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Acid-Fast Bacillus (AFB) Tests

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 certain other infections. Tuberculosis, commonly known as TB, is a serious bacterial infection that mainly affects the lungs. It can also affect other parts of the body, including the brain, spine, and kidneys. TB is 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. If you have active TB, you’ll have symptoms of the disease and could spread the infection to others.

AFB tests are usually ordered for people with symptoms of active TB. The tests look for the presence of AFB 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 two main types of AFB tests:

  • AFB smear. In this test, your 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 can’t provide a definite diagnosis.
  • AFB culture. In this test, your sample is taken to a lab and put in a special environment to encourage the growth of bacteria. An AFB culture can positively confirm a diagnosis of TB or other infection. But it takes 6–8 weeks to grow enough bacteria to detect an infection.

Other names: AFB smear and culture, TB culture and sensitivity, mycobacteria smear and culture

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What are Acid-Fast Bacillus (AFB) Tests used for?

AFB tests are most often used to diagnose 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 loss of feeling.
  • An infection is similar to TB that mostly affects people with HIV/AIDS 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 Acid-Fast Bacillus (AFB) Tests?

You may need an AFB test if you have symptoms of active TB. These include:

  • Cough that lasts for three weeks or more
  • Coughing up blood and/or sputum
  • Chest pain
  • Fever
  • Fatigue
  • Night sweats
  • Unexplained weight loss

Active TB can cause symptoms in other parts of the body besides the lungs. Symptoms vary depending on which part of the body is affected. So you may need testing if you have:

You may also need testing if you have certain risk factors. You may be at higher risk of getting TB if 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. These include homeless shelters, nursing homes, and prisons.
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What happens during Acid-Fast Bacillus (AFB) testing?

Your health care provider will need a sample of your sputum for both an AFB smear and an AFB culture. The two tests are usually done at the same time. To get sputum samples:

  • You will be asked to cough deeply and spit into a sterile container. You will need to do this for two or three days in a row. This helps make sure your sample has enough bacteria for testing.
  • 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?

You don’t any special preparations for an AFB smear or culture.

Are there any risks to the test?

There is no risk of 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 health care provider to make a diagnosis.

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If your AFB smear was positive, it means you probably have TB or other infection, but an AFB culture is needed to 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. 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 questions about your results, talk to your health care 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 health care 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 TB, you need to take antibiotics for six to nine months. The length of time depends 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.


Centers for Disease Control and Prevention [Internet]. Atlanta: U.S. Department of Health and Human Services; Basic TB Facts

Lab Tests Online [Internet]. Washington D.C.: American Association for Clinical Chemistry

Mayo Clinic [Internet]. Mayo Foundation for Medical Education and Research

UF Health: University of Florida Health [Internet]. Gainesville (FL): University of Florida

University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center

UW Health [Internet]. Madison (WI): University of Wisconsin Hospitals and Clinics Authority