A 28-year-old man presents with urethral discharge and dysuria for 3 days. On examination, mucopurulent urethral exudate is noted. He denies systemic symptoms. Which is the most appropriate investigation to differentiate between gonococcal and non-gonococcal urethritis?

## Investigation of Choice for Gonococcal vs Non-Gonococcal Urethritis ### Why NAAT is Superior **High-Yield:** NAAT (PCR, TMA, SDA) is the gold standard for diagnosing both *Neisseria gonorrhoeae* and *Chlamydia trachomatis* simultaneously. It is the most sensitive (>95%) and specific (>99%) test, and can differentiate between the two organisms in a single test. **Key Point:** NAAT is recommended by CDC, WHO, and Indian STI guidelines as the investigation of choice for suspected urethritis because it: - Detects both organisms in one assay - Works on first-void urine, urethral swabs, or endocervical swabs - Has superior sensitivity compared to culture or microscopy - Allows rapid, simultaneous diagnosis and appropriate targeted therapy ### Comparison of Investigations | Investigation | Sensitivity | Specificity | Differentiates N.g. from C.t.? | Limitations | |---|---|---|---|---| | **NAAT** | >95% | >99% | **Yes** | Requires PCR equipment; expensive | | Gram stain | 90% (N.g. only) | 95% | **No** (only shows gram-negative diplococci) | Cannot detect C. trachomatis; operator-dependent | | Culture (Thayer-Martin) | 80–90% | 100% | **Yes** | Fastidious organism; requires special media; slow (48–72 hrs) | | Wet mount | 60% | 70% | **No** | Low sensitivity; cannot differentiate organisms | ### Clinical Pearl Gram stain showing intracellular gram-negative diplococci in polymorphonuclear leucocytes is highly suggestive of gonorrhea, but **cannot exclude chlamydia**. Up to 30% of gonococcal infections are co-infected with chlamydia; therefore, simultaneous testing is essential. **Mnemonic:** **NAAT = NAA Test** (Nucleic Acid Amplification Test) — the gold standard for STI diagnosis in the modern era. [cite:Harrison 21e Ch 137]