A 33-year-old man, who had been diagnosed as congenital factor X deficiency having familial traits, was hemostatically and immunologically investigated in detail. Factor X activity (F.X: C) was below 1% by the determination using prothrombin time method, activated partial thromboplastin time method and Russell's viper venom time method. F.X: C was also below 1% by amidolytic method. Factor X antigen (F.X: Ag) was below 6.25%, below 1% and below 0.1%, by the assay using EIA, ELISA-P (polyclonal), and ELISA-M (monoclonal), respectively. Thus the patient was diagnosed as cross-reacting material negative (CRM—). This is the second case in Japan.
Because his mother's F.X: C is 40% and F.X: Ag is 32%, she may be heterozygous. Although his father's F.X can't be determined, the patient is probably homozygous.
