Acid Fast Bacilli Test
About Acid Fast Bacilli (AFB) Test:
Acid fast bacilli (AFB) is a type of bacteria that causes tuberculosis and other similar infections. Tuberculosis, commonly known as TB, is a lung infection caused due to bacteria named Mycobacterium tuberculosis. TB infection can either be active or latent. An active TB shows symptoms and is contagious whereas, latent TB shows no symptoms. TB can also affect other parts of the body such as brain, spine and kidneys, apart from lungs.
An AFB test is usually advised for people who show active symptoms of TB. It involves checking the sputum for presence of AFB bacteria. Sputum, as opposed to spit or saliva, is a viscous mucus that is coughed up.
Who should get tested?
You might need AFB testing, if you have been
In close contact with someone who has been diagnosed with TB
Affected with HIV or other diseases that weakens your immune system
Living in a place with high rate of TB infection
Coughing for 3 weeks or more
Experiencing unexplained weight loss
Experiencing night sweats
Coughing up blood or sputum
Tests included under AFB test
AFB test includes two types of tests under it. They are:
1. AFB smear:
In this test, the sputum sample is smeared on a glass slide and examined under a microscope. Although it cannot offer a conclusive diagnosis, it can assist in determining the possible or likely infection of Tuberculosis.
2. AFB culture:
In this test, the sputum sample is kept under special environment to promote the bacterial growth. It can help confirm the diagnosis of Tuberculosis or other infections.
Test preparation
No special preparation is required for AFB testing.
Both tests, AFB smear and AFB culture will need a sample of your sputum. These two tests are usually performed simultaneously.
To collect sputum samples:
You are advised to collect your sputum sample in a sterile container for at least 2-3 days in a row to ensure that enough bacteria have been collected. If you are finding difficulty with collecting sputum, your healthcare provider will collect it through a procedure called bronchoscopy. In this procedure, a medication will be given to you before the procedure to ensure that you don't experience any pain. A thin, lit tube will then be inserted through your mouth or nose and into your airways. The sample is then gathered using a tiny brush or by suction.
Interpretation of results
Results for AFB test are generally given out as positive or negative.
AFB smear: A positive test results of AFB test indicates you probably have active TB or other infection. However, you will be needing AFB culture to confirm the diagnosis of infection.
Results from culture can take time. Therefore, your healthcare provider may decide to treat your infection in the meantime based on the severity of symptoms of the infection.
A negative result indicates no infection.
AFB culture: A positive culture indicates you have active TB or any other AFB infection. This helps in identifying the type of infection you are affected with.
A negative culture test indicates no infection.
Once the diagnosis is made through positive AFB culture, you will be advised for susceptibility test of your sputum sample. This susceptibility test helps in determining the most effective antibiotic treatment to your infection.
FAQs
What is the turnaround time for AFB smear test?
The results for AFB smear testing are usually given within 1-2 days after the sample collection. Although, it may take a little longer at times.
What is the turnaround time for AFB culture test?
The turnaround time for the test results of AFB culture is usually 6 to 8 weeks. The sample is kept under special environment to encourage the bacterial growth which takes time. Once there is enough growth of bacteria, the culture is examined and results are given.
How accurate is AFB testing for the diagnosis of tuberculosis (TB) infection?
The Acid-fast bacilli testing is about 96-97.5% accurate in terms of detecting Tuberculosis. You are always advised to go for sputum culture once your AFB smear shows positive results. AFB cultures are used to confirm tuberculosis or any other similar infections diagnosis.
Can I get false-positive reports in AFB testing?
False-positive to AFB testing can be because of several reasons. Other than mycobacterium tuberculosis several other bacteria are also acid-fast and may give false-positive results to AFB testing. However, sometimes, due to errors from laboratory such as scratched slides, old stains and contaminated water can also cause false-positive reports.
What are the limitations to AFB smear test?
Though an AFB smear test is a simple, rapid and inexpensive procedure it has few limitations. These include:
· Under ideal circumstances, the cut off for AFB detection in sputum samples is between 104 and 105 bacilli per ml. If the bacterial count is lower than the above-mentioned count, it cannot detect the infection. This may generate false-negative results to the AFB test.
· Compared to AFB culture, the AFB smear has a lower sensitivity. If an AFB smear reveals tuberculosis, then an AFB culture is done to confirm the diagnosis.
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