If the screening PT or aPTT are prolonged, which other laboratory tests should be ordered?
When a prolonged PT or aPTT is encountered in the preoperative evaluation, the primary concern is to deter-mine whether the prolonged coagulation time is caused by a deficiency of one or more clotting proteins and to deter-mine whether the abnormality can be corrected by plasma. If the medical history and list of current medications is not informative, the most efficient way to make this assessment is to perform an in vitro assay called a mixing study. In this test, patient plasma is mixed with normal control plasma in a 1:1 ratio. The test is then repeated with the mixture immediately after mixing and after incubation at 37°C for 2 hours. When factor deficiency is present, addition of 50% normal plasma to the patient’s plasma will shorten the patient’s coagulation time, usually bringing it into the normal range. This correction is maintained after incubation. If the prolonged coagulation test is not corrected by the addition of 50% normal plasma, or if it corrects immediately after mix-ing but becomes prolonged again after incubation, an inhibitor to a coagulation factor is likely to be present. Many inhibitors are autoantibodies to coagulation factors. In dys-proteinemias, the abnormal immunoglobulin may nonspecif-ically inhibit several coagulation factors in in vitro assays.
When the prolonged coagulation time is corrected, one should measure the activity of the individual coagulation proteins involved in the screening test. When an inhibitor is suspected, a hematologist should be consulted to guide the laboratory evaluation and to help plan treatment.