Relative FVIII activity and inhibitor level in HAT mice. (A) The relative FVIII antigen expression in HAT mice. The FVIII protein level was determined by the ELISA method using a specific antibody. (B) The relative FVIII activity in HAT mice. The FVIII activity was determined by the COATEST assay according to the manufacturer's protocol. (C) The standard curve of normal pooled plasma FVIII activity versus clotting time in seconds. The results show that the response of the aPTT reagent to both mouse and human plasma are comparable. (D) The aPTT coagulation assay. The aPTT values of pooled human and WT, HA, and HAT mice are shown. The aPTT values of WT mice were 15.5 to 19.3 seconds, which was increased to 62.5 to 75.3 seconds in HA mice. The aPTT values were significantly lower (25.3-41.6 seconds) in HAT mice. *P < .01. (E) Relative FVIII activity. The relative percentage of FVIII activities in plasma samples from HAT mice and patients were determined. The normal range of mouse FVIII activity is indicated by the gray shaded region. (F) FVIII inhibitors in HAT mice and patients plasma. Mice and patient plasma samples, described in panel E, were tested for anti-FVIII activity by the Bethesda assay. The results show the absence of inhibitor in all the plasma samples of HAT mice. H indicates normal human; HA, hemophilic A mouse; HAP, hemophilic A patient; and HAP (I), hemophilic A patient-inhibitor.