A 32-year-old man with newly diagnosed HIV infection (CD4 count 180 cells/μL) presents with fever, cough, and dyspnea for 2 weeks. Chest X-ray shows bilateral interstitial infiltrates. Which investigation is most appropriate to confirm the suspected diagnosis?

Explanation

## Clinical Presentation Analysis The patient presents with: - CD4 count 180 cells/μL (severe immunosuppression) - Bilateral interstitial infiltrates on CXR - Subacute presentation (2 weeks) This clinical picture is classic for **Pneumocystis jirovecii pneumonia (PCP)**, the most common opportunistic infection at this CD4 threshold. ## Investigation of Choice for PCP **Key Point:** Induced sputum or bronchoalveolar lavage (BAL) with Giemsa or immunofluorescence staining is the gold standard for PCP diagnosis in HIV patients with CD4 <200 cells/μL. ### Why This Investigation? | Feature | Induced Sputum/BAL | Sputum AFB | Blood Culture MAC | HRCT | |---------|-------------------|-----------|-------------------|------| | **Sensitivity for PCP** | 80–95% | N/A | N/A | High but non-specific | | **Specificity** | >95% | Detects TB, not PCP | Detects MAC, not PCP | Cannot diagnose PCP alone | | **Invasiveness** | Minimally invasive | Non-invasive but low yield | Non-invasive | Non-invasive but imaging only | | **Diagnostic yield at CD4 <200** | Excellent | Poor for PCP | Used for prophylaxis decisions | Suggestive, not diagnostic | **High-Yield:** PCP presents with bilateral interstitial infiltrates (ground-glass appearance) on imaging, but imaging alone cannot confirm diagnosis — microbiological confirmation is mandatory before starting treatment. ## Staining Techniques 1. **Giemsa stain** — reveals trophozoites and cysts 2. **Immunofluorescence (IF) with monoclonal antibodies** — highest sensitivity and specificity 3. **Silver stain (Grocott–Gomori)** — alternative but less sensitive than IF **Clinical Pearl:** If induced sputum is non-diagnostic and clinical suspicion remains high, BAL should be performed immediately — delaying diagnosis in a CD4 <200 patient risks rapid deterioration. ## Prophylaxis Consideration Once PCP is confirmed and treated, **trimethoprim–sulfamethoxazole (TMP-SMX)** prophylaxis is started and continued until CD4 recovers to >200 cells/μL on antiretroviral therapy. [cite:Harrison 21e Ch 197]