A 32-year-old woman from Mumbai presents with a 4-month history of progressive dyspnea, bilateral leg swelling, and recurrent miscarriages (two spontaneous abortions in the second trimester). She reports a history of deep vein thrombosis in her left leg 18 months ago. On examination, she has mild facial edema and bilateral lower limb pitting edema. Laboratory findings reveal: platelet count 95,000/μL (normal 150,000–400,000), aPTT 45 seconds (normal 25–35), PT normal, lupus anticoagulant positive, anticardiolipin IgG antibody 42 GPL units (normal <15), and anti-β2 glycoprotein-I antibody positive. ANA is negative. Serum creatinine is 1.8 mg/dL. What is the most likely diagnosis?

Explanation

## Diagnosis: Primary Antiphospholipid Syndrome (PAPS) ### Clinical Presentation This patient presents with the classic triad of antiphospholipid syndrome (APS): **Key Point:** APS is characterized by thrombosis (venous or arterial) and/or pregnancy morbidity in the presence of persistent antiphospholipid antibodies. ### Clinical Features Present | Feature | Clinical Significance | |---------|----------------------| | DVT (18 months ago) | Venous thrombosis — major criterion | | Recurrent miscarriages (2nd trimester) | Pregnancy morbidity — major criterion | | Thrombocytopenia (95,000/μL) | Mild; occurs in ~30% of APS patients | | Dyspnea + edema | Suggestive of pulmonary embolism or renal involvement | | Elevated creatinine (1.8 mg/dL) | May indicate thrombotic microangiopathy or renal artery stenosis | ### Serologic Profile — Diagnostic Criteria Met **High-Yield:** Diagnosis of APS requires BOTH clinical criteria (thrombosis or pregnancy morbidity) AND laboratory criteria (persistent antiphospholipid antibodies on ≥2 occasions, ≥12 weeks apart). | Antibody | Result | Interpretation | |----------|--------|----------------| | Lupus anticoagulant | Positive | Prolonged aPTT; paradoxically causes thrombosis | | Anticardiolipin IgG | 42 GPL (normal <15) | Elevated; high-risk category (>40 GPL) | | Anti-β2 glycoprotein-I | Positive | Highly specific for APS; associated with thrombosis | | ANA | Negative | Rules out SLE; indicates PRIMARY APS | **Clinical Pearl:** The presence of lupus anticoagulant (prolonged aPTT that does NOT correct with normal plasma) is the strongest predictor of thrombotic risk in APS. The paradoxical name "anticoagulant" reflects its in vitro effect; in vivo it causes thrombosis. ### Pathophysiology of APS ```mermaid flowchart TD A[Antiphospholipid antibodies]:::outcome --> B[Binding to β2-glycoprotein-I<br/>on endothelial/platelet surfaces]:::outcome B --> C[Activation of complement<br/>and tissue factor pathway]:::outcome C --> D[Endothelial dysfunction<br/>Platelet activation]:::outcome D --> E[Thrombosis in veins/arteries]:::outcome D --> F[Placental infarction<br/>Fetal loss]:::outcome E --> G[DVT, PE, Stroke]:::outcome F --> G2[Recurrent miscarriage]:::outcome ``` ### Primary vs. Secondary APS | Feature | Primary APS | Secondary APS | |---------|-------------|---------------| | ANA | Negative | Positive (SLE, other CTD) | | Anti-dsDNA | Absent | Often present | | Complement levels | Normal | Often low | | Clinical context | Isolated APS | Features of underlying autoimmune disease | | Prevalence | ~50% of APS cases | ~50% of APS cases | **Key Point:** This patient has NEGATIVE ANA, which excludes secondary APS associated with SLE. The diagnosis is PRIMARY APS. ### Diagnostic Criteria for APS (Sydney 2006, updated 2023) **Clinical Criteria (≥1 required):** 1. Vascular thrombosis (venous or arterial) — ✓ DVT present 2. Pregnancy morbidity (recurrent early loss or late loss with placental insufficiency) — ✓ Recurrent 2nd trimester losses **Laboratory Criteria (≥1 required, on ≥2 occasions ≥12 weeks apart):** 1. Lupus anticoagulant — ✓ Positive 2. Anticardiolipin antibody (IgG or IgM, medium/high titer) — ✓ IgG 42 GPL (high-risk) 3. Anti-β2 glycoprotein-I antibody (IgG or IgM) — ✓ Positive **High-Yield:** Triple positivity (lupus anticoagulant + anticardiolipin + anti-β2GPI) confers the highest thrombotic risk (~10-year thrombosis rate ~50%). ### Management Implications - **Anticoagulation:** Long-term anticoagulation with warfarin (target INR 2–3) or DOAC (if arterial thrombosis) - **Pregnancy:** Aspirin + LMWH throughout pregnancy and 6 weeks postpartum - **Monitoring:** Repeat antibody testing at 12 weeks to confirm persistence (required for diagnosis) **Mnemonic: HITT vs. APS** - **H**eparin-induced thrombocytopenia: acute onset, HIT antibody positive, temporal relation to heparin - **A**ntiphospholipid syndrome: chronic, persistent antibodies, history of thrombosis/pregnancy loss [cite:Harrison 21e Ch 179; Robbins 10e Ch 6]