A 28-year-old man with a history of recurrent oral aphthous ulcers and genital ulcers presents with acute anterior uveitis with fibrinous exudate and hypopyon. He also has a history of recurrent thrombophlebitis. Which investigation is most specific for confirming the diagnosis of Behçet disease?

Explanation

## Clinical Presentation of Behçet Disease This patient presents with the **classic triad of Behçet disease**: 1. **Recurrent oral aphthous ulcers** (mandatory criterion) 2. **Genital ulcers** (recurrent) 3. **Ocular involvement** (anterior uveitis with fibrinous exudate and hypopyon — characteristic of Behçet) 4. **Vascular involvement** (recurrent thrombophlebitis) ## The Pathergy Test: Gold Standard for Behçet **Key Point:** The **pathergy test** is the most specific diagnostic test for Behçet disease. It demonstrates abnormal skin reactivity to minor trauma (needle prick). **High-Yield:** Pathergy Test Procedure: - A 20-gauge needle is inserted intradermally on the volar forearm - Read at **24 and 48 hours** - **Positive result:** papule >2 mm in diameter (erythematous or pustular) - **Sensitivity:** 60–90% (varies by ethnicity; highest in Mediterranean and Middle Eastern populations) - **Specificity:** >95% (highly specific for Behçet disease) **Clinical Pearl:** The pathergy test is included in the **International Study Group (ISG) diagnostic criteria for Behçet disease** as a major criterion. A positive pathergy test + recurrent oral ulcers + any two of (genital ulcers, ocular lesions, skin lesions) = Behçet diagnosis. ## Comparison of Investigations | Investigation | Sensitivity | Specificity | Clinical Use | |---|---|---|---| | **Pathergy test** | 60–90% | >95% | **Gold standard; highly specific; part of ISG criteria** | | HLA-B51 typing | 40–90% | Low (present in healthy controls) | Supports diagnosis but not diagnostic; useful in endemic regions | | Anti-HSV serology | Variable | Low | Excludes HSV-1/2 uveitis; not specific for Behçet | | Conjunctival biopsy | N/A | Low | Non-specific; shows chronic inflammation | **Mnemonic: PATHERGY = Prick And Tissue Hyperreactivity (Erythema/pustule) — Reaction at 48 hours — Guides Yardstick for Behçet diagnosis** ## Why Pathergy Over HLA-B51? Although **HLA-B51 positivity** is associated with Behçet disease (especially in Mediterranean and Asian populations), it is **NOT diagnostic**: - Present in 40–90% of Behçet patients (sensitivity varies by ethnicity) - Also present in healthy controls in endemic regions (low specificity) - Cannot be used alone to confirm diagnosis - Useful for risk stratification but not confirmation [cite:Park 26e Ch 12; Kanski Clinical Ophthalmology 9e Ch 7]