13.9 seconds (Reference range, 11.8 to 14.5)

168.6 seconds (Reference range, 23.0 to 35.0)

The patient's PTT is extremely elevated. Three questions come to mind. Is the patient on heparin? Is there a circulating anticoagulant present? Does the patient have a congenital acquired factor deficiency? A thrombin time was performed in the unlikely event that the patient was somehow receiving heparin (most likely, low-molecular-weight heparin, which can be administered on an outpatient basis). The thrombin time was normal, so the hematologist then ordered a PTT mixing study.

Mixing study: Immediate PTT = 32.9

50:50 mix:

1-Hour incubated 50:50 mix: PTT = 34.3

Based on the mixing study results, one could conclude that the patient has a factor deficiency. Additionally, the incubated mixing study demonstrated that no slow-acting inhibitor is present. Because only the PTT is affected, the most likely factor would be one or more from the intrinsic pathway (factors XII, XI, IX, or VIII; HMWK; or prekallikrein). The hematologist then ordered factor assays, with the following results: Factor VIII 109% activity (Reference range,

55% to 145%) Factor IX 121% activity (Reference range,

61% to 140%) Factor XI 86% activity (Reference range,

65% to 135%) Factor XII 33% activity (Reference range,

50% to 150%) As can be seen from the laboratory data, this patient was factor XII deficient. Unlike for factors VIII, IX, and XI, patients with a factor XII deficiency do not have bleeding problems. Factor XII-deficient patients tend to have very long PTTs, however, because the clotting time of a PTT is dependent on the in vitro activation of factor XII. Similar to HMWK and prekallikrein deficiency, factor XII-deficient patients may even have a tendency toward thrombosis. This young man had his surgery with no complications.

[Case submitted by Wendy Sutula, MS, MT(ASCP), SH, Washington Hospital Center.]

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