The consensus on preoperative laboratory screening in moving toward fewer tests being performed in the perioperative setting. Although the yield is low, preoperative coagulation studies can be vitally important. When they are ordered, these tests must be correctly interpreted for good care. Any abnormality of the partial thromboplastin time (PTT) in the setting of a normal prothrombin time (PT) cannot be dismissed as a lab error, as it can be a marker for an underlying coagulation factor disorder that can cause fatal bleeding.

PTT measures the coagulation ability of the intrinsic pathway, and to a lesser extent, the common pathway in the clotting cascade. Factors in the intrinsic pathway include VIII, IX, XI, XII, and prekallikrein. Factors in the common pathway include fibrinogen, II, V, VII, and X. Anticoagulation with heparin (including the low-molecular-weight heparins if used in sufficiently high doses), hirudins, danaparoid, and argatroban cause a prolonged PTT in the setting of a normal PT. However, a prolonged PTT with normal PT in the absence of these drugs must prompt further laboratory investigation using mixing studies.

Virtually every laboratory has a protocol for performing a PTT mixing study. For PTT mixing studies, the patient’s serum is mixed with pooled serum of normal controls and then incubated for 1 to 2 hours at an elevated temperature. The PTT is measured twice after mixing—before and after incubation. There are three possible outcomes of this test: