Comparison of Clot-Based vs Chromogenic Factor Xa Procoagulant Phospholipid Activity Assays

We compared 2 commercial plasma procoagulant phospholipid activity (PPA) assays, chromogenic, using bound annexin V to capture phosphatidylserine-containing microparticles, and clot-based. In both, anionic phospholipids accelerated activation of prothrombin by factor Xa. PPA levels were lower in the chromogenic vs the clot-based assay, with poor correlation between methods: normal samples, mean +/- SD, 27 +/- 17 vs 590 +/- 414 nmol/L (n = 24; r(2) = 0.29) and patient samples, mean +/- SD, 45 +/- 44 vs 401 +/- 330 nmol/L (n = 51; r(2) = 0.26). Recovery of phosphatidylserine added to normal, heparinized, and warfarin plasma samples averaged 109% +/- 39% using the chromogenic assay but was higher and more varied (mean +/- SD, 176% +/- 59%) in the clot-based assay. Lupus anticoagulants caused low recovery in both assays. Removal of microparticles by 0.22-mu m filtration reduced PPA by 91% in the clot-based and 65% in the chromogenic assay. The clot-based assay showed higher correlation (r(2) = 0.82 vs 0.23) with flow cytometric platelet microparticle counts. The 2 assays measure different aspects of PPA in plasma, with the chromogenic assay primarily measuring smaller microparticles.