Factor assays can be used to determine the cause of an unexpected, prolonged PT or aPTT. They are performed after mixing studies because they can identify specific factor deficiencies or inhibitors.
Mixing studies are screening tests, while factor assays are confirmatory tests for specific factor deficiencies.
The test 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, and the resultant time is compared to a standard curve. The percent of activity and amount of correction with normal plasma determines the specific factor deficiencies.
