A 35-year-old man with a 10-year history of recurrent deep vein thrombosis and pulmonary embolism despite therapeutic anticoagulation is investigated. Platelet count is normal, aPTT is prolonged and does not correct with normal plasma mixing, and PT is normal. Which finding best distinguishes antiphospholipid syndrome from Factor V Leiden in this patient?

Explanation

## Distinguishing Antiphospholipid Syndrome from Factor V Leiden ### Clinical Context Both antiphospholipid syndrome (APS) and Factor V Leiden cause recurrent thrombosis and are hypercoagulable states. However, their laboratory and immunological profiles differ fundamentally. ### Comparison Table | Feature | Antiphospholipid Syndrome | Factor V Leiden | | --- | --- | --- | | **Mechanism** | Autoimmune (antibodies against phospholipid-binding proteins) | Genetic (Factor V mutation R506Q) | | **aPTT** | Prolonged (paradoxically) | Normal | | **aPTT mixing** | Does NOT correct (inhibitor) | N/A (aPTT normal) | | **PT** | Normal | Normal | | **Platelet count** | May be low (thrombocytopenia in 20–30%) | Normal | | **Anticardiolipin/LA** | Positive | Negative | | **Activated Protein C resistance** | Negative (unless dual defect) | Positive | | **Thrombosis despite anticoagulation** | Yes (high recurrence) | Yes (if homozygous or dual defect) | | **Obstetric complications** | Recurrent miscarriage, preeclampsia | No | ### Key Point: **Positive anticardiolipin antibodies or lupus anticoagulant (LA) is the single best discriminator.** This is the defining immunological marker of APS and is absent in Factor V Leiden. ### High-Yield: The **"lupus anticoagulant paradox"**: LA prolongs aPTT in vitro (mimicking a coagulation defect) but causes thrombosis in vivo (paradoxically hypercoagulable). This is pathognomonic for APS. ### Mnemonic: **APLS** = **A**ntiphospholipid syndrome: **P**hospholipid antibodies, **L**upus anticoagulant, **S**ystemic (autoimmune). ### Clinical Pearl: APS is diagnosed by the combination of: 1. **Clinical criteria**: Recurrent thrombosis (venous or arterial) or pregnancy morbidity. 2. **Laboratory criteria**: Positive LA, anticardiolipin IgG/IgM, or anti-β~2~-glycoprotein I antibodies on ≥2 occasions ≥12 weeks apart. Factor V Leiden is a genetic thrombophilia detected by activated protein C resistance (APCR) testing or Factor V gene sequencing—not by antibody assays. ### Why Other Options Are Insufficient: - **Recurrent thrombosis despite anticoagulation:** Both APS and homozygous Factor V Leiden can present this way. Not specific. - **Prolonged aPTT not correcting with mixing:** This is characteristic of LA in APS, but Factor V Leiden has a **normal aPTT**. This finding alone does not distinguish them; it points to APS but does not rule out Factor V Leiden as a co-existent defect. - **Normal platelet count with normal PT:** Both conditions typically have normal PT and platelet count (though APS may have mild thrombocytopenia). Not discriminatory.