A 32-year-old man from Delhi presents with a 3-week history of fever, productive cough, and dyspnea. He is newly diagnosed with HIV (CD4 count 85 cells/μL). Chest X-ray shows bilateral interstitial infiltrates with a ground-glass appearance. Sputum examination is negative for acid-fast bacilli. Serum LDH is elevated at 450 U/L. What is the most likely diagnosis?

## Clinical Diagnosis: PCP in Advanced HIV **Key Point:** Pneumocystis jirovecii pneumonia (PCP) is the most common opportunistic infection in HIV patients with CD4 <100 cells/μL, presenting with subacute dyspnea, nonproductive or minimally productive cough, and bilateral interstitial infiltrates. ### Diagnostic Features Supporting PCP | Feature | PCP | TB | CMV | |---------|-----|----|---------| | CD4 threshold | <100 | <50 (late) | <50 | | Onset | Subacute (weeks) | Chronic (months) | Acute/fulminant | | Cough character | Nonproductive/minimally productive | Productive, often hemoptysic | Minimal cough | | CXR pattern | Bilateral interstitial (ground-glass) | Upper lobe cavitation/infiltrates | Diffuse infiltrates or normal | | Sputum AFB | Negative | Positive | Negative | | LDH elevation | Marked (>400) | Mild | Variable | **High-Yield:** The combination of CD4 <100, bilateral interstitial infiltrates, elevated LDH, and **negative sputum AFB** makes PCP the diagnosis. The ground-glass appearance is pathognomonic for PCP. ### Pathophysiology 1. CD4 count falls below 100 cells/μL → loss of cellular immunity 2. Pneumocystis colonization → trophozoite proliferation in alveoli 3. Foamy exudate accumulates → impaired gas exchange 4. Bilateral interstitial inflammation → ground-glass opacities on imaging **Clinical Pearl:** PCP typically presents with **dyspnea out of proportion to physical findings** — chest auscultation may be clear despite severe hypoxemia. Pulse oximetry desaturation with exertion is characteristic. **Mnemonic: PCP RED FLAGS** — Respiratory distress, Elevated LDH, Dyspnea subacute, Flat/ground-glass infiltrates, Lymphopenia (CD4 <100), Albumin often low, Granulomas absent. ### Confirmatory Investigations - **Induced sputum** or **bronchoalveolar lavage (BAL)** with Giemsa or immunofluorescence staining - **Chest CT** (high-resolution): bilateral ground-glass opacities, often with subpleural sparing - **Arterial blood gas**: hypoxemia, elevated A-a gradient ### Management **First-line:** Trimethoprim-sulfamethoxazole (TMP-SMX) 15–20 mg/kg/day (based on TMP component) for 21 days. Add corticosteroids if PaO₂ <70 mmHg or A-a gradient >35 mmHg. **Prophylaxis:** TMP-SMX single-strength daily once CD4 <100 cells/μL; discontinue when CD4 >200 for >3 months on ART. [cite:Harrison 21e Ch 197]