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

## Clinical Presentation & Diagnosis The patient presents with classic features of **Pneumocystis jirovecii pneumonia (PCP)**: - CD4 count <200 cells/µL (major risk factor) - Subacute onset (2 weeks) of nonproductive cough and dyspnea - Bilateral interstitial infiltrates on CXR ### Investigation of Choice **Key Point:** Induced sputum or bronchoalveolar lavage (BAL) with special staining (Giemsa, immunofluorescence, or silver stain) is the gold standard for diagnosing PCP in resource-limited and resource-rich settings alike. **High-Yield:** PCP diagnosis requires visualization of the organism: - **Induced sputum**: First-line in stable patients; sensitivity ~90% when performed by experienced personnel - **BAL**: Reserved for induced sputum-negative cases or when diagnosis remains uncertain; sensitivity >95% - Staining methods: Giemsa (shows trophozoites), immunofluorescence (most specific), or methenamine silver (shows cyst wall) ### Why This Investigation? | Feature | Induced Sputum/BAL | Alternatives | |---------|-------------------|---------------| | **Sensitivity** | 90–95% | AFB smear <5% for PCP; galactomannan for *Histoplasma* | | **Specificity** | >95% | Galactomannan: useful but not diagnostic alone | | **Invasiveness** | Minimally invasive | BAL more invasive but higher yield | | **Organism visualization** | Direct visualization | Culture-based methods miss PCP | **Clinical Pearl:** In an Indian setting with high TB prevalence, AFB smear may appear attractive, but TB does not typically present with this CD4 count and clinical picture. PCP is the leading diagnosis here and requires organism visualization. ## Diagnostic Algorithm ```mermaid flowchart TD A[CD4 < 200 + fever, nonproductive cough, bilateral infiltrates]:::outcome --> B{Suspect PCP}:::decision B -->|Confirm diagnosis| C[Induced sputum with special staining]:::action C -->|Positive| D[PCP confirmed]:::outcome C -->|Negative, high suspicion| E[BAL with staining]:::action E -->|Positive| D E -->|Negative| F[Consider alternative diagnosis]:::decision F -->|TB risk| G[AFB smear, culture]:::action F -->|Fungal risk| H[Serum/CSF antigen, culture]:::action ```