Acid-Fast Bacillus (AFB) Tests

Acid-Fast Bacillus (AFB) tests are a type of laboratory test used to diagnose a variety of respiratory tract and other infections, such as tuberculosis (TB). These tests are intended to detect the presence of mycobacterium, a type of bacteria present in TB infections, as well as other bacterial, fungal, and parasitic infections. AFB tests may also be used to monitor the effectiveness of tuberculosis treatment.

Preparation

No specific preparation is needed for this type of test, though the facility performing the test may have specific instructions to follow.

Procedure

AFB tests typically involve a medical professional taking a sample from bodily fluid or tissue of the person being tested. This sample is then placed on a slide and sent to a laboratory. The sample is placed in a stain which makes the bacteria more visible to the microscope. The laboratory technician then examines the sample under a microscope. An Acid-fast bacillus test is usually performed in a series of tests in order to better detect bacterial presence. These include:

Gram Stain – An examination of the bacterial cells for size, shape, and arrangement.
Acid-fast Stain – The sample is examined under a microscope for the presence of mycobacterium.
Culture – This involves growing the sample in order to identify bacteria, fungi, and/or parasites.
Molecular testing - This uses either the polymerase chain reaction (PCR) or nucleic acid amplification tests (NAATs) to detect the presence of various TB proteins.

Risks

Acid-Fast Bacillus tests don't usually pose any specific risks, though the test sample may involve contact with a small amount of the patient’s bodily fluids.

Types

The most common types of AFB tests are:

Ziehl-Neelsen impression: This is the most commonly used AFB test. It involves staining the sample with a special dye that binds to acid-fast bacteria.
Fluorescent-labeled antibody: This involves exposing the sample to antibodies that are tagged with fluorescent dye in order to identify acid-fast bacteria.

Why

AFB tests are used to detect the presence of mycobacterium, a type of bacteria present in TB infections, as well as other bacterial, fungal, and parasitic infections. AFB tests are also often used to monitor the effectiveness of TB treatment.

When

AFB tests may be ordered whenever there is suspicion of an infection caused by Mycobacterium, such as TB. Additionally, they may be ordered periodically to monitor the effectiveness of treatment for TB.