The Lupus Inhibitor (Lupus Anticoagulant)
The correct answer is C: The lupus inhibitor is an acquired anti-phospholipid antibody that paradoxically causes thrombosis rather than hemorrhage, despite prolonging clotting times in vitro. 1
Key Characteristics
The lupus anticoagulant represents a diagnostic paradox in coagulation medicine:
It is an acquired immunoglobulin (part of the antiphospholipid antibody family) that interferes with phospholipid-dependent coagulation tests in vitro, causing prolongation of the aPTT and other clotting assays 2, 3
Despite its name as an "anticoagulant," it does NOT cause bleeding—instead, it is strongly associated with thrombotic complications including arterial and venous thrombosis, stroke, deep vein thrombosis, pulmonary emboli, and pregnancy morbidity 2, 4, 5
It is the strongest predictor of clinical thrombotic events among the antiphospholipid antibodies, particularly when associated with other antiphospholipid antibodies 4
Clinical Significance
The lupus anticoagulant is NOT a contraindication to anticoagulation therapy—patients with confirmed lupus anticoagulant who meet criteria for antiphospholipid syndrome actually require anticoagulation for thrombosis prevention 6
Why the Other Options Are Incorrect:
Option A is incorrect: An acquired anti-factor antibody that causes hemorrhage describes acquired hemophilia A (Factor VIII inhibitor), which is a completely different entity that requires opposite management—these patients need hemostatic therapy with bypassing agents, not anticoagulation 1, 7
Option B is incorrect: A naturally occurring inhibitor protein that regulates hemostasis would describe physiologic anticoagulant proteins like protein C, protein S, or antithrombin—not the lupus anticoagulant 1
Option D is incorrect: An IgG anti-platelet antibody describes immune thrombocytopenia (ITP) or other platelet antibody disorders, not lupus anticoagulant 1
Critical Diagnostic Distinction
When encountering a prolonged aPTT, distinguishing between lupus anticoagulant and acquired hemophilia A is life-critical because they require opposite management approaches 8:
- Lupus anticoagulant: Continue or initiate anticoagulation (thrombotic risk) 8, 6
- Acquired hemophilia A: Immediate hemostatic therapy with bypassing agents (hemorrhagic risk) 8, 7
The mixing study characteristics differ: lupus anticoagulants are not time-dependent, while Factor VIII inhibitors show time and temperature-dependent prolongation (worsening after 1-2 hour incubation) 1