Mixing studies may be ordered after an unexpected, prolonged PT or aPTT is encountered to determine if the problem stems from a factor deficiency or the presence of an inhibitor.
To perform the test, the patients' plasma is mixed with an equal volume of pooled normal plasma, and then a PT and aPTT are performed on the mixture. If the addition of the pooled plasma brings the resultant values into normal range, then the pooled plasma contained factors the patient's sample was deficient in, and the patient has a factor deficiency. If the results are not "corrected" or brought back into normal range after the addition of pooled normal plasma, then an inhibitor may be present.
The next step in the diagnostic sequence of events, if correction has occurred, is to perform a factor assay to determine which specific factor is lacking.