Acid-Fast Bacillus (AFB) Smear Testing:  What You Need to Know

AFB smear testing - Healthians
Contributed by – Rachana Arya

What is the AFB test?

AFB is a type of bacteria that causes tuberculosis and certain other infections. Tuberculosis (TB) is a serious bacterial infection caused by Mycobacterium tuberculosis (MTb) bacteria and it mainly affects the lungs. TB is transmitted from person to person through the air when an infected person sneezes, coughs or speaks.

AFB is a well-recognized and highly recommended diagnostic approach employed as a primary method to test if the patient is suffering from TB or some bacterial infection, such as leprosy and a TB-like disease that affects people with HIV/AIDS. The test looks for the presence of AFB bacteria in your sputum by microscopic examination of a fluorochrome stain of the clinical specimen.


Who should get the test?

Specialists recommend this screening to estimate your likelihood of mycobacterial infection. Likewise, persons suspected of having an active TB infection are advised to have their sputum specimens collected for an AFB smear and culture. TB disease should be suspected in persons who have the following symptoms:

  • Unexplained weight loss
  • Loss of appetite
  • Night sweats
  • Fever
  • Chills and weakness
  • Fatigue


Moreover, if TB disease is in the lungs, symptoms may include:

  • Chronic coughing that produces phlegm or sputum
  • Hemoptysis (coughing up blood)
  • Pain in chest


Why is the AFB test needed?

The AFB test may be needed:

  • to check if you have an active TB infection
  • when you have a positive pulmonary TB screen test
  • you are in a high-risk group for progressing to active disease and/or have characteristic lung involvement as shown by X-ray
  • when you have been in close contact with a person who has been diagnosed with TB
  • when you have any infection that may be due to mycobacteria lung infection
  • to evaluate the effectiveness of TB treatment


What does the test result mean?

Acid-fast bacilli (AFB) smear-positive sputum is usually the first indication of mycobacterial infection and potential TB disease.

A negative AFB smear means that no infection is present, that symptoms are caused by something other than mycobacteria, or that the mycobacteria were not present in sufficient numbers to be seen under the microscope.

Usually, at least three samples are collected to increase the probability that the organisms will be detected. Nevertheless, if AFB smears are negative and there is still a strong suspicion of mycobacterial infection, then additional samples may be collected and tested on different days.


How is the test done?

Acid-fast microscopy is easy and quick. 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 because some acid-fast-bacilli are not M. tuberculosis. Therefore, a culture must be performed on all initial samples to confirm the diagnosis.


Take the AFB Smear test now 

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