A 45-year-old HIV-positive man on ART with undetectable viral load and CD4 count 180 cells/µL develops new fever, nonproductive cough, and weight loss over 6 weeks. CXR shows miliary infiltrates. Serum LDH is elevated. Sputum AFB smear is negative × 3. The best diagnostic test to confirm disseminated Mycobacterium avium complex (MAC) is:

• A Bone marrow biopsy for AFB culture
• B Urine lipoarabinomannan (LAM) antigen
• C Blood mycobacterial culture (lysis-centrifugation/BACTEC) ✓
• D Serum interferon-gamma release assay (IGRA)

Explanation

Disseminated MAC (dMAC) is diagnosed by positive blood mycobacterial cultures (using lysis-centrifugation or BACTEC radiometric blood culture), which are positive in >90% of cases. This is the gold standard for dMAC diagnosis, as MAC seeds the bloodstream early in dissemination. Bone marrow biopsy can also yield the organism but is more invasive and reserved for cases where blood cultures are negative. Urine LAM detects M. tuberculosis in severely immunosuppressed patients. IGRA detects TB sensitization but not MAC.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.