Acid-Fast Bacillus (AFB) Tests
Reviewed
AFB tests detect bacteria that cause tuberculosis (TB) and other mycobacterial infections. They are mainly done on sputum and include smear, culture, molecular (NAAT), and susceptibility tests. Results help diagnose active infection and guide treatment, but no single result is diagnostic by itself.
Preparation and collection
There are no special preparations needed for AFB testing.
Understanding this test
Use the reference interval and units printed by the laboratory that performed the test. A result outside that interval is not a diagnosis by itself, and a result inside it does not rule out every condition. Your symptoms, history, medicines, timing, and related tests can change what the result means.
- Negative results do not completely rule out infection, especially if the sample had few bacteria.
- AFB smear cannot confirm the specific type of mycobacterium; culture is required.
- AFB culture takes 6-8 weeks, delaying final diagnosis.
- Molecular tests may not detect all mycobacterial species.
What to do next
If you are diagnosed with TB, you will need to take antibiotics as prescribed, usually for 6-12 months. It is crucial to complete the full course even if you feel better to prevent relapse. A susceptibility test may guide the choice of antibiotics. Talk to your provider about any follow-up testing needed to monitor treatment.
Evidence source
U.S. National Library of Medicine: Acid-Fast Bacillus (AFB) Tests