A 32-year-old man from Delhi presents with a 3-week history of fever, progressive dyspnea, and dry cough. He reports unprotected sexual contact 18 months ago. On examination, he is tachypneic (RR 28/min) and SpO₂ is 88% on room air. Chest X-ray shows bilateral interstitial infiltrates with a ground-glass appearance. CD4 count is 85 cells/μL. Sputum smear for acid-fast bacilli is negative. What is the most likely diagnosis?

Explanation

## Clinical Presentation and Diagnosis **Key Point:** PCP is the most common serious opportunistic infection in HIV patients with CD4 <100 cells/μL, presenting with subacute dyspnea, dry cough, fever, and bilateral interstitial infiltrates on imaging. ### Why PCP? The clinical triad of: 1. **CD4 count 85 cells/μL** — profound immunosuppression 2. **Bilateral interstitial (ground-glass) infiltrates** — classic radiologic pattern 3. **Negative sputum AFB** — rules out TB as primary diagnosis 4. **Subacute presentation** (3 weeks) — typical for PCP ### Diagnostic Features of PCP | Feature | PCP | TB | CMV | Cryptococcal | |---------|-----|----|----|---------------| | **CD4 threshold** | <100 | <50 (but can occur at any CD4) | <50 | <100 | | **Imaging** | Bilateral interstitial/ground-glass | Cavitary (upper lobe) or nodular | Focal consolidation | Nodular or normal | | **Sputum AFB** | Negative | Positive | Negative | Negative | | **Onset** | Subacute (weeks) | Insidious (months) | Acute | Insidious | | **Respiratory rate** | Often elevated | Variable | Variable | Variable | **High-Yield:** Ground-glass infiltrates + CD4 <100 + negative AFB = **PCP until proven otherwise**. ### Diagnostic Confirmation - **Induced sputum** or **bronchoalveolar lavage (BAL)** with Giemsa or immunofluorescence staining - **LDH elevation** (>400 IU/L) supports diagnosis - **Arterial blood gas** typically shows hypoxemia with widened A-a gradient **Clinical Pearl:** PCP prophylaxis (TMP-SMX) is indicated when CD4 <200 cells/μL; this patient likely did not receive it, explaining the presentation. ### Management 1. High-dose TMP-SMX (15–20 mg/kg/day IV/PO in divided doses) for 21 days 2. Adjunctive corticosteroids if PaO₂ <70 mmHg or A-a gradient >35 mmHg 3. Concurrent antiretroviral therapy (ART) initiation 4. PCP prophylaxis continuation until CD4 >200 cells/μL for ≥3 months on ART [cite:Harrison 21e Ch 197]