Acid-fast bacteria (e.g., Mycobacterium tuberculosis and non‑tuberculous mycobacteria) are treated with combinations such as isoniazid, rifampicin (rifampin), ethambutol and pyrazinamide; streptomycin is used in some regimens. Non‑tuberculous infections often use macrolides (clarithromycin, azithromycin), ethambutol, rifabutin, amikacin or fluoroquinolones. Partially acid‑fast Nocardia typically responds to trimethoprim‑sulfamethoxazole, sometimes with amikacin or imipenem. Treatment depends on species and susceptibility; regimens are prolonged and specialist‑guided.

Any visible acid‑fast bacillus (AFB) on a sputum smear is considered smear‑positive for TB. For quantitation: scanty = 1–9 AFB per 100 fields; 1+ = 10–99 AFB/100 fields; 2+ = 1–10 AFB per field in at least 50 fields; 3+ = >10 AFB per field in at least 20 fields. Culture or molecular tests confirm infection and guide treatment.

Acid-fast bacilli (AFB) smear microscopy has relatively low sensitivity—typically about 20–60% compared with culture. Overall average sensitivity is often cited near 50%, improving with multiple sputum samples and higher bacillary load, and reduced in paucibacillary settings (e.g., HIV, early or extrapulmonary TB). Culture or molecular tests (e.g., NAATs) are more sensitive for diagnosis.

An AFB test detects acid-fast bacilli—bacteria with waxy cell walls, notably Mycobacterium tuberculosis and non-tuberculous mycobacteria—in sputum, tissue, or body fluids. It helps diagnose tuberculosis, guide isolation and treatment decisions, monitor response to therapy, and assess infectiousness. Smear microscopy, culture and molecular methods are used; culture or molecular tests confirm species and drug susceptibility.

Classic acid-fast bacteria include Mycobacterium tuberculosis, Mycobacterium leprae, Mycobacterium avium–intracellulare (MAC) and Nocardia species. Mycobacteria are strongly acid-fast due to mycolic acids in their cell walls; Nocardia is variably or weakly acid-fast with modified staining. These organisms are identified using acid-fast stains such as Ziehl–Neelsen or modified Kinyoun techniques and are clinically important.

Mycobacterium tuberculosis infection (tuberculosis) is the most common acid‑fast bacillus (AFB) infection worldwide. It most often causes pulmonary TB, presenting with persistent cough, weight loss, fever, night sweats and sometimes hemoptysis. TB spreads via airborne droplets, requires prolonged combination antibiotic therapy under medical supervision, and is diagnosed by sputum AFB smear, culture and molecular tests.