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By Dr Madhuri Alhat
Consultant Physician & Diabetologist
Intensivist & Cardiologist
What is acid-fast bacilli (AFB) test? What are the uses of the acid-fast bacilli test? What happens during Acid Fast Bacilli Test? Understanding results of acid-fast bacilli test Frequently Asked Questions

Acid Fast Bacilli Test

What is acid-fast bacilli (AFB) test?

The acid-fast bacilli (AFB) test is a laboratory procedure used to detect the presence of acid-fast bacteria in a sample. Acid-fast bacteria are a group of bacteria that have a unique cell wall structure, which makes them resistant to certain staining techniques used in microbiology.

The AFB test is commonly used to diagnose tuberculosis (TB), as the bacterium that causes TB, Mycobacterium tuberculosis, is an acid-fast bacterium. This test can also diagnose other types of mycobacterial infections, such as leprosy and some forms of atypical mycobacterial infections.

During the AFB test, a sample (such as sputum or a tissue sample) is collected and treated with a special staining technique involving heat and acid. Acid-fast bacteria will retain the stain despite the treatment, while other bacteria will not. The stained sample is then examined under a microscope to look for the presence of acid-fast bacilli.

If acid-fast bacilli are present in the sample, further tests may be done to identify the specific type of bacteria and determine the appropriate treatment. The AFB test is an important tool in the diagnosis and management of mycobacterial infections.

What are the uses of the acid-fast bacilli test?

The acid-fast bacilli (AFB) test is a laboratory test used to identify the presence of acid-fast bacteria, which include Mycobacterium tuberculosis and other closely related species. The AFB test is primarily done for the diagnosis of tuberculosis (TB) and other mycobacterial infections.

Here are some of the uses of the acid-fast bacilli test:

Diagnosis of tuberculosis:

The AFB test is commonly used to diagnose active tuberculosis, particularly in patients who have symptoms of TB such as cough,fever, and weight loss.

Screening for latent tuberculosis:

This test can also screen for latent tuberculosis in people exposed to TB but who do not have active disease.

Diagnosis of other mycobacterial infections:

In addition to TB, the AFB test can also be used to diagnose other mycobacterial infections such as leprosy, Mycobacterium avium complex (MAC) infection, and other atypical mycobacterial infections.

Monitoring response to treatment:

This test can be used to monitor the effectiveness of TB treatment by tracking the number of acid-fast bacilli in a patient's sputum.

Infection control:

This test is an important tool for infection control in healthcare settings where TB is a risk. It can help identify individuals who may be contagious and require isolation.

What happens during Acid Fast Bacilli Test?

During the AFB test, a sample of sputum, urine, or other bodily fluids is collected from the patient and smeared onto a glass slide. The slide is then stained with a special dye, which makes the bacteria appear red or pink under the microscope.

The slide is examined under a microscope to look for the presence of acid-fast bacilli. Acid-fast bacteria, including Mycobacterium tuberculosis, have a unique cell wall that is resistant to standard staining techniques, which makes them stand out from other bacteria.

Understanding results of acid-fast bacilli test

The test results can provide important information for the diagnosis and management of tuberculosis (TB) and other mycobacterial infections.

Interpreting the results of the AFB test involves analyzing the microscopy and culture results. Here is a brief explanation of how to understand the results:

Microscopy

The AFB test typically involves staining a sample of sputum, tissue, or other body fluids with a special dye and then examining it under a microscope. Acid-fast bacilli appear as red or pink rods against a blue or purple background. The results are reported as the number of acid-fast bacilli per field of view.

  • Negative result: No acid-fast bacilli are seen under the microscope.
  • Scanty result: Only one to nine acid-fast bacilli are seen per field of view.

Culture

In addition to microscopy, the AFB test also involves culturing the sample on a special media that supports the growth of acid-fast bacilli. The results are reported as positive or negative, and the type of mycobacterial species can also be identified.

  • Positive result: Acid-fast bacilli are detected in the culture.
  • Negative result: No acid-fast bacilli are detected in the culture.

It is important to remember that the AFB test results must be interpreted in the context of other clinical and laboratory findings, such as the patient's symptoms, medical history, and radiographic imaging.

A positive AFB test does not necessarily mean that a patient has active TB, as other mycobacterial infections and even non-infectious conditions can also yield positive AFB test results. Therefore, the interpretation of the AFB test results should always be made by a healthcare professional with expertise in TB and mycobacterial infections.

Frequently Asked Questions

1. What is the AFB test used for?

The AFB test is primarily used for the diagnosis of tuberculosis (TB) and other mycobacterial infections. It can also be used to monitor response to treatment and infection control in healthcare settings.

2. How is the AFB test performed?

The AFB test typically involves collecting a sample of sputum, tissue, or other body fluids, staining the sample with a special dye called Ziehl-Neelsen stain, and examining it under a microscope. The sample is also cultured on a special media that supports the growth of acid-fast bacilli.

3. How long does it take to get the results of the AFB test?

The results of the AFB test can take several days to several weeks, depending on the laboratory and the type of test performed.

4. Can the AFB test be used to diagnose all types of TB?

The AFB test is most useful for diagnosing pulmonary TB, which affects the lungs. It may be less sensitive for diagnosing extrapulmonary TB, which affects other parts of the body.

5. Are there any risks associated with the AFB test?

There are generally no risks associated with the AFB test. However, collecting sputum samples may cause discomfort or trigger coughing in some patients.

6. How accurate is the AFB test?

The accuracy of the AFB test depends on the quality of the sample and the expertise of the laboratory performing the test. False-negative and false-positive results can occur in some cases.

7. Is the AFB test the only test used to diagnose TB?

The AFB test is not the only test used to diagnose TB. Other tests, such as the tuberculosis skin test, interferon-gamma release assays, chest X-rays, and CT scans, may also be used in conjunction with the AFB test to make a diagnosis.

8. What is the cost of the AFB test?

An AFB test costs around Rs. 300 to Rs. 500, which can vary from place to place.

9. Where can I get an AFB test?

You can get an AFB test at Medicover Hospitals.