PRESENTED at EAHAD 2026 and awarded third best poster prize, a multicentre study has shown that product-specific calibration significantly improves factor VIII activity measurement accuracy and reduces variability across commonly used assay systems.
Introduction
Efanesoctocog alfa is an ultra-long half-life recombinant factor VIII molecule developed to provide sustained factor replacement for patients with haemophilia A. Potency assignment for this therapy was established using a one-stage activated partial thromboplastin time assay based on the Actin FSL reagent. Clinical trial data have previously demonstrated that chromogenic assays overestimate factor VIII activity by approximately 2.5-fold. Additional concern has arisen regarding the reliability of factor VIII activity measurement when one-stage activated partial thromboplastin time assays use reagents other than Actin FSL, highlighting the need for improved standardisation across laboratories.
Factor VIII Activity Measurement and Study Design
To assess the impact of assay calibration on factor VIII activity measurement, 40 centres were distributed three lyophilised plasma samples. One sample served as a plasma calibrator with a defined concentration of 100 IU/dL, based on the labelled potency of the efanesoctocog alfa vial. Two further samples were designed to mimic post-treatment plasma levels at approximately 15 IU/dL and 50 IU/dL. Participating centres were asked to calibrate their routine one-stage activated partial thromboplastin time assay or chromogenic assay using the provided 100 IU/dL plasma calibrator, rather than their standard calibrator material.
Assay Performance and Conclusions
A total of 41 one-stage activated partial thromboplastin time assay results were returned using the supplied plasma calibrator, alongside eight results generated using alternative calibrator materials. In addition, 19 chromogenic assay results were reported using the plasma calibrator, with five results obtained using different calibrators. Some centres submitted more than one factor VIII result. The data were consistent with previously reported deviations from target potency when certain activated partial thromboplastin time reagents or chromogenic assay kits are used. Importantly, use of a product-specific plasma calibrator reduced both overestimation and underestimation of factor VIII activity recovered from in vitro spiked samples at 15 IU/dL and 50 IU/dL. These findings reinforce the value of tailored calibration strategies to improve factor VIII activity measurement accuracy across assay platforms.
Reference
Williams A et al. UK NEQAS BC survey outcome of the laboratory measurement of Altuvoct (efanesoctocog alfa) samples using an in vitro-spiked product specific calibrator. Abstract PO003. EAHAD 2026 Annual Meeting; 3-6th February 2026.
