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Factor Levels in Hemophilia

Mary Paoli of Palmetto Health asks, “When hemophilia is classified as severe, moderate or mild, are factor VIII activity levels (FVIII:C), reported in percentage, or factor VIII antigen (FVII:Ag), reported in U/dL? 
Thank you for your question, Mary. When a newborn boy is at risk of hemophilia, his plasma is tested using the plasma-based factor VIII (or in hemophilia B, factor IX) assay. In limited instances, laboratories may use a chromogenic factor VIII or factor IX assay in place of the plasma-based assay, but both measure factor activity. The results may be reported in percent or in international units (IU)/mL or IU/dL. Most laboratories publish the normal factor range as 50-150%, 0.5-1.5 IU/mL, or 50-150 IU/dL. Half of hemophilia A patients are classified as severe, with less than 1 IU/dL factor VIII. About 10% are moderate, from 1 to 5 IU/dL factor VIII, and 40% have mild hemophilia, meaning they have symptoms with factor VIII levels greater than 5 IU/dL but below the normal range.

The FVIII:Ag and FIX:Ag immunoassays are used to differentiate hemophilia mutations that produce a qualitative Vs. a quantitative factor deficiency. If the FVIII:C, for instance, is less than 50 IU/dL but the FVIII:Ag is within the normal range, the patient possesses a qualitative defect. Less than 10% of hemophilia A or B are quantitative defects. The immunoassay results may be expressed as percentages, IU/mL or UI/dL, and the two assay results are often expressed as a ratio.

Thank you for your question, Mary, and I hope this is helpful. Geo

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