A 38-year-old woman presents with recurrent deep vein thrombosis (DVT) of the left lower limb and a spontaneous pulmonary embolism 6 months ago. She has no family history of thrombophilia and is not on oral contraceptives. Physical examination reveals mild splenomegaly. Routine coagulation studies (PT, aPTT, platelet count) are normal. Which investigation is most appropriate to establish the diagnosis?

Explanation

## Clinical Presentation Analysis This patient presents with: - Recurrent venous thromboembolism (DVT + PE) - Normal routine coagulation studies - Mild splenomegaly - No obvious acquired risk factors This clinical picture is highly suggestive of **antiphospholipid syndrome (APS)**, a thrombophilia characterized by thrombosis and/or pregnancy morbidity in the presence of antiphospholipid antibodies. ## Why Antiphospholipid Antibody Testing? **Key Point:** Antiphospholipid syndrome is the most common **acquired thrombophilia** and accounts for 5–15% of recurrent venous thromboembolism cases in young patients without traditional risk factors. **High-Yield:** The diagnostic criteria for APS require: 1. Clinical criterion: thrombosis (venous or arterial) or pregnancy morbidity 2. Laboratory criterion: LAC, aCL (IgG or IgM), or anti-β~2~-glycoprotein-I antibodies on ≥2 occasions ≥12 weeks apart **Clinical Pearl:** Splenomegaly in a thrombophilic patient should raise suspicion for APS or other autoimmune thrombotic disorders. The normal routine coagulation studies do NOT exclude APS — in fact, the LAC paradoxically *prolongs* the aPTT in vitro but causes thrombosis in vivo. ## Investigation Hierarchy for Thrombophilia Workup | Investigation | Indication | Sensitivity in APS | |---|---|---| | LAC and aCL | Recurrent VTE + normal PT/aPTT; suspected APS | 80–90% | | APCR/Factor V Leiden | First VTE, family history, or young age | Common but not present here | | Prothrombin G20210A | Heterozygous carriers; less common | Lower pretest probability | | Antithrombin/Protein C/S | Deficiency states; rarer | Not suggested by presentation | **Mnemonic:** **APS** = **A**ntiphospholipid **P**rotein **S**yndrome — remember the three main antibodies: **LAC**, **aCL**, **anti-β~2~-GPI**. ## Diagnostic Approach ```mermaid flowchart TD A[Recurrent VTE + Normal PT/aPTT]:::outcome --> B{Clinical context?}:::decision B -->|No obvious acquired risk| C[Suspect inherited or acquired thrombophilia]:::outcome C --> D{Splenomegaly or autoimmune features?}:::decision D -->|Yes| E[Test for APS first: LAC + aCL]:::action D -->|No| F[Test for inherited: Factor V Leiden, PT G20210A]:::action E --> G[LAC/aCL positive?]:::decision G -->|Yes| H[Repeat testing at 12 weeks for diagnosis]:::action G -->|No| F ``` ## Next Steps If LAC/aCL testing is positive, repeat testing ≥12 weeks later to confirm diagnosis. Initiate anticoagulation (warfarin target INR 2–3, or DOAC with caution) pending confirmation.