Factor VIII Activity and Inhibitor Assays in the Diagnosis and Treatment of Hemophilia A
The treatment of a patient with a factor VIII (FVIII) deficiency can be complicated. The mainstay of therapy is factor replacement. Replacement therapy can be given prophylactically, with the goal of decreasing hemarthroses and spontaneous hemorrhage, or on-demand for the bleeding patient. Intra- and interindividual variability in a patient's response to treatment has been well documented by the differences in observed half-lives of infused product. Although weight-based dosing nomograms are most often used, personalized therapies are coming into use to ease the burden of therapy and cost. The most significant complication of treatment is the formation of inhibitors to FVIII. The role of the laboratory is to provide results for FVIII activity that accurately reflects a patient's baseline level and response to treatment. However, factor activity assays have many components that can contribute to result variability. These include the methodology and reagent components used to measure the FVIII activity, the reference standard employed, algorithm used to interpret the dilutions, and the replacement factor being measured. An understanding of assay variables and their impact will assist in providing accurate factor activity results and appropriate patient care.