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AFB Smear Test & Culture

This post is also available in: हिन्दी (Hindi)

Why the AFB Smear test is done?

• This test is done to diagnose tuberculosis (TB) by detecting the mycobacterium tuberculosis bacteria.

• To diagnose infections caused by other Mycobacterium species, collectively called as acid-fast bacilli (AFB).

• To monitor the efficacy of the treatment.

When AFB Smear Test is done?

• When a person has signs and symptoms of lung infection, such as chronic cough, fever, weight loss, chills and weakness, which might be due to tuberculosis or non-tuberculous mycobacterial (NTM) infection.

• This is done when a person has positive IGRA blood test (TB gold test) or Mantoux test and has high risk of progressing to active TB.

• This can also be done when a person is suspected to have tubercular infection in parts of the body other than lungs.

• This is also done during the treatment of tuberculosis (TB).

What sample is needed and how is it collected?

• Sputum is the most preferred sample taken for the testing. Sputum is made of phlegm, thick mucus which comes out of the lungs when a person coughs.

• In suspected case of tuberculosis, typically three sputum samples are taken in the morning into sterile container on different days. Sample taken during these times increase the chance of detecting the bacteria.

• In case you are unable to produce enough sputum, then a procedure called bronchoscopy is done where an instrument called bronchoscope is used to collect fluid from the bronchi or bronchioles. Sometime, the lab technician will use a small amount of saline through the tubing to collect bronchial washing.

• In children, gastric washings/ aspirates is used to collect sample. Here saline is inserted through a tube followed by fluid aspiration.

• Based on symptoms, urine, cerebrospinal fluid (CSF), aspirate from the site of infection, biopsy tissue samples, body fluids can be collected for AFB examination.

What preparations are required before the AFB Smear test? / What are the preparations needed before the AFB Smear test?

No preparation is required, except rinse the mouth with water before the AFB Smear test.

What is tested?

• Typically, the acid fast bacilli (AFB) testing is done to check for the presence of the mycobacterium tuberculosis which causes tuberculosis. The AFB Smear test also detects other types of infection causing mycobacteria which are much less common in comparison to mycobacterium tuberculosis. These mycobacteria are also known as acid fast bacilli (AFB) which have the tendency to get stained with a special acid dye, which gives the name to the test. After the acid staining the sample is checked under the microscope where the bacteria appear as rod shaped microbes of pink color. By detecting the mycobacteria the AFB Smear testing allows to establish the presence of tuberculosis infection in a person.

• Tuberculosis is an airborne disease which easily spreads through small suspended particles in the air. It spreads in the air when someone infected coughs, sneezes, sing or speak in front of other people. It gets spread in those people who share common area with each other such as room, school, home, jails and nursing homes. This disease spread in those people who are elderly or very young, or have predisposed conditions and diseases such as AIDS which affect their immune system. AFB Smear testing helps to diagnose, check and minimize expansion of tuberculosis in these vulnerable people. It also helps in monitoring the effectiveness of the ongoing treatment.

• Another group of mycobacteria called as nontuberculous mycobacteria (NTM) may also lead to infections. Among these 60 mycobacteria identified so far, only few of them causes infection in humans. For instance Mycobacterium avium-intracellulare complex (MAC) can cause infections in people with weak immunity. Apart from tuberculosis, AFB testing can also find out these infections caused by nontuberculous mycobacteria.


Read MoreAFB & Gram Stain Testing and Results


What does the AFB Smear test result mean?

AFB Smear Test and NAAT

AFB Smear Negative result:

• If the result of the AFB smear comes as negative, then it suggests that no acid fast bacilli or mycobacteria could be found in the given sample. This could mean absence of an infection by mycobacteria or too low level of mycobacteria in the sample. To reduce the chances of missing the bacteria, the doctor advises you to give 3 early morning sputum samples. This could also be In presence of infection or symptoms it could also mean presence of some other infection.

• In case of strong suspicion for tuberculosis, more samples can be taken on different days. Moreover, the sample can also be send for culture were low number of bacteria can grow into large colonies on a special medium which provides them suitable nutrition to grow.

AFB Smear Positive result:

• Positive AFB smear suggests the possibility of mycobacterial infection. However, a culture should be done to confirm the diagnosis and to identify the type of mycobacteria.

• For those who have signs and symptoms of active tuberculosis infection, AFB smear test results are correlated with the results of NAAT for TB, as advised by the Centers for Disease Control and Prevention. Although definitive diagnosis needs results from a culture, results of smear and NAAT can be helpful in taking further course of action. For instance, if a person is considered to have tuberculosis on quick AFB Smear test result, then most of the doctors would like treat the infection.

• Interpretation of smear and NAAT result can be seen in the below table. All results should be correlated with the results from culture before reaching to a conclusion.


AFB smear findingsNAAT findings for tuberculosisDefinition
PositivePositivePresumptive diagnosis for tuberculosis
NegativeNegativeSymptoms possibly not present due to active mycobacterial infection.
NegativePositiveNAAT is more sensitive than an AFB smear, so this may happen in infected people; this may need to test for extra samples using NAAT. If more than one sample is tested positive for NAAT, then it suggest presumptive diagnosis for tuberculosis.
PositiveNegativeDoubtful results for tuberculosis; the AFB seen on the smear are not M. tuberculosis.

AFB Culture

Positive result:

• Positive AFB cultures helps in identifying the specific mycobacterium which is causing the infection.

• If the AFB cultures comes as positive even after receiving treatment, then it means that the treatment is not working and needs to be changed. It also gives indication that the person is still actively infected and can pass the infection to other through coughing or sneezing.

Negative result:

• If AFB cultures comes as negative then it means the person does not have active AFB infection or bacteria were too low to be detected or not present in that particular sample. Thus to reduce the chances of missing on bacteria in the sample the doctor asks for at least 3 sputum samples. The growth of bacteria on the cultures is observed for 6 to 8 weeks before reporting it as negative. The person may have latent infection which caused TB screening test to come out as positive, but the negative culture AFB Smear test would suggest unlikelihood of active tuberculosis.

• If tuberculosis has spread to another part of body, then a sample from the site of infection should taken for testing and identification of infection.

• If the culture result continues to come as negative after treatment, then it means tuberculosis infection is responding to the treatment.

Susceptibility Testing

Susceptibility testing is done on the bacteria grown to check for the effectiveness of the antibiotic in killing these bacteria. Thus, the AFB Smear test helps to find out the effective treatment for the infection. Isoniazid and Rifampin are two drugs which are commonly used for the treatment to tuberculosis. If the bacteria are resistant to more than one antibiotic or the primary drugs, then it is called as “multi-drug-resistant tuberculosis”. And if the organisms are resistant to multiple drugs approved for first and second lines of therapy, then it is called as “extensively drug resistant tuberculosis”.

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