rosner index and the percent correction formulas for interpretation of mixing studies are complementary and can help to reduce misclassification of LA-positive or LA-negative samples.