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Acid-Fast Bacilli Smear with Culture Sensitivity

Last updated Oct. 11, 2015


What are the other Names for this Test? (Equivalent Terms)

  • Acid Fast Bacilli (Staining, Smear and Culture Sensitivity)
  • Mycobacterial Smear with Culture Sensitivity
  • Tuberculosis (TB) Smear with Culture Sensitivity

What is Acid-Fast Bacilli Smear with Culture Sensitivity Test? (Background Information)

  • Acid-Fast Bacilli Smear with Culture Sensitivity is meant to detect, grow, isolate, identify, and then test, the sensitivity to antibiotics of acid-fast bacilli (AFB)
  • AFB are a group of rod-shaped bacteria; the most common and clinically important of which, belong to the genus mycobacterium. AFB bacteria types, when smeared on a slide and processed with special acid-fast staining methods, is visible under a microscope
  • An AFB smear may be combined with nucleic acid amplification test (NAAT). The results of both of these tests are available within hours, which can help the healthcare provider with a presumptive diagnosis and start early treatment. NAAT for mycobacterium is a molecular test
  • A definitive diagnosis can be made with AFB cultures (which are prepared along with the smears); however, the results may take from several days, to weeks to obtain
  • AFB culture results can also help the healthcare provider monitor the effectiveness of the treatment provided, until the patient is cured
  • The bacteria is also tested for sensitivity to different antibiotics, to help the physician make a decision about the most effective treatment plan, as the development of antibiotic resistance is very common with mycobacteria

Some of the species of mycobacteria, which causes infections in humans, are:

  • Mycobacterium tuberculosis: It is one of the most common and infectious mycobacteria that causes tuberculosis
  • Mycobacterium avium-intracellulare complex (MAC): It causes lung infection in the immunocompromised individuals, like the elderly adults, patients on immunosuppressive drugs, HIV patients, etc. The infection is difficult to treat, due to antibiotic resistance
  • Mycobacterium bovis: It generally causes an infection in animals, but the infection may be transferred to humans too
  • Mycobacterium leprae: This mycobacterium species causes leprosy
  • Mycobacterium africanum: In some parts of the world, this bacterium species causes a tuberculosis-like infection

What are the Clinical Indications for performing the Acid-Fast Bacilli Smear with Culture Sensitivity Test?

Individuals in the following category may be prescribed an Acid-Fast Bacilli (AFB) Smear with Culture Sensitivity:

  • TB skin test is positive, and an X-ray of the individual shows characteristic lung involvement
  • The individual has symptoms of pulmonary tuberculosis or lung infection, caused by other mycobacterial species. Some of these symptoms may include:
    • A persistent cough producing sputum that could have blood streaks
    • Night sweats
    • Fever
    • Weakness
    • Chest pain
    • Weight loss and loss of appetite
  • The individual has signs and symptoms of extra-pulmonary (outside the lungs) tuberculosis, or infection caused by other mycobacterial species. Some of these signs and symptoms include:
    • Joint pain
    • Back pain and abdominal pain
    • Anemia
    • Blood in urine
    • Weight loss
    • Fever, headache
    • Paralysis
    • Altered mental status
  • The individual was in close contact with someone, who had tuberculosis, and now has symptoms of the disease. Else, the individual has a predisposing condition, which puts them at an increased risk of TB, due to weak immune system, such as HIV/AIDS, old age, chronic disease, hospitalization, immunosuppressive therapy, etc.
  • The patient is currently being treated for TB (tuberculosis) - in such cases, the test aids in assessing the effectiveness of treatment and also when to stop treatment

How is the Specimen Collected for Acid-Fast Bacilli Smear with Culture Sensitivity Test?

Following is the specimen collection process for Acid-Fast Bacilli (AFB) Smear with Culture Sensitivity:

Sample required: Sputum

Though sputum is the most common sample type used (mycobacterium mostly involves the lungs); however, other samples, such as cerebrospinal fluid, blood, urine, stools, joint fluid, and fluid and tissue from other body parts may also be collected, depending on the area involved and the signs and symptoms presented.

Process: Collection of sputum samples in individual sterile cups.

  • Early morning samples are collected on consecutive days, at least 3-5 times, in individual sterile containers
  • In cases, where the individual is unable to produce sputum, a bronchoscopy will be performed
  • In young children, gastric aspirates may be collected, as they are unable to produce sputum

Preparation required: No special preparation is needed, prior to the test.

What is the Significance of the Acid-Fast Bacilli Smear with Culture Sensitivity Test Result?

The significance of Acid-Fast Bacilli Smear with Culture Sensitivity is explained below:

  • A positive AFB smear test result is an indication that the patient may have a mycobacterial infection. However, the test result only provides a presumptive diagnosis, and in order to confirm the diagnosis, a culture must be performed
  • As per recommendations of the US Centers for Disease Control and Prevention (CDC), both acid-fast bacilli (AFB) smear and nucleic acid amplification test (NAAT) results should be considered together, when deciding on the treatment. The healthcare provider usually starts treatment, based on the positive results of both the AFB smear and NAAT tests. This is called presumptive treatment for tuberculosis
  • A negative AFB Smear result is an indication that the collected specimen does not have any mycobacteria, or an AFB infection. However, TB may still be present in another part of the body, or the sample may have insufficient number of mycobacteria to be detected in the smear test. To increase the probability of detecting the bacteria, 3 such samples are obtained
  • The NAAT test is more reliable than AFB smear test and more than one positive NAAT test, despite negative results for AFB smear, usually gives a presumptive diagnosis of TB
  • AFB culture is the definitive way to diagnose the type of mycobacteria. Mycobacterium grows very slowly and it usually takes several days to weeks, to detect any growth, and to identify the particular type. Negative results are confirmed only after there is no growth on the testing culture plates for 6-8 weeks
  • Positive AFB cultures are supplemented by sensitivity testing, to help the healthcare provider treat the condition with appropriate antibiotics. Hence, sensitivity testing for AFB is very important
  • A negative culture report indicates the absence of a mycobacterium infection, or it may simply mean that the collected specimen did not have any mycobacteria present in it. Again, multiple samples are collected, to increase the probability of a test result
  • Culture reports are also requested at regular intervals after the start of treatment, to monitor the effectiveness of treatment. This also helps in understanding, if the treatment needs to be changed, modified, or stopped. Sometimes, an individual may develop drug-resistant tuberculosis. Sensitivity testing will help diagnose a drug-resistant tuberculosis
  • A negative AFB culture report (during the treatment course) does not indicate a cure. It simply means that the individual is no longer infectious (spreading infection). The individual may still have mycobacteria in his body and is not fully cured. This is called latent tuberculosis. Antibiotics may have to be continued for some more weeks, after negative culture reports, in order to achieve a full cure.

The laboratory test results are NOT to be interpreted as results of a "stand-alone" test. The test results have to be interpreted after correlating with suitable clinical findings and additional supplemental tests/information. Your healthcare providers will explain the meaning of your tests results, based on the overall clinical scenario.

Additional and Relevant Useful Information:

Certain medications that you may be currently taking may influence the outcome of the test. Hence, it is important to inform your healthcare provider, the complete list of medications (including any herbal supplements) you are currently taking. This will help the healthcare provider interpret your test results more accurately and avoid unnecessary chances of a misdiagnosis.

There are several other tests for diagnosing tuberculosis (TB). The following article links will help you understand these laboratory procedures:

http://www.dovemed.com/common-procedures/procedures-laboratory/tb-screening-test-interferon-gamma-release-assays/
http://www.dovemed.com/common-procedures/procedures-laboratory/tuberculin-skin-test-for-tuberculosis/

What are some Useful Resources for Additional Information?

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References and Information Sources used for the Article:

http://www.longbeach.gov/health/epi_lab/phlab/lab_tests/afb_culture.asp (accessed on 04/13/2014)

http://www.georgetownhospitalsystem.org/stw/Page.asp?PageID=STW2974447 (accessed on 04/13/2014)

http://labtestsonline.org/understanding/analytes/afb-culture/tab/test (accessed on 04/13/2014)

http://health.mo.gov/lab/smearculture.php (accessed on 04/13/2014)

Reviewed and Approved by a member of the DoveMed Editorial Board
First uploaded: April 16, 2014
Last updated: Oct. 11, 2015