Factor VIII (FVIII) activity can be measured with different methods including one-stage clotting assay (OSA) or chromogenic substrate assay (CSA). Some persons with mild hemophilia A (HA) have a significant discrepancy between the assays which has been previously reported. Discrepancy between these two methods, albeit less frequent, can also lead to a switch in diagnosis from mild to moderate HA. Herein, we report 3 persons with HA (PwHA) whose diagnosis changed from mild to moderate HA after obtaining FVIII levels assessed by CSA.

Materials and Methods

Data were extracted from the medical records of the PwHA including demographics, bleeding rates, and factor levels. FVIII activity was measured using the OSA on an ACL Top 500 with SynthaSil aPTT reagent (Instrumentation Laboratories Bedford, MA, USA). Factor VIII activity was also determined photometrically via the Chromogenix Coatest® SP4 FVIII chromogenic assay kit (bovine chromogenic FVIII activity, Diapharma Group, West Chester, OH).


Data were collected from 3 PwHA who were previously diagnosed with mild HA yet were noted to have a bleeding phenotype worse than expected for mild HA (Table-1). Case 1 is 26 years old and had been experiencing multiple joint bleeds. His OSA FVIII level was 6.8%. He was on clotting factor concentrate (CFC) on demand. After having severe hemophilic arthropathy affecting his social life, prophylaxis with CFC was commenced. Significant improvement was observed after the initiation of prophylaxis. His CSA FVIII level was 2.7%, consistent with his bleeding phenotype, and he has been on emicizumab prophylaxis for a year without any bleeding events. Case 2 is 22 years old and had been experiencing frequent muscle bleeds. His OSA FVIII level was 6.1% and CSA FVIII level was 2.7%. He remains on CFC on demand based on his preference despite the frequent bleeding. Case 3 is 15 years old and was diagnosed with mild HA at the age of 8 after recurrent joint bleeds. His OSA FVIII level was 13.8% and CSA FVIII level was 1.5%. He has been on emicizumab prophylaxis for 10 months without any bleeding event.


While discrepancies between assays in mild hemophilia have been widely reported, there are fewer reports on an assay discrepancy changing a diagnosis from mild to moderate HA. As such, we strongly suggest performing both OSA and CSA for the diagnosis and classification of HA especially if the PwHA has more bleeding episodes than expected.

Kizilocak:Genentech/Roche: Honoraria. Young:Bayer: Consultancy, Honoraria; Biomarin: Consultancy, Honoraria, Speakers Bureau; Genentech: Research Funding, Speakers Bureau; Hema Biologics: Consultancy, Speakers Bureau; Novo Nordisk: Consultancy, Honoraria; Pfizer: Consultancy, Honoraria; Sanofi: Consultancy, Honoraria, Speakers Bureau; Roche: Consultancy; Takeda: Consultancy, Honoraria, Research Funding; Spark: Consultancy, Research Funding, Speakers Bureau; Grifols: Research Funding; CSL Behring: Consultancy, Honoraria; Genentech/Roche: Consultancy, Honoraria; LFB: Consultancy; Viatris: Patents & Royalties.

Author notes


Asterisk with author names denotes non-ASH members.

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