Utilizing Inhibitor Screening Assay Results to Predict Bethesda Titers in Patients With Hemophilia A: Cost-Effective Strategy in Resource-Constrained Setting.

Abstract

Background: The classical or modified Nijmegen-Bethesda assay measures FVIII inhibitor titers and requires testing residual FVIII levels at multiple serial dilutions, making it prohibitively expensive in resource-constrained settings. The study aimed to develop a cost-effective strategy to predict inhibitor titers (Bethesda units [BU]) in patients with hemophilia A (HA) using the inhibitor screening assay.

Methods: Inhibitor screening and Bethesda assay were performed on samples of suspected HA with inhibitors. The laboratory data were analyzed to develop a model to predict inhibitor titers from inhibitor screening (normalized ratio of APTT values of the fresh mix [FM] or incubated mix [IM]). The model was then prospectively validated on new inhibitor-positive samples.

Results: One hundred and four samples from 83 patients with HA and inhibitors were analyzed. A linear regression analysis was performed, and the normalized APTT ratio of FM (R² = 0.815) proved to be highly predictive of inhibitor titers (BU) compared with IM (R² = 0.536). A predictive model was developed based on the normalized APTT ratio of FM values. This predictive model was then validated on a new cohort of 27 inhibitor-positive samples, where the predicted inhibitor titers strongly correlated with the observed titers (R² = 0.843).

Conclusion: The FM of the screening assay serves as a potential indicator of the expected inhibitor titers (BU) and guides the selection of dilutions likely to yield near-50% residual FVIII activity for estimating the Bethesda titers. This approach substantially reduces assay costs, particularly in cases with > 2 BU. Additional prospective studies are needed to validate these results.