A 32-year-old man from Delhi presents with a 3-week history of fever, productive cough, and dyspnea. He is known to be HIV-positive with a CD4 count of 85 cells/μL (measured 2 weeks ago). On examination, he is febrile (38.5°C), tachypneic (RR 28/min), and has bilateral fine crackles on auscultation. Chest X-ray shows bilateral interstitial infiltrates with a ground-glass appearance. Arterial blood gas shows PaO₂ 65 mmHg on room air. LDH is elevated at 520 U/L. What is the most likely diagnosis?

Explanation

## Clinical Presentation **Key Point:** The constellation of CD4 <100 cells/μL, subacute presentation (3 weeks), bilateral interstitial infiltrates with ground-glass appearance, elevated LDH, and hypoxemia (PaO₂ 65 mmHg) is pathognomonic for Pneumocystis jirovecii pneumonia (PCP). ## Diagnostic Criteria for PCP | Feature | PCP | TB | CMV pneumonitis | |---------|-----|----|-----------------| | CD4 threshold | <200 (usually <100) | Any CD4 | <50 | | Onset | Subacute (1–4 weeks) | Insidious (weeks–months) | Acute/fulminant | | CXR pattern | Bilateral interstitial, ground-glass | Upper lobe infiltrates, cavitation | Diffuse infiltrates, less typical | | LDH | Markedly elevated (>500) | Normal or mildly elevated | Mildly elevated | | PaO₂ at rest | Often <70 mmHg | Variable | Variable | | Sputum/BAL | Trophozoites/cysts on stain | AFB positive | CMV inclusions | **High-Yield:** PCP is the most common serious opportunistic infection in advanced HIV (CD4 <200). The **ground-glass appearance on CXR** combined with **markedly elevated LDH** is the classic radiological-biochemical signature. ## Pathophysiology 1. CD4 count <200 cells/μL → loss of cellular immunity 2. Pneumocystis jirovecii colonizes lungs (previously thought to be P. carinii in animals) 3. Trophozoites and cysts accumulate in alveolar spaces 4. Inflammatory response → alveolar edema and protein exudation 5. Bilateral interstitial infiltrates and hypoxemia result ## Diagnostic Confirmation **Clinical Pearl:** Diagnosis is confirmed by: - Induced sputum with Giemsa or immunofluorescence staining (70–90% sensitivity) - Bronchoalveolar lavage (BAL) with silver stain or direct fluorescent antibody (>95% sensitivity if sputum negative) - PCR is increasingly used but not routine in resource-limited settings ## Management Approach ```mermaid flowchart TD A[Suspected PCP<br/>CD4 <200, ground-glass CXR]:::outcome --> B{Severity?}:::decision B -->|Mild-moderate<br/>PaO₂ >70| C[TMP-SMX 15-20 mg/kg/day<br/>+ Prednisone]:::action B -->|Severe<br/>PaO₂ <70| D[TMP-SMX IV<br/>+ Prednisone 40 mg BD]:::action C --> E[Response in 5-7 days]:::outcome D --> E E --> F[Continue 21 days total]:::action F --> G[Start ART if CD4 <50]:::action G --> H[Prophylaxis: TMP-SMX<br/>until CD4 >200 × 3 months]:::action ``` **Tip:** Always give **corticosteroids** (prednisone 40 mg BD × 5 days, then taper) in moderate-to-severe PCP (PaO₂ <70 mmHg) to reduce inflammation and mortality. **Mnemonic:** **PILS** — PCP, Isoniazid (TB prophylaxis), Lymphoma, Sepsis — are the top 4 causes of fever + respiratory symptoms in CD4 <100.