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.
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