Acid-Fast Bacillus (AFB) Tests
Reviewed
Also called: AFB Smear and Culture, TB Culture and Sensitivity, Mycobacteria Smear and Culture, TB NAAT, Acid-Fast Bacillus Smear and Culture and Sensitivity
Acid-fast bacillus (AFB) is a type of bacteria that causes tuberculosis and other types of mycobacterial infections, such as leprosy (Hansen's disease).
Specimen: Sputum (or skin/nerve biopsy if leprosy is suspected)
Why this test may be ordered
- Symptoms of active TB: cough lasting >3 weeks, coughing up blood or sputum, chest pain, chills, fever, night sweats, weight loss, fatigue.
- Higher risk: close contact with TB patient, HIV or weakened immune system, living/working in high-TB settings, substance use, silicosis, diabetes, severe kidney disease, organ transplant, head/neck cancer.
Preparation and collection
There are no special preparations needed for AFB testing.
Your health care provider will need a sample of your sputum for all AFB tests. The AFB smear and AFB culture are usually done at the same time. To get sputum samples: You will be asked to breathe deeply and then cough and spit into a sterile container. If you still can't cough up enough sputum, your provider may perform a procedure called a bronchoscopy.
There is no risk to providing a sputum sample by coughing into a container. If you have a bronchoscopy, your throat may feel sore after the procedure. For a skin/nerve biopsy, there is also a small risk of infection and bleeding at the site where the sample is taken.
Understanding results
If you have the signs and symptoms of an active AFB infection, your provider will consider both your AFB smear and NAAT molecule test results when deciding whether to treat you for TB or any other mycobacterial infection while you wait for the culture results.
- Negative (AFB smear or culture): If your results on an AFB smear or culture were negative, you probably don't have active TB. Limitation: But it could also mean there weren't enough bacteria in the sample for your provider to make a diagnosis. Next step: If you have questions about your results, talk to your provider.
- Positive (AFB smear): If your AFB smear was positive, it means you probably have TB or another mycobacterial infection, but an AFB culture is needed to confirm the diagnosis. Limitation: Culture results can take several weeks, so your provider may decide to treat your infection in the meantime. Next step: It's important to take the antibiotics for as long as your provider tells you, even if you feel better.
- Positive (AFB culture): If your AFB culture was positive, it means you have active TB or another type of AFB infection. Limitation: The culture can identify which type of infection you have. Next step: A susceptibility test is used to help determine which antibiotic will provide the most effective treatment.
- Positive (Molecular / NAAT test): If your molecular (NAAT) test was positive, it means you probably have TB or another kind of AFB infection. Limitation: Once you have been diagnosed, your provider may order a "susceptibility test" on your sample. Next step: It's important to take the antibiotics for as long as your provider tells you, even if you feel better.
- 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 not treated, TB can be deadly. But most cases of TB can be cured if you take antibiotics as directed by your provider. It's important to take the antibiotics for as long as your provider tells you, even if you feel better. Stopping early can cause the infection to come back.
- What are the symptoms that would require an AFB test?
- How is the sputum sample collected if I have trouble coughing?
- What does a positive AFB smear mean while waiting for culture results?
- How long does it take to get results from each AFB test?
- Is it possible to have a false negative or false positive result?
Evidence source
Source: U.S. National Library of Medicine (MedlinePlus). Practices and reference ranges may differ in your country.
U.S. National Library of Medicine: Acid-Fast Bacillus (AFB) Tests