A 45-year-old HIV-positive patient (CD4 count 40 cells/µL) presents with fever, non-productive cough, and progressive dyspnoea over 3 weeks. Chest X-ray shows bilateral perihilar haze. LDH is 650 U/L. What is the MOST specific investigation to confirm the likely diagnosis?

• A Serum beta-D-glucan
• B Serum LDH >500 U/L
• C CD4 count <200 cells/µL
• D BAL with methenamine silver stain showing cysts ✓

Explanation

Pneumocystis jirovecii pneumonia (PCP) in HIV is most specifically confirmed by BAL with Gomori methenamine silver (GMS) or immunofluorescence staining demonstrating cysts/trophic forms. Serum beta-D-glucan is sensitive (~95%) but not specific (positive in other fungal infections). Elevated LDH and low CD4 count are suggestive but non-specific. PCR on BAL is highly sensitive and increasingly used. The clinical picture of subacute dyspnoea, bilateral infiltrates, and very low CD4 is classic for PCP, but microscopic identification remains the diagnostic gold standard.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.