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Factor Activity Assays: Concentration or Function?

When performing factor activity assays, are you determining the concentration or the function? There has been some discussion lately on this and while we thought we knew, we’ve become confused. Thanks.

Diane Thornton
Section Supervisor Hematology
St. John’s Regional Medical Center Laboratory
Joplin, MO

Hello, Diane, and thank you for your question. When you perform a clot-basedcoagulation factor assay using factor-depleted plasma and a partial thromboplastin time (PTT)-based, or prothrombin time (PT)-based assay, you are assaying coagulation factor activity or function. Likewise, a chromogenic factor assay measures function. In contrast, when you perform an immunoassay, such as an enzyme-linked immunoassay (ELISA), you are measuring factor concentration, just the amount of protein present.

Using factor VIII as an example: most cases of hemophilia A, factor VIII deficiency, arise from mutations that cause a quantitative factor VIII deficiency in which factor VIII production is reduced. The factor VIII activity assay and immunoassay are proportionally reduced. In slightly less than 10% of mutations, the function is reduced but the factor VIII concentration remains normal or near-normal. This implies a functional mutation in the reactive site, and the activity assay, either clot-based or chromogenic, would be reduced but the immunoassay would not.

In practice, the distinction between an immunoassay and clot-based assay result becomes vague, as the two results may be near the lower limit of the reference interval. Typically, immunoassays are reproducible, but CAP surveys illustrate there is some variability in clot-based assays, often traceable to the nature of the factor-depleted plasma reagent. In order to make a crisp distinction between a quantitative and qualitative deficiency, you need to ensure your clot-based assay is carefully calibrated, usually an ongoing process. I hope this helps. Geo.

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