Abstract Number: PB0190
Meeting: ISTH 2022 Congress
Background: Inhibitor antibodies remain a major complication of hemophilia A. However, clinical assays do not adequately predict bleeding risk. We recently demonstrated that factor VIII activity (fVIII:C) supported by activated platelets is inhibited by anti-C2 mAbs in a manner that has no significant correlation with the degree of inhibition in a standard aPTT-based assay. These findings suggest that a platelet-based assay may better assess bleeding risk.
Aims: To measure residual fVIII:C in the presence of patient inhibitors utilizing the activated platelet time (aPT) adapted for the clinical laboratory.
Methods: Cryopreserved platelets were thawed at 37˚C and purified on a density gradient. The aPT was initiated by combining i) platelets reconstituted in fVIII-deficient plasma, ii) a fVIII sample in 10% plasma (with or without inhibitory antibodies and using either fVIII-deficient plasma or inhibitor patient plasma), and iii) an activation mix (factor XIa, PAR1/4 agonist peptides, Ca++). Time to fibrin polymerization was measured using viscosity-based clot detection (STA Compact Max, Diagnostica Stago).
Results: A 3rd-order relationship between time to fibrin formation and the log of fVIII concentration was reliable over a 3.5-log range (Fig 1A). The assay was robust over platelet concentrations of 300,000-10,000,000 platelets/mL and 2-60 pM fXIa. A panel of anti-fVIII-C2 antibodies caused increased inhibition of the aPT vs. the aPTT (Fig 1B) consistent with prior results (Fig 1C). Out of 6 patient inhibitor plasmas, 5 showed lower factor VIII residual activity when measured using the aPT assay (Fig 2). For three samples, the aPT measured fVIII:C ≤ 1% while the aPTT-based assay indicated fVIII:C ≥ 5% for all samples.
Conclusion(s): The aPT, refined for clinical use, measures inhibitor-suppressed fVIII:C low enough to rationalize a high bleeding risk for some patients. Further studies will be needed to indicate whether the aPT more accurately predicts bleeding risk than current clinical assays.
To cite this abstract in AMA style:Gilbert G, Novakovic V, Chatterjee M. Platelet-dependent factor VIII activity is more susceptible to inhibitor antibodies than phospholipid vesicle-dependent factor VIII activity [abstract]. https://abstracts.isth.org/abstract/platelet-dependent-factor-viii-activity-is-more-susceptible-to-inhibitor-antibodies-than-phospholipid-vesicle-dependent-factor-viii-activity/. Accessed November 30, 2023.
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