A 28-year-old woman is evaluated prior to an elective cholecystectomy. She has no history of abnormal bleeding, and her physical examination is unremarkable. Routine preoperative laboratory testing reveals the following:
| Laboratory Test | Result | Reference Range |
|---|---|---|
| Platelet count | ||
| Prothrombin time (PT) | ||
| Activated partial thromboplastin time (aPTT) |
A mixing study is performed, and the addition of normal plasma fails to correct the patient's aPTT to within the reference range. Which of the following is the most likely explanation for these laboratory findings?
- Lupus anticoagulantAnswer
- BFactor VIII deficiency
- CVon Willebrand disease
- DFactor VII deficiency
- EVitamin K deficiency
Answer
Lupus anticoagulant
The correct answer is lupus anticoagulant. This patient presents with an isolated prolonged aPTT. The next best step in evaluating an isolated prolonged aPTT is a mixing study, which involves mixing the patient's plasma 1:1 with normal plasma. If the patient has a factor deficiency (such as Factor VIII, IX, XI, or von Willebrand disease), the normal plasma will supply the missing factors, and the aPTT will 'correct' to a normal reference range. If the patient has an inhibitor (such as a lupus anticoagulant or a specific factor inhibitor), the inhibitor will act on the factors in the normal plasma as well, and the aPTT will 'fail to correct' (remain prolonged). Lupus anticoagulant is an antiphospholipid antibody that binds to phospholipids in the testing assay, causing an in vitro prolongation of the aPTT that does not correct with mixing.
Step-by-Step Solution
Key Concept
A mixing study differentiates between coagulation factor deficiencies (which correct with normal plasma) and coagulation inhibitors (which fail to correct). Lupus anticoagulant is a common inhibitor causing an isolated, uncorrected aPTT.