A 28-year-old woman with HIV (CD4 count 180 cells/μL) on irregular ART presents with progressive dyspnea, fever, and dry cough for 3 weeks. Chest X-ray shows bilateral interstitial infiltrates with a ground-glass appearance. What is the most common cause of pneumonia in this patient?

Explanation

## Most Common Cause of Pneumonia in Advanced HIV/AIDS **Key Point:** Pneumocystis jirovecii pneumonia (PCP) is the most common opportunistic infection of the lungs in patients with CD4 count <200 cells/μL, occurring in 70–80% of untreated patients in this range. ### Clinical Features of PCP - **Presentation:** Subacute (weeks) progressive dyspnea, fever, dry cough - **Chest imaging:** Bilateral interstitial infiltrates with ground-glass opacification (classic) - **Oxygenation:** Hypoxemia disproportionate to radiographic findings (hallmark) - **Lactate dehydrogenase (LDH):** Elevated (>400 IU/L), correlates with severity ### Diagnostic Approach | Test | Sensitivity | Specificity | Notes | |------|-------------|-------------|-------| | Chest X-ray | 85–90% | Moderate | Ground-glass, interstitial pattern typical | | Induced sputum with silver stain | 50–80% | High | Non-invasive; organism appears as "cup-shaped" | | Bronchoalveolar lavage (BAL) with stain | 95–100% | Very high | Gold standard if sputum negative | | PCR (BAL or sputum) | 95–100% | Very high | Most sensitive; not routinely available | | Serum LDH | 80–90% | Low | Elevated but non-specific | ### Pathophysiology - *Pneumocystis jirovecii* is an atypical fungus (formerly classified as protozoan) - Colonizes lungs → cyst rupture → alveolar inflammation → impaired gas exchange - CD4 count <200 cells/μL is the critical threshold ### Management 1. **First-line:** Trimethoprim-sulfamethoxazole (TMP-SMX) 15–20 mg/kg/day (based on TMP) for 21 days 2. **Adjunctive corticosteroids:** Prednisone 40 mg BD × 5 days, then taper (for moderate-to-severe PCP with PaO₂ <70 mmHg or A-a gradient >35 mmHg) 3. **Alternative agents:** Pentamidine, atovaquone, dapsone + pyrimethamine (for sulfa allergy) 4. **Prophylaxis:** TMP-SMX daily or thrice weekly until CD4 >200 cells/μL for ≥3 months on ART **High-Yield:** PCP prophylaxis should be initiated in ALL patients with CD4 <200 cells/μL. TMP-SMX is preferred (also covers *Toxoplasma* and *Streptococcus pneumoniae*). **Mnemonic: PCP Red Flags** — **P**aO₂ low disproportionate to CXR, **C**D4 <200, **P**rogressive dyspnea over weeks. **Clinical Pearl:** The A-a gradient (alveolar-arterial oxygen gradient) is more sensitive than PaO₂ alone for detecting PCP. Calculate as: A-a = (FiO₂ × (760 − 47) − PaCO₂/0.8) − PaO₂. An A-a >35 mmHg on room air suggests PCP.