A 28-year-old man from Delhi presents with recurrent episodes of deep vein thrombosis (DVT) in the left lower limb over the past 18 months. He has no family history of thrombophilia and is a non-smoker with normal BMI. His recent investigations show: PT 14 sec (control 12 sec), aPTT 48 sec (control 35 sec), thrombin time normal, platelet count 250,000/μL, fibrinogen 320 mg/dL. Mixing study (1:1 patient:normal plasma) shows aPTT remains prolonged at 42 sec. What is the most likely diagnosis?

Explanation

## Diagnosis: Antiphospholipid Syndrome with Lupus Anticoagulant ### Clinical Presentation **Key Point:** Recurrent venous thrombosis (DVT) in a young, otherwise healthy patient without traditional risk factors is a hallmark of antiphospholipid syndrome (APS). ### Laboratory Interpretation | Test | Result | Interpretation | |------|--------|----------------| | PT | 14 sec (mildly prolonged) | Mild prolongation in APS | | aPTT | 48 sec (prolonged) | Suggests inhibitor (not factor deficiency) | | Thrombin time | Normal | Rules out hypofibrinogenemia or FDP | | Fibrinogen | 320 mg/dL (normal) | Rules out DIC or consumption | | Platelet count | 250,000/μL (normal) | No thrombocytopenia | | **Mixing study** | **aPTT remains prolonged (42 sec)** | **Diagnostic: Inhibitor present (not factor deficiency)** | ### Why the Mixing Study is Diagnostic **High-Yield:** The **mixing study is the key differentiator:** - **Normal plasma correction** (aPTT normalizes) → Factor deficiency (e.g., Factor V, VIII, IX, XI, XII deficiency). - **No correction** (aPTT remains prolonged) → Inhibitor present (e.g., lupus anticoagulant, heparin contamination, acquired factor VIII inhibitor). In this patient, the aPTT remains prolonged at 42 sec (only slight improvement from 48 sec), indicating an **inhibitor**, not a factor deficiency. ### Antiphospholipid Syndrome: Pathophysiology **Mnemonic: APLS** - **A** — Anticardiolipin antibodies (IgG, IgM) - **P** — Phospholipid-binding proteins (β2-glycoprotein I, prothrombin) - **L** — Lupus anticoagulant (most thrombogenic) - **S** — Serine protease inhibitor (protein C, protein S) **Key Point:** Lupus anticoagulant (LA) is a **misnomer**: - It is NOT an anticoagulant in vivo; it causes **thrombosis**. - It is called "anticoagulant" because it prolongs aPTT *in vitro* (by interfering with phospholipid-dependent coagulation tests). - It is NOT specific to lupus; it occurs in APS, SLE, infections, and malignancy. ### Diagnostic Criteria for APS (Sydney Criteria) **Clinical criteria:** 1. Vascular thrombosis (arterial or venous) — **this patient has DVT**. 2. Pregnancy morbidity (≥3 consecutive unexplained miscarriages, preeclampsia, placental insufficiency). **Laboratory criteria (must be present on ≥2 occasions, 12 weeks apart):** 1. Lupus anticoagulant (prolonged aPTT, positive mixing study, positive confirmatory test). 2. Anticardiolipin antibodies (IgG or IgM, moderate to high titer). 3. Anti-β2-glycoprotein I antibodies (IgG or IgM). **Diagnosis:** ≥1 clinical criterion + ≥1 laboratory criterion = APS. ### Confirmatory Tests for Lupus Anticoagulant 1. **Dilute aPTT** (dilute patient plasma 1:1 with saline): - LA prolongs aPTT even when diluted (phospholipid-dependent). - Factor deficiency corrects with dilution. 2. **Hexagonal phase phospholipid test** (or dilute PT): - Adds excess phospholipid to neutralize LA. - Corrects aPTT if LA is present. 3. **Tissue thromboplastin inhibition (TTI) test**: - Alternative confirmatory test. **Clinical Pearl:** The **prolonged aPTT in APS is a paradox**: the patient has a prolonged aPTT (suggesting anticoagulation) but presents with **thrombosis** (suggesting hypercoagulability). This is because LA interferes with phospholipid-dependent tests in vitro but promotes thrombosis in vivo by: - Inhibiting protein C and protein S (natural anticoagulants). - Activating platelets and endothelial cells. - Promoting tissue factor expression. ### Management 1. **Anticoagulation:** Warfarin (target INR 2–3) or DOAC (apixaban preferred in recent trials). 2. **Avoid heparin** in acute thrombosis (may paradoxically worsen thrombosis in APS). 3. **Repeat testing** at 12 weeks to confirm diagnosis (transient LA can occur with infections). 4. **Screen for other thrombophilias** (Factor V Leiden, prothrombin mutation, protein C/S deficiency) — APS can coexist with inherited thrombophilia.