AFB Culture

An AFB culture detects acid‑fast bacilli—most often Mycobacterium tuberculosis and non‑tuberculous mycobacteria—in clinical specimens such as sputum, tissue, or body fluids. It identifies viable mycobacteria to confirm infection, enables species identification and drug‑susceptibility testing, and guides treatment. Cultures are more sensitive than smears but require specialized media and several weeks for growth and final results.

Culture media for AFB include egg-based Lowenstein–Jensen, Middlebrook solid agars (7H10, 7H11), and liquid media such as Middlebrook 7H9 with OADC enrichment and automated MGIT systems. These support growth of Mycobacterium species; cultures are incubated at 35–37°C and monitored for several weeks because mycobacteria grow slowly. Selective supplements (e.g., antibiotics) reduce contamination.

Acid-fast bacilli (AFB) positive bacteria include Mycobacterium species (M. tuberculosis complex, M. leprae and non-tuberculous mycobacteria such as M. avium complex, M. kansasii and rapid growers). Other partially acid-fast organisms include Nocardia species and related actinomycetes (Rhodococcus, Tsukamurella, Gordonia). Modified acid-fast stains detect these weakly or variably acid-fast bacteria. Clinically important pathogens are mainly Mycobacterium and Nocardia.

Collect appropriate clinical specimens (sputum, BAL, tissue) aseptically. Digest–decontaminate with N‑acetyl‑L‑cysteine–NaOH, neutralize, centrifuge and resuspend the pellet. Inoculate onto solid media (Lowenstein‑Jensen) and into automated liquid systems (e.g., MGIT). Incubate at 35–37°C with biosafety precautions; monitor for growth up to 6–8 weeks (solid) or ~42 days (liquid). Confirm isolates by microscopy, molecular tests, or biochemical ID.

AFB-positive means acid-fast bacilli were seen on a smear, which suggests a mycobacterial infection such as tuberculosis but is not diagnostic by itself. Further tests (culture, nucleic acid amplification/GeneXpert) and clinical and radiologic correlation are needed to distinguish Mycobacterium tuberculosis from non-tuberculous mycobacteria and to guide treatment. Treatment decisions shouldn’t be based on smear alone.

The cost of an AFB test varies by country, lab and test type. Simple AFB smear microscopy commonly costs about $5–$50 (roughly ₹200–₹1,000), culture and sensitivity testing about $20–$200 (₹500–₹3,000), while molecular/NAAT assays often cost $30–$200 or more. Public health programs frequently offer testing free or at reduced cost; private lab prices fall within these ranges.