In a factor VIII activity assay, the patient's plasma should be mixed with what type of plasma?

In a factor VIII activity assay, the patient's plasma should be mixed with factor VIII deficient plasma to accurately determine the activity level of factor VIII in the patient's sample. Factor VIII deficient plasma lacks the factor VIII protein entirely, which allows for a clear evaluation of the patient's plasma for any functional activity of this clotting factor.

When the patient's plasma is mixed with the deficient plasma, any presence of functional factor VIII from the patient will correct the coagulation defect, leading to a measurable increase in clotting activity. The results obtained can help clinicians understand whether the patient has a deficiency in factor VIII and, if so, to what extent. This approach contrasts with mixing with normal control plasma or plasma with high concentrations of factor VIII, which would not serve the purpose of isolating and assessing the factor VIII activity in the patient's sample specifically.