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Acid-Fast Bacillus (AFB) Tests

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

What are they used for?

AFB tests are most often used to diagnose an active tuberculosis (TB) infection. They may also be used to help diagnose other types of AFB infections. These include:

  • Leprosy, a once feared, but rare and easily treatable disease that affects the nerves, eyes, and skin. The skin often becomes red and flaky, with a loss of feeling.
  • An infection similar to TB, such as Mycobacterium avium, that mostly affects people with HIV and others with weakened immune systems.

AFB tests may also be used for people who have already been diagnosed with TB. The tests can show if the treatment is working and whether the infection can still be spread to others.

Why do I need an AFB test?

You may need an AFB test if you have symptoms of active TB (TB disease). Which symptoms you have will depend on where in the body the TB bacteria are growing.

If TB disease is in the lungs (pulmonary), symptoms may include:

  • Cough that lasts for three weeks or more
  • Coughing up blood and/or sputum
  • Chest pain

Other symptoms of TB disease may include:

Generally, certain people are more likely to develop TB disease. They include:

  • People who have recently been infected with TB bacteria
  • People with medical conditions that weaken the immune system

Babies and young children often have weakened immune systems. Others can have weakened immune systems, too, especially people with any of these conditions:

People can also have weakened immune systems due to certain treatments:

You may also need testing if you are at higher risk of getting TB because you:

  • Have been in close contact with someone who has been diagnosed with TB
  • Have HIV or another disease that weakens your immune system
  • Live or work in a place with a high rate of TB infection, such as a homeless shelter, nursing home, or prison

If you are currently being treated for TB, you might need testing to see if your treatment is working and if you could still spread the TB bacteria to others.

What happens during 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 have trouble coughing up enough sputum, your provider may ask you to breathe in a sterile saline (salt) mist that can help you cough more deeply.
  • If you still can't cough up enough sputum, your provider may perform a procedure called a bronchoscopy. In this procedure, you will first get medicine so you won't feel any pain. Then, a thin, lighted tube will be put through your mouth or nose and into your airways. The sample may be collected by suction or with a small brush.

Will I need to do anything to prepare for the test?

There are no special preparations needed for AFB testing.

Are there any risks to the test?

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. There is also a small risk of infection and bleeding at the site where the sample is taken.

What do the results mean?

If your results on an AFB smear or culture were negative, you probably don't have active TB. But it could also mean there weren't enough bacteria in the sample for your provider to make a diagnosis.

If your AFB smear was positive, it means you probably have TB or other infection, but an AFB culture is needed confirm the diagnosis. Culture results can take several weeks, so your provider may decide to treat your infection in the meantime.

If your AFB culture was positive, it means you have active TB or another type of AFB infection. The culture can identify which type of infection you have.

If your molecular (NAAT) test was positive, it means you probably have TB.

Once you have been diagnosed, your provider may order a "susceptibility test" on your sample. A susceptibility test is used to help determine which antibiotic will provide the most effective treatment.

If you have the signs and symptoms of an active TB infection, your provider will consider both your AFB smear and NAAT molecule test results when deciding whether to treat you for TB while you wait for the culture results. If you have questions about your results, talk to your provider.

Is there anything else I need to know about AFB testing?

If not treated, TB can be deadly. But most cases of TB can be cured if you take antibiotics as directed by your provider. Treating TB takes much longer than treating other types of bacterial infections. After a few weeks on antibiotics, you will no longer be contagious, but you will still have TB. To cure active TB, you usually need to take antibiotics for six to twelve months. How long you need to take it will depend on your overall health, age, and other factors. 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.


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The information on this site should not be used as a substitute for professional medical care or advice. Contact a health care provider if you have questions about your health.