Factor assays may be part of the testing process to determine the cause of an unexpected, prolonged PT or aPTT. This test is performed after mixing studies have been performed and a deficiency of one or more coagulation factors is suspected. Factor assays are able to identify specific factor deficiencies or inhibitors. Think of mixing studies as being the screening test, while factor assays are confirmatory tests for specific factor deficiencies.
The test itself involves performing a PT and aPTT, except that plasma known to be deficient in a specific factor type is combined with the patient's plasma, comparing the resultant time to a standard curve. The percent of activity, and amount of correction with normal plasma determines the specific factor deficiencies.