Introduction: In hemophilia A (HA), females are diagnosed using F8 genotype, factor VIII (FVIII) level, and bleeding symptom information. Females who have HA-causing DNA variants have variable FVIII levels, with approximately 30% expected to have low FVIII levels and HA. Many genotype-positive females have been reported to have bleeding symptoms, including females with factor levels within the normal range. More information is needed to better understand female risk in hemophilia. 8CHECK is a free F8 gene variant testing service funded by Octapharma. 8CHECK is available to any person in the U.S. with HA and to females who are known or at-risk to have inherited a HA-causing genetic variant. Octapharma has no access to individual identifying information.

Methods: Individuals are consented for genetic testing by a U.S. provider and samples sent to Bloodworks Northwest for F8 genotyping. Inclusion criteria included all individuals with a FVIII level < 40% or a female with a family history of HA in a first degree relative or, occasionally, a more distant relative. Genetic testing is performed and a clinical report is returned to the provider. The 8CHECK program began in 2021. In February 2024, additional clinical information was requested to aid in F8 variant testing and results interpretation including family information, baseline FVIII levels, and presence of bleeding symptoms. Data on females participating in the program were abstracted and analyzed.

Results: Since its inception in May 2021, 2764 individuals have had HA genotyping through the 8CHECK program, including 1449 females. Between March 2024 and July 2025, 531 eligible females had a sample submitted for testing. Ages ranged from <1 to 83 (25.1 +/- 16.6 years). A F8 variant was identified in 62.0% (329/531) of participating females. The same types ofvariants previously reported in males with HA were found in females, with the most common type being missense (47.1%) followed by intron 1 or 22 inversions (28.6%), stop-gain (6.7%), frameshift (4.9%), large structural variants (4.6%), synonymous (4.6%), and splice (2.7%) variants. Three females had more than one deleterious variant found. A family history of hemophilia was reported in 88.1% (468/531) of females. Genotyping was much lower yield in females reported not to have a family history of hemophilia (18%, n=11/59) compared to females with a positive family history (67.5%, n=316/468). Nearly all females with a variant found (96.3%, n=316/329) had a positive family history, while 25.4% (51/202) of females who tested negative had no family history. Overall, 231 females had baseline FVIII activity levels reported, of whom 57.5% (133/231) had a variant identified. In genotype-positive females FVIII levels varied widely from <1% to 213%, with 44.4% (59/133) having FVIII levels <40% and HA. Two females had severe HA (FVIII <1%), one of whom was heterozygous for an intron 22 inversion and one of whom had a missense and a complex large structural variant. Average FVIII levels were lower in females with a variant identified compared to females with no variant found (50.4% +/- 32.9% vs. 66.4% +/- 46.2%, p=0.004), with considerable overlap in range. Providers reported on the presence or absence of bleeding in 432 females. In females with a variant identified and bleeding data, 49.3% (133/270) had excessive bleeding. There were 123 individuals with a variant, bleeding data, and FVIII levels reported. FVIII levels were statistically significantly lower in those with a variant and bleeding than those without bleeding (44.6% +/- 26.9% vs 59.3% +/- 38.4, p=0.02). Notably, 38/73 (52%) of females with a HA-causative variant and bleeding symptoms had a FVIII > 40%.

Conclusion: Overall, females have comprised 52% of individuals tested in the F8 genotyping 8CHECK program. This is lower than the expected ~1.5 females for every male with HA in the population. Genotyping can be positive in the absence of a family history of HA but is lower yield. Females with a F8 variant had statistically lower FVIII levels, but levels varied widely making FVIII a poor predictor of genotype status. Of females with a causative variant and bleeding information, nearly half (49%) had excessive bleeding, supporting that it is common for females with an HA-causative variant to have excessive bleeding at normal FVIII levels. These findings show the importance of access to F8 genotyping for females known or at-risk to inherit an HA-causative genotype.

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2025
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