A 35-year-old man presents with a 6-week history of chronic follicular conjunctivitis, mild mucoid discharge, and photophobia. He reports a history of urethritis 8 weeks ago. What is the most appropriate investigation to confirm the diagnosis?

Explanation

## Diagnosis: Chlamydial Conjunctivitis (Inclusion Conjunctivitis) **Key Point:** Chlamydia trachomatis causes chronic follicular conjunctivitis with mild mucoid discharge. The organism is an obligate intracellular pathogen that cannot be cultured on routine bacterial media. Diagnosis requires molecular or immunological methods. **High-Yield:** Direct immunofluorescence (DIF) using monoclonal antibodies against chlamydial antigens or nucleic acid amplification tests (NAAT) — particularly PCR — are the gold standard investigations. PCR is the most sensitive and specific, especially for asymptomatic infection. ### Chlamydial Conjunctivitis: Clinical and Diagnostic Features | Feature | Finding | |---------|----------| | Duration | Chronic (>2 weeks); often 6–8 weeks | | Discharge | Mucoid, scanty | | Follicles | Present (lower tarsal conjunctiva) | | Systemic signs | History of urethritis, cervicitis, or PID | | Giemsa stain | Intracytoplasmic inclusions (but low sensitivity) | | Culture | Not possible on routine media; requires special cell culture | | DIF/PCR | Gold standard; high sensitivity and specificity | **Mnemonic:** **CHLAMYDIA** = **C**hronic follicular conjunctivitis, **H**istory of urethritis, **L**ow sensitivity of Giemsa, **A**ntigen detection (DIF) or **M**olecular tests (PCR), **Y**ield intracytoplasmic inclusions, **D**iagnosis requires immunological methods, **I**ncluded in STI screening, **A**lways treat partner. ### Investigation Algorithm for Chronic Follicular Conjunctivitis ```mermaid flowchart TD A[Chronic follicular conjunctivitis]:::outcome --> B{Duration & discharge type?}:::decision B -->|Mild mucoid, >2 weeks| C[Suspect Chlamydia]:::outcome B -->|Purulent, acute| D[Suspect bacterial]:::outcome C --> E[DIF or PCR for Chlamydia]:::action D --> F[Gram stain + culture]:::action E --> G{Chlamydia detected?}:::decision G -->|Yes| H[Treat with azithromycin/doxycycline + partner therapy]:::action G -->|No| I[Consider other causes]:::outcome ``` **Clinical Pearl:** Chlamydial conjunctivitis is part of the spectrum of sexually transmitted infections. Patients require systemic treatment (azithromycin 1 g single dose or doxycycline 100 mg BD for 7 days) and partner notification/treatment to prevent reinfection. Topical antibiotics alone are insufficient. **Warning:** ~~Giemsa stain~~ has low sensitivity (~30%) for chlamydial conjunctivitis because many infected cells do not contain visible inclusions. Do not rely on Giemsa alone to exclude Chlamydia. [cite:Park 26e Ch 7; Khurana Ophthalmology Ch 3]