Factor Assays can be used to determine the cause of an unexpected, prolonged PT or APTT. They are performed after mixing studies have been run, because 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 is involves performing a PT and APTT, except that plasma known to be deficient in a specific factor type is combined with the patients 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.