A Clot Waveform Analysis of Thrombin Time Using a Small Amount of Thrombin Is Useful for Evaluating the Clotting Activity of Plasma Independent of the Presence of Emicizumab

Objective: Although emicizumab is a bispecific, monoclonal antibody that has led to a significant improvement of treatment for hemophilia A patients with inhibitors, the routine monitoring of patients treated with emicizumab is difficult. Thrombin time (TT) reflects thrombin burst, which mainly depends on activation of factor V (FV) and FVIII. Methods: We, therefore, developed a method for evaluating clotting activity independent of the presence of emicizumab. Normal plasma (NP) or FVIII-deficient plasma (FVIIIDP) with and without emicizumab was measured using clot waveform analysis (CWA)-activated partial thromboplastin time (APTT) and TT. Results: Emicizumab caused clot formation in FVIIIDP using the CWA-APTT; however, the coagulation peaks of plasma with and without emicizumab measured by the CWA-TT did not differ to a statistically significant extent. Regarding the mixing tests with NP and FVIIIDP, CWA-APTT showed large differences between each mixing test in plasma with and without emicizumab, whereas the CWA-TT showed similar patterns in mixing plasma with and without emicizumab. Regarding the standard curve of FVIII activity, the CWA-APTT showed an FVIII-concentration-dependent increase; however, the values with each concentration of FVIII differed between samples with and without emicizumab, whereas CWA-TT showed FVIII-concentration-dependent fluctuations independent of the presence of emicizumab, and the values with each concentration of FVIII were similar in samples with and without emicizumab. Conclusions: As CWA-TT using a small amount of thrombin (0.5 IU/mL) can reflect thrombin burst and be useful for evaluating FVIII activity, independent of the presence of emicizumab, it is useful for monitoring clotting activity in patients with an anti-FVIII inhibitor treated with emicizumab.