A team of French scientists successfully measured the plasma concentration of the hemophilia treatment emicizumab with 4 different reagents and on 2 different coagulation analyzers: the CK-Prest, Cephascreen, and STA-PTT Automate reagents on the STAR MAX analyzer and the Actin FS reagent on the ATELLICA COAG 360 analyzer.

The study, recently published in Haemophilia, recruited 21 patients with severe hemophilia with or without inhibitors between June 2020 and February 2021. To evaluate the reliability of these assays in real-life conditions, the researchers collected venous blood samples and measured the medication levels before each administration of 4 loading doses of 3.0 mg/kg of emicizumab per week.

For the purpose of this study, the authors used factor VIII (FVIII)-derived methodology with high sample dilutions of 1:40 for CK-Prest, Cephascreen, and STA-PTT Automate and 1:80 for Actin FS, as well as a specific emicizumab calibrator (r2 diagnostics).

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The baseline emicizumab concentrations before emicizumab injection were under 2 μg/mL for all participants, except for 4 who had a residual 1-stage clotting FVIII:C of greater than 0.15 IU/mL. The mean emicizumab concentration on day 7 was 21 μg/mL, followed by 37 μg/mL on day 14 when 81% of patients had already reached the therapeutic maintenance dose levels.

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The mean concentration on day 21 was 50 μg/mL, which is considered within the usual plasma concentrations at the steady state (30 to 80 μg/mL). There was 1 adult patient whose emicizumab concentration was above the therapeutic level at 90 μg/mL with CK-Prest.

The emicizumab concentrations measured with the 4 reagents did not differ significantly. Interassay variabilities for pathologic and normal controls were 7.0% and 7.2% with CK-Prest, 6.6% and 6.4% with Cephascreen, 5.9% and 5.0% with STA-PTT Automate, and 1.9% and 5.0% with Actin FS, respectively.

“Biological monitoring of patients treated by emicizumab is a challenge because emicizumab interferes with all coagulation assays based on . . . activated partial thromboplastin times (aPTT) and on chromogenic factor VIII activity (FVIII:C) assays using human recombinant reagents,” Launois and colleagues wrote.

“The determination of emicizumab concentration is not required routinely but may be necessary in case of trauma, urgent surgery, unusual bleeding symptoms and suspicion of anti-drug antibody against emicizumab.”

The new prophylactic treatment for patients with severe hemophilia is likely to improve their quality of life as it shows great efficacy in lowering annualized bleed rates and is administered by a few subcutaneous injections.


Launois A, De Raucourt E, Martin-Toutain I, et al. Emicizumab assays evaluations with four different reagents in severe haemophilia A patients: concentration from baseline to maintenance therapy. Haemophilia. Published online November 29, 2022. doi:10.1111/hae.14703