ABSTRACT Background Efanesoctocog alfa (EFA) is an ultra‐long half‐life FVIII product. However, measuring the level of EFA using standard one stage (OSA) and Chromogenic assays (CA) is affected by both underestimation and overestimation when using some commonly used APTT reagents for OSA and overestimation when using CA assays. Objectives ECAT and NEQAS BC distributed samples containing EFA to centres for measurement of EFA using assays available locally. The aim was to provide method specific data and recommendations for additional OSA or CA assays. Methods In Autumn 2024, four samples with levels of EFA between 5 and 100 IU/dL (based on potency label) were buffered and lyophilized for distribution to 300 hospital and university laboratories. Results Two‐hundred participants returned 174 OSA and 148 CA results for samples EFA 24:01‐24:04. Twelve different APTT reagents, six were used by ( n ≥ 10) and 10 different chromogenic assays were employed, of which four were used by ( n ≥ 10). Conclusion The NEQAS BC and ECAT exercise confirmed the field study: over‐estimation for Actin FS OSA and all CA assays at all EFA levels plus an under‐estimation for SynthasIL OSA at EFA levels of: 20, 50 and 100 IU/dL. Centres using Actin FSL on a Siemens/Sysmex or Stago analyser plus centres using Synthafax on a Werfen analysers had median recoveries within ±25% compared to the assigned potency for samples at EFA levels of: 20; 50 and 100 IU/dL. Centres using CK Prest and Pathromtin had median recoveries within ±25% compared to the assigned potency for samples at an EFA levels of 100 IU/dL.
Reilly‐Stitt et al. (2025) studied this question.