O Troubleshooting

What Do I Do When Preoperative Coagulation Studies Are Abnormal?

Preoperative testing was ordered on a 43-year-old woman scheduled for an elective hysterectomy. She has suffered with dysfunctional uterine bleeding for 6 months. Rather than go to the hospital setting, she went to a physician office laboratory that accepted her insurance. Her surgeon ordered a CBC with platelet count and a PT and PTT. Her CBC was within reference range but the results of her PT and aPTT were:

PT 10.6 seconds (Reference range, 10 to 14)

aPTT 53 seconds (Reference range, 28 to 38)

The elevated PTT was an unexpected result. Possibilities for an elevated PTT include a factor deficiency, the presence of a circulating anticoagulant, or a patient on heparin. Heparin was eliminated as a possible contributor to the prolonged PTT since there was no patient history of anticoagulation therapy. Mixing studies are familiar screening tests in the clinical laboratory to determine whether there is a factor deficiency or a circulating anticoagulant. The technologist decided to perform mixing studies on this patient and proceeded with the laboratory protocol. In mixing studies, the patient's plasma is mixed with pooled normal plasma, in a 1:1 ratio and the elevated test is repeated. Pooled normal plasma contains all clotting factors and technologists use normal quality control material as the source of pooled plasma. Once the test is repeated, if the result returns to the normal range, then it is assumed that the source of aPTT elevation was a clotting factor deficiency and factor assay tests on the plasma should be ordered. If the repeated test does not return to the reference range, then it is assumed that the patient plasma contains a circulating anticoagulant. As an additional screening procedure, the aPTT test was incubated for 1 to 2 hours. The rationale behind this additional step is to determine if there is a weak or time-dependent circulating inhibitor. Certain inhibitors such as factor VIII inhibitor have a stronger inhibitory effect with prolonged incubation. These pathological circulating inhibitors will be thoroughly discussed in Chapter 19.

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