Which method is considered quantitative for determining the amount of Factor VIII?

The factor assay is the method recognized as quantitative for determining the amount of Factor VIII in the blood. This assay involves diluting the patient's plasma with a standard plasma that contains a known concentration of Factor VIII and then assessing the ability of the patient's plasma to correct a coagulation defect. The results are expressed in terms of percentage of Factor VIII activity compared to a standard reference, allowing for precise quantification.

In contrast, the mixing study is primarily a qualitative assessment used to identify the presence of inhibitors that may affect Factor VIII activity, rather than quantifying the factor itself. Prothrombin time and thrombin time are both part of the broader coagulation testing but do not specifically measure Factor VIII levels quantitatively. Prothrombin time assesses the pathway of coagulation involving factors I, II, V, VII, and X, while thrombin time evaluates the final step in the coagulation cascade involving fibrinogen, making them unsuitable for quantitative assessment of Factor VIII.