Objective:To establish a modified one-stage clotting assay (mOSA) based on the STA-R Evolution coagulation analyzer for quantifying emicizumab (EMI) concentration and to preliminarily evaluate its analytical performance; meanwhile to explore the clinical utility of the standard one-stage clotting assay (sOSA) in indirectly predicting EMI levels through surrogate factor Ⅷ (FⅧ) activity.Methods:A total of 30 pediatric patients with hemophilia A (HA) treated with EMI in the Hemophilia Treatment Center of Shenzhen Children′s Hospital from January 2023 to March 2025 were enrolled, and 48 post-treatment plasma samples were collected. EMI standards (2.5~100 μg/ml) were prepared using FⅧ-deficient plasma to establish the mOSA detection system. The linearity, accuracy, and precision of the method were evaluated. Surrogate FⅧ activity was measured by sOSA to estimate EMI concentrations, and its correlation with mOSA-derived EMI concentrations was analyzed using Spearman correlation analysis. The equivalent FⅧ activity in patient plasma samples was measured using a human chromogenic substrate assay-based FⅧ activity detection reagent, and Spearman correlation analysis was employed to evaluate its correlations with both the EMI concentrations measured by the mOSA method and estimated by the sOSA method respectively.Results:The established mOSA method for EMI detection showed excellent linearity in the range of 2.5?100 μg/ml ( Y=1.047 X?1.033, R 2=0.995, P<0.001). Average spike recovery rates at 25, 50, and 75 μg/ml were 101.55%(25.39/25.00), 105.31%(52.66/50.00), and 98.20%(73.65/75.00), respectively. Coefficients of variations of within-and inter-batch were 3.47%?4.80% and 6.30%?8.96%, respectively. A prediction model for EMI concentration was established as follows: estimated EMI concentration (μg/ml)=0.095×[alternative FⅧ activity (%) measured by sOSA]+2.652 ( R2=0.999, P<0.001). Validation demonstrated a strong correlation between the EMI concentration measured by the mOSA method and the EMI concentration estimated by the sOSA method ( r=0.989, P<0.001), with good consistency ( Y=1.014 X+0.684, R2=0.972, P<0.001). Both the EMI concentration measured by the mOSA method and the EMI concentration estimated by the sOSA method showed extremely strong correlations with the equivalent FⅧ activity ( r=0.986 and 0.987, respectively; P<0.001 for both). Conclusions:The mOSA system established on the STA-R Evolution analyzer demonstrates robust linearity, accuracy, and reproducibility, fulfilling clinical requirements for therapeutic drug monitoring of EMI. The sOSA method provides reliable indirect estimation of EMI concentrations through surrogate FⅧ activity, offering critical support for emergency decision-making.

Objective:To establish a modified one-stage clotting assay (mOSA) based on the STA-R Evolution coagulation analyzer for quantifying emicizumab (EMI) concentration and to preliminarily evaluate its analytical performance; meanwhile to explore the clinical utility of the standard one-stage clotting assay (sOSA) in indirectly predicting EMI levels through surrogate factor Ⅷ (FⅧ) activity.Methods:A total of 30 pediatric patients with hemophilia A (HA) treated with EMI in the Hemophilia Treatment Center of Shenzhen Children′s Hospital from January 2023 to March 2025 were enrolled, and 48 post-treatment plasma samples were collected. EMI standards (2.5~100 μg/ml) were prepared using FⅧ-deficient plasma to establish the mOSA detection system. The linearity, accuracy, and precision of the method were evaluated. Surrogate FⅧ activity was measured by sOSA to estimate EMI concentrations, and its correlation with mOSA-derived EMI concentrations was analyzed using Spearman correlation analysis. The equivalent FⅧ activity in patient plasma samples was measured using a human chromogenic substrate assay-based FⅧ activity detection reagent, and Spearman correlation analysis was employed to evaluate its correlations with both the EMI concentrations measured by the mOSA method and estimated by the sOSA method respectively.Results:The established mOSA method for EMI detection showed excellent linearity in the range of 2.5?100 μg/ml ( Y=1.047 X?1.033, R 2=0.995, P<0.001). Average spike recovery rates at 25, 50, and 75 μg/ml were 101.55%(25.39/25.00), 105.31%(52.66/50.00), and 98.20%(73.65/75.00), respectively. Coefficients of variations of within-and inter-batch were 3.47%?4.80% and 6.30%?8.96%, respectively. A prediction model for EMI concentration was established as follows: estimated EMI concentration (μg/ml)=0.095×[alternative FⅧ activity (%) measured by sOSA]+2.652 ( R2=0.999, P<0.001). Validation demonstrated a strong correlation between the EMI concentration measured by the mOSA method and the EMI concentration estimated by the sOSA method ( r=0.989, P<0.001), with good consistency ( Y=1.014 X+0.684, R2=0.972, P<0.001). Both the EMI concentration measured by the mOSA method and the EMI concentration estimated by the sOSA method showed extremely strong correlations with the equivalent FⅧ activity ( r=0.986 and 0.987, respectively; P<0.001 for both). Conclusions:The mOSA system established on the STA-R Evolution analyzer demonstrates robust linearity, accuracy, and reproducibility, fulfilling clinical requirements for therapeutic drug monitoring of EMI. The sOSA method provides reliable indirect estimation of EMI concentrations through surrogate FⅧ activity, offering critical support for emergency decision-making.