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A Guide to the AFB Laboratory Test

written by: Lashan Clarke•edited by: Emma Lloyd•updated: 6/18/2010

AFB stands for acid fast bacilli. This term is used to describe rod-shaped bacteria, with the most well known bacilli being Mycobacterium tuberculosis. This bacterium is responsible for causing the infectious disease tuberculosis. The AFB sputum culture test detects bacilli in the lungs.

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    Test Overview

    The AFB sputum test is ordered because it is used to diagnose if a person has become infected with tuberculosis. If an infection has occurred, it will be necessary to quickly treat the person, as tuberculosis can be very resistant to treatment. It is also necessary to quickly start treatment of anyone infected as tuberculosis is highly contagious and spreads in the same manner as the common cold. Therefore, the test is mainly to detect any of sixty plus species of the Mycobacteria family that can cause lung disease.
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    What is Tuberculosis?

    AFB stands for “Acid Fast Bacilli" and this term is used to describe rod shaped bacteria that can readily be stained with dye using a glass slide. The most well known bacilli are Mycobacterium tuberculosis, which is responsible for causing the highly infectious disease tuberculosis. The bacteria enter the body through the nose and mouth, and go on to infect the lungs. When the bacteria enter the lungs, they use the cells of the lungs to produce more bacteria. Tuberculosis is a type of granulomatous infection in which a hard tumor like structure is formed inside the tissue. This granuloma reduces the bacteria’s ability to be destroyed by white blood cells, or killed by antibiotics.

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    How Is The AFB Sputum Test Performed?

    The common area of infection for this bacteria is the cells of the lungs, so the sputum that is produced inside the lungs is tested for the presence of tuberculosis bacilli. Sputum is a clear mucus-like substance that is constantly being expelled from the lungs. When it arrives up into the mouth, it mixes with the saliva produced there and this is known as “spit."

    The problem with carrying out the AFB sputum test is once the sputum has combined with saliva, it becomes contaminated with oral bacteria. The true test for someone that produces the most accurate AFB sputum culture test results should be contaminated with very little saliva. To test for the presence of bacilli, the person will expel three collections of sputum into a container over a three-day period. Afterwards, the sputum is placed on a glass slide and stained to see if any tuberculosis bacilli are apparent.

    If someone is not able to cough up sputum, then the physician has the option of collecting a sample of sputum using a bronchoscope inserted into the person’s throat under mild anesthesia. For young children without any sputum production, then the contents of the stomach can be used instead to test for the presence of bacteria.

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    What Does The Test Results Mean?

    If someone receives a positive AFB sputum culture test results this is indicative of a potential infection with a Mycobacterium. However the physician will need to review tissue samples, and carry out a physical examination to record signs and symptoms for a definite diagnosis. With a definite diagnosis, treatment can quickly begin with the necessary infection control measures.

    If the test is negative, then one of two situations is plausible. The person is either not infected with the bacteria, or the bacteria is located another part of the body other than the lungs.

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    Web Source: American Association for Clinical Chemistry. "AFB Culture: The Test". 2010. Available at: