A 35-year-old woman with a history of recurrent oral ulcers, genital ulcers, and ocular inflammation presents with acute thrombosis of the superior vena cava. Behçet disease is suspected. Which investigation is most specific for confirming the diagnosis?

Explanation

## Diagnosis of Behçet Disease ### Pathergy Test: Gold Standard for Behçet Disease **Key Point:** The pathergy test is the most specific diagnostic test for Behçet disease. A positive result (papule or pustule at needle prick site after 24–48 hours) is highly specific and supports the diagnosis when combined with clinical criteria. ### Pathergy Test Mechanism 1. **Procedure:** Intradermal needle prick (20–22 gauge) on forearm or back 2. **Timing:** Examination at 24 and 48 hours 3. **Positive result:** Erythematous papule or pustule ≥2 mm diameter 4. **Interpretation:** Reflects abnormal neutrophil function and vascular hyperreactivity ### Specificity and Sensitivity **High-Yield:** Pathergy test is **highly specific (>95%)** for Behçet disease but **sensitivity varies (40–80%)** depending on ethnicity and disease activity. A positive test strongly supports diagnosis; a negative test does not exclude it. **Clinical Pearl:** Pathergy positivity is more common in patients from the Mediterranean, Middle East, and East Asian regions compared to Northern Europeans. ### Diagnostic Criteria for Behçet Disease (International Study Group) **Mnemonic: **RRGO** — **R**ecurrent oral ulcers (mandatory), **R**ecurrent genital ulcers, **G**enital ulcers, **O**cular involvement + Pathergy test positive = Behçet** - **Mandatory criterion:** Recurrent oral ulcers (≥3 episodes in 12 months) - **Plus 2 of the following:** - Recurrent genital ulcers - Eye lesions (typically posterior uveitis) - Skin lesions (erythema nodosum, pseudofolliculitis) - Positive pathergy test ### Table: Investigations in Behçet Disease | Investigation | Specificity | Sensitivity | Role | | --- | --- | --- | --- | | **Pathergy test** | **>95%** | **40–80%** | **Most specific diagnostic test** | | HLA-B51 typing | Low | Moderate (60–80%) | Genetic marker, not diagnostic | | Ophthalmologic exam | Moderate | High | Identifies ocular involvement | | Venography/imaging | Moderate | Moderate | Detects vascular thrombosis | **Warning:** HLA-B51 positivity is associated with Behçet disease but is neither necessary nor sufficient for diagnosis—many HLA-B51-positive individuals never develop the disease, and some Behçet patients are HLA-B51-negative. ### Why Pathergy Test is Superior **Tip:** Pathergy test is the **only test that is specific to Behçet disease**. It reflects the underlying pathophysiology (neutrophil hyperfunction and vascular inflammation) and is not seen in other vasculitides.