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What are acid-fast bacillus (AFB) tests?
Acid-fast bacillus (AFB) is a type of bacteria that causes tuberculosis and other types of mycobacterial infections, such as leprosy (Hansen's disease). AFB tests are usually ordered for people with symptoms of active tuberculosis, commonly known as TB. TB is a serious bacterial infection caused by Mycobacterium tuberculosis. It mainly affects the lungs. But can also affect other parts of the body, including the brain, spine, and kidneys. TB bacteria spread from person to person through coughing or sneezing.
TB can be latent or active. If you have latent TB, you'll have TB bacteria in your body but won't feel sick and can't spread the disease to others because the germs aren't active. If you have active TB, you'll have symptoms of the disease and could spread the infection to others.
AFB tests look for FB bacteria in your sputum. Sputum is a thick mucus that is coughed up from the lungs. It is different from spit or saliva. There are four tests used to identify AFB:
- AFB smear. In this test, your sputum sample is "smeared" on a glass slide and looked at under a microscope. It can provide results in 1-2 days. These results can show a possible or likely infection but cannot be used to provide a diagnosis.
- AFB culture. In this test, your sample is sent to a lab to allow the cells to grow until there are enough to test. An AFB culture can positively confirm a diagnosis of TB or other mycobacterial infection. But it takes 6-8 weeks to grow enough bacteria to detect an infection.
- Molecular test. In this test, your sample is tested using nucleic acid amplification (NAAT) with AFB smear and culture. The NAAT test is done directly on sputum samples and can detect TB bacteria. If there is TB bacteria, the test can also check whether the bacteria are resistant to rifampicin, a common treatment for TB. This test can be done in less than two hours.
- Susceptibility test. This test is usually ordered along with an AFB culture. It is used to figure out the most effective antibiotic to treat your infection. Mycobacterium tuberculosis may be resistant to one or more of the medicines that are used to treat TB.
Other names: AFB smear and culture, TB culture and sensitivity, mycobacteria smear and culture, TB NAAT, Acid-Fast Bacillus Smear and Culture and Sensitivity, Mycobacterium tuberculosis Nucleic Acid Amplification Test
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