A 42-year-old man with a 10-year history of recurrent thrombosis (3 DVTs, 2 PEs) is found to have a prolonged activated partial thromboplastin time (aPTT) that does not correct on mixing study. Protein C, protein S, antithrombin, and factor assays are normal. Which single laboratory finding best distinguishes antiphospholipid syndrome (APS) from factor deficiencies in this patient?

Explanation

## Distinguishing Antiphospholipid Syndrome from Factor Deficiencies ### The Mixing Study Paradox in APS **Key Point:** Antiphospholipid syndrome presents with a PARADOXICAL laboratory finding: prolonged aPTT (suggesting anticoagulation) but THROMBOSIS clinically (not bleeding). The aPTT does NOT correct on mixing study because the lupus anticoagulant interferes with the phospholipid-dependent coagulation test itself, not because of a factor deficiency. ### Mechanism of aPTT Prolongation in APS 1. **Lupus anticoagulant** (anti-β2-glycoprotein-I, anticardiolipin antibodies) binds to phospholipid surfaces 2. In vitro: Inhibits the phospholipid-dependent coagulation cascade → prolonged aPTT 3. In vivo: Antibodies activate endothelial cells and platelets → thrombosis (paradoxical) 4. Mixing study: Patient plasma + normal plasma → aPTT remains prolonged (antibody persists) ### Diagnostic Comparison Table | Feature | Antiphospholipid Syndrome | Factor Deficiency (e.g., Factor VIII deficiency) | | --- | --- | --- | | **aPTT** | Prolonged | Prolonged | | **Mixing study** | Does NOT correct | CORRECTS (factor supplied by normal plasma) | | **Clinical presentation** | Thrombosis (paradox) | Bleeding | | **Platelet count** | Normal or low | Normal | | **Anticardiolipin/anti-β2-GP1** | Positive | Negative | | **Lupus anticoagulant** | Positive | Negative | | **Prothrombin time (PT)** | May be prolonged | Normal | ### High-Yield Mnemonic **Mnemonic:** **CLOT** — **C**ardiolipin antibodies, **L**upus anticoagulant, **O**ver-coagulation (thrombosis), **T**est does NOT correct (mixing study) ### Clinical Pearl **Clinical Pearl:** The "lupus anticoagulant" is a misnomer. It is NOT an anticoagulant in vivo — it is a thrombophilia. Patients with APS and prolonged aPTT are at HIGH risk for thrombosis, not bleeding. This is a classic NEET PG trap: students incorrectly assume prolonged aPTT = bleeding risk. ### Confirmatory Tests for APS 1. **Lupus anticoagulant** (mixing study + confirmatory test) 2. **Anticardiolipin IgG/IgM** (ELISA) 3. **Anti-β2-glycoprotein-I IgG/IgM** (ELISA) 4. **Dilute aPTT** (more sensitive than standard aPTT) [cite:Harrison 21e Ch 181]