Acid-Fast Bacillus (AFB) Tests Test
An AFB test helps diagnose active tuberculosis (TB) and other mycobacterial infections like leprosy. It looks for acid-fast bacillus bacteria in your sputum (mucus coughed up from the lungs).
What this test measures
AFB tests detect acid-fast bacillus bacteria in sputum. There are four types: AFB smear (quick but not definitive), AFB culture (confirms diagnosis but takes weeks), molecular test (NAAT, detects TB and drug resistance in hours), and susceptibility test (finds the best antibiotic).
What your results mean
Reference ranges vary by laboratory and by your age and sex. MedlinePlus does not publish a single normal range for this test — always read your result against the range printed on your own lab report and discuss it with your provider.
Higher-than-normal results
A positive AFB smear means you probably have TB or another mycobacterial infection. A positive AFB culture confirms active TB or other AFB infection. A positive molecular test means you probably have TB or another AFB infection.
Lower-than-normal results
A negative AFB smear or culture means you probably don't have active TB, but it could also mean there weren't enough bacteria in the sample to detect.
Frequently asked questions
What is an AFB test?
An AFB test looks for acid-fast bacillus bacteria in your sputum to diagnose tuberculosis (TB) and other mycobacterial infections.
How long does it take to get AFB test results?
AFB smear results are available in 1-2 days. Molecular (NAAT) results can be done in less than two hours. AFB culture takes 6-8 weeks.
What does a positive AFB smear mean?
A positive AFB smear means you probably have TB or another mycobacterial infection, but a culture is needed to confirm the diagnosis.
What does a negative AFB culture mean?
A negative AFB culture means you probably don't have active TB, but it could also mean there weren't enough bacteria in the sample.
Can AFB tests detect drug resistance?
Yes, the molecular (NAAT) test can check if TB bacteria are resistant to rifampicin, a common TB treatment. A susceptibility test can also find the most effective antibiotic.
Related lab tests
Sources
- MedlinePlus: Acid-Fast Bacillus (AFB) Tests — NIH MedlinePlus
Last updated . Information is aggregated from official public sources and is not a substitute for professional medical care.