PB0564 - Evaluation of the significance of mixing study in lupus anticoagulant testing using a new CMC algorithm

Background: Laboratory criteria for Lupus Anticoagulant (LA) include prolonged clotting time of a phospholipid dependent test, failure of correction on mixing study, shortening of clotting time in the presence of high phospholipid concentrations, and absence of other inhibitors.
Elevated Screening test without mixing studies does not prove inhibitor effect and it wastes time and resource to perform confirmatory tests skipping the mixing step. Hence, we devised Christian Medical College (CMC) Algorithm (Figure-1) which starts with dRVVT((Dilute Russell’s viper venom time) Screen Mix. To identify interferences due to anticoagulants/FDP, thrombin time (TT) was also included.

Aims: To evaluate the significance of mixing study in lupus anticoagulant testing using a new CMC algorithm.

Methods: 200 samples were analysed over a period of 2 months. The new CMC Algorithm was tested simultaneously along with ISTH Guideline based lupus testing ((DRVVT screen and APTT were done as the initial steps)
LA testing was performed using DRVVT screen and confirm reagents (STA-Staclot DRVV Screen & STA-Staclot DRVV Confirm) and/or APTT reagents (Synthasil &Actin-FS).
LA test was considered positive when Screen/Confirm(S/C) ratio was >1.2.

Results: Out of the 200 cases,48 cases were positive and 149 cases were negative by both methods.
Discrepant cases (Table 1): In 3 cases Screen mix was more prolonged than Screen & Screen fell below the cut off. These 3 cases were positive only by new algorithm. Other Anti-Phospholipid antibodies was positive in 1 case. This could represent false negative lupus if done by routine method. There were 2 cases where TT was prolonged (patient was on anticoagulant). One case among these had prolonged S/C ratio and this could represent false positive lupus.

Conclusion(s): The new algorithm highlights the importance of mixing study in lupus testing which helped to reduce false negatives. The addition of TT helped in identification of interferences which can give false positive results.