Evaluating the Role of a New Dilute Prothrombin Time Assay ACTICLOT dPT in Lupus Anticoagulant Testing.

The current ISTH recommendations for Lupus Anticoagulant (LA) testing include screening with two or more sensitive assays with different assay principles. Many laboratories utilize a single Global assay such as the Hexagonal Phase Neutralization (STACLOT-LA) to satisfy the diagnostic criteria of a prolonged screen, inhibitor on mixing study, and demonstration of phospholipid dependence. Recent data presented at the SSC meeting of the ISTH suggests that laboratories may be relying too heavily on global assay to make the determination of LA. The aim of this study was to evaluate LA positivity in a pre-selected cohort of LA positive plasmas in which the diagnosis was based only upon a single positive result from a hexagonal phase global assay using a new dilute Prothrombin time test ACTICLOT dPT™

Methods: 100 LA positive plasmas characterized as STACLOT-LA positive and dRVVT negative were collected. A STACLOT-LA delta > 8 sec was considered positive. The sample mean delta was 13.3 (SD +/− 5.5). ACTICLOT dPT screen and confirm was performed according to package insert instructions on a Beckman Coulter ACL automated coagulation analyzer. Additionally, 1:1 immediate and 1 hour incubated mixing studies were performed. The samples were also characterized for antiphospholipid antibodies using IMUCLONE aCL-HS and IMUCLONE anti-B2GPI IgM and IgG ELISAs. Thirty dRVVT Positive LA controls were also tested using the same ELISAs. Results are tabulated below. In graphs that will be presented, the STACLOT-LA Positive/dRVVT Negative samples have a stronger association with IgM antiphospholipid antibodies, whereas the dRVVT positive control cases demonstrate a stronger association with IgG antiphospholipid antibodies.

Conclusions: The hexagonal phase positive/dRVVT negative samples examined in this study (n=100) were characterized as “weak” positive LA by hexagonal phase assay and contained mostly low titer IgM antiphospholipid antibodies by ELISA. These results suggest that many of the LA plasmas pre-selected in this cohort by hexagonal phase alone are likely to be transient LA positives. About 25% of the samples were considered LA positive using the ACTICLOT dPT screening, confirmatory and mixing protocols. The diagnosis of LA in the remaining samples may still be equivocal based upon the hexagonal phase results alone. Adding the dPT test which functions through the extrinsic pathway may improve current LA screening panels comprised of dRVVT and hexagonal phase assays. The third LA test may also identify certain LA samples that are not picked up by the dRVVT or hexagonal phase assays. Additional studies (in progress) of other LA cohorts such as STACLOT-LA Negative/dRVVT positive and prospective evaluations will help determine if single global assays overestimate LA positivity.