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Acid-Fast Bacteria Culture

Does this test have other names?

Acid-fast bacillus smear and culture, AFB smear and culture, TB culture and sensitivity, mycobacterial culture

What is this test?

An acid-fast bacteria (AFB) culture is done to find out if you have tuberculosis (TB) or another mycobacterial infection. Besides tuberculosis, the other main mycobacterial infections are leprosy and a TB-like disease that affects people with HIV/AIDS.

To do an AFB culture, health care providers take a sample of phlegm or sputum you're coughing up or a tiny bit of your tissue. They "culture" it by putting it in a special container with food the bacteria needs to grow. They then check it over a few weeks' time to see whether the bacteria grow. If they do, you have a mycobacterial infection. 

Why do I need this test?

These are reasons you might need this test:

  • You have symptoms of a lung infection, such as chronic cough, coughing up blood, weight loss, fever, chills, and tiredness.

  • You have a positive TB skin or blood test and you are at high risk for exposure to TB or its progression to active disease. This includes those with HIV/AIDS or another condition that weakens your immune system. People who have been in hospitals, nursing homes, or correctional facilities are also at high risk.  

  • You have symptoms of TB outside the lungs, a condition called extrapulmonary TB. These symptoms vary based on the site of infection. Most people think of TB as only a lung disease. But it can show up in other parts of the body. If it infects the spinal cord, for example, it can cause back pain and paralysis; in the kidneys it often causes blood in the urine.

  • You have a condition like HIV/AIDS that puts you at increased risk of getting TB. If you have also been in close contact with someone who has TB, doctors will want to test you for the disease.

Your health care providers may also give you this test from time to time if you are being treated for TB. This is to see if the treatment is working and to find out whether you are still infectious. 

What other tests might I have along with this test?

You will likely have a chest X-ray if doctors suspect you have pulmonary TB, or TB in the lungs. The X-ray can help your health care provider decide whether it is likely you have TB. The X-ray doesn't give a definitive diagnosis though.

The results of a culture usually take two to six weeks. You may be treated for TB during this time if your doctor suspects pulmonary TB. More rapid but less definitive tests are usually first done on phlegm coughed up from deep inside your lungs. These help your health care provider decide whether to isolate and treat you before the culture results are available. The most common of these tests are an AFB smear and a nucleic acid amplification (NAA) test, both of which also require a sample of your phlegm.

The AFB smear will show whether acid-fast bacteria are present. But it doesn't identify the species of bacteria or tell you which treatment it will respond to. The NAA is a relatively new test that detects the genetic material of the bacteria. The CDC now recommends that the NAA be done on at least one sputum sample when doctors suspect TB and when the test result may affect treatment decisions.

If the AFB culture is positive, the lab will do what is known as a susceptibility test. This determines which antibiotic will work best to treat the disease. The germ M. tuberculosis is often resistant to one or more of the drugs commonly used to treat it.  For this reason, susceptibility testing improves the chances of finding the right antibiotic in the first round. 

What do my test results mean?

Many things may affect your lab test results. These include the method each lab uses to do the test. Even if your test results are different from the normal value, you may not have a problem. To learn what the results mean for you, talk with your health care provider.

A negative result means you likely don't have active TB or another mycobacterial infection.

A positive AFB culture for M. tuberculosis means you do have TB. A separate susceptibility report will tell you which antibiotics should best treat it.

If you are being treated for TB, you will usually have the AFB culture done monthly until it comes back negative two months in a row. This means the treatment is working. A positive AFB culture several weeks after drug treatment has started may mean the antibiotic is not working and needs to be changed. It also means you are still likely to be infectious and can pass the infection to others through coughing or sneezing. 

How is this test done?

If you are being tested for pulmonary TB, the AFB culture is done on sputum, or phlegm coughed up from deep inside your lungs. You will provide sputum in a sterile cup at least two days in a row, preferably early in the morning. A health care provider may help you get a good sputum sample. You will need to cough deeply several times to make sure you aren't just coughing up saliva or postnasal drip. 

If you are unable to cough up enough sputum on your own, you may need to inhale a special warm, sterile saline solution to trigger a cough that will produce enough sputum. If that doesn't work, you may need gastric aspiration. This is a procedure in which a tube is put through your nose or mouth into your stomach to collect sputum that has been coughed up and swallowed.

As a last resort, health care providers may do a bronchoscopy. This involves putting a bronchoscope through your mouth or nose right into your lung to remove sputum or tissue. It requires the use of anesthesia in a hospital setting.

If doctors suspect you have TB outside your lungs or another mycobacterial infection, tests may be done on urine, cerebrospinal fluid, or tissue samples. Tests may also be done on other body fluids. 

Does this test pose any risks?

A phlegm culture doesn't pose any risks. A blood sample may be needed. Taking a blood sample with a needle carries risks that include bleeding, infection, bruising, or feeling dizzy. When the needle pricks your arm, you may feel a slight stinging sensation or pain. Afterward, the site may be slightly sore.

What might affect my test results?

AFB cultures are quite accurate. Rarely, an AFB smear will be positive when the culture is negative. In these cases, the smear results are usually wrong because the sputum was contaminated. An AFB smear could also be falsely positive if you are taking antibiotics for a mycobacterial infection.

How do I get ready for this test?

You should prepare to take the test early in the morning, preferably before breakfast. Rinse your mouth out with water before coughing up the phlegm.