Laboratory Assessment of Emicizumab Levels in Hemophilia A: Influence of Assay Selection on Reported Results

Background Emicizumab has emerged as a promising therapy for hemophilia A (HA), employing a bypassing mechanism to restore hemostasis. However, since the traditional factor assays cannot be used for measuring the effect of emicizumab, treatment monitoring is difficult. Objectives To assess the impact of emicizumab on global hemostatic potential (OHP), as well as on chromogenic and modified one-stage (mOSA) FVIII assays calibrated for emicizumab in patient samples. Methods Peripheral blood samples from patients with HA on emicizumab were utilized for a validation (n = 28) and a clinical cohort (n = 9). Emicizumab concentration was measured by chromogenic FVIII and mOSA (Actin FS, Actin FSL, and PTT-LA), while overall haemostatic potential (OHP) was used to assess global hemostasis. Results Significantly lower emicizumab concentrations were observed using chromogenic FVIII assay compared to mOSA ( P < .005, P < .005, and P < .005 for mOSA Actin FSL, mOSA Actin FS, and mOSA PTT-LA respectively) in the validation cohort. In the clinical cohort, emicizumab concentrations were significantly lower with chromogenic FVIII assay compared to mOSA Actin FS ( P < .005) and mOSA PTT-LA ( P = .009), but not mOSA Actin FSL. In the clinical cohort, no correlation was seen between OHP and emicizumab. Conclusion Accurate measurement of emicizumab concentration may have the potential to understand assay variability, which is important for informed clinical decision-making in HA patients. Our findings underscore the need for introducing new methods for measuring emicizumab concentration and suggest chromogenic-based methodologies as a viable option for mitigating assay interference.