Acid-Fast Bacilli (AFB) Test

The acid-fast method is a special staining technique that is particularly useful when identifying mycobacteria in sputum specimens, which often contain a variety of organisms.

Test Description

The acid-fast method is a special staining technique that is particularly useful when identifying mycobacteria in sputum specimens, which often contain a variety of organisms. Examples of mycobacteria are those causing leprosy, tuberculosis, and respiratory infection in patients with acquired immunodeficiency syndrome (AIDS). Mycobacteria retain stain coloring even after treatment with a decolorizing acidalcohol solution. Once bacilli are determined to be acid-fast, a culture is done to differentiate the type of mycobacteria, along with sensitivity testing to determine appropriate pharmacologic treatment. 
AFB

THE EVIDENCE FOR PRACTICE

Any patient with a cough lasting ≥2 to 3 weeks, with at least one additional symptom including fever, night sweats, weight loss, or hemoptysis, should have a chest radiograph. If suggestive of tuberculosis, three consecutive morning sputum specimens for AFB should be collected.
Read: CA 19-9 (Cancer Antigen 19-9, CA 19-9 Tumor Marker)

Normal Values

Negative for bacilli

Possible Meanings of Abnormal Values

Positive

  • AIDS
  • Leprosy
  • Tuberculosis

Contributing Factors to Abnormal Values

Collection of saliva, rather than sputum, will provide inaccurate test results.

Interventions/Implications

Pretest

  • The sputum should be collected before antimicrobial therapy is begun.
  • Explain to the patient the purpose of the test and the need for a sputum specimen.
  • Explain the procedure to the patient:
    1. An early morning specimen is best, because sputum is most concentrated at that time.
    2. The patient should brush the teeth and rinse the mouth with water before collecting the sputum to reduce contamination of the sample.
    3. The sputum must be from the bronchial tree. The patient must understand this is different from saliva in the mouth.
    4. The sample is collected in a sterile sputum container.
  • If tuberculosis is suspected, three consecutive morning specimens may be ordered. This
  • increases the chance of isolating the microbes.
  • If the sputum is very thick, it can be thinned by inhaling nebulized saline or water or by increasing fluid intake the evening before sample collection. Postural drainage and chest physiotherapy may also prove helpful.

Procedure

  • The patient should take several deep breaths and then cough deeply to obtain the sputum. At least one teaspoon of sputum is needed.
  • If specimen collection via coughing is ineffective, endotracheal suctioning and fiberoptic bronchoscopy are other options.
  • After collection of the sputum, the sample is sent to the laboratory for a Gram stain. This is used to differentiate between true sputum and saliva, which contains many epithelial cells. Decolorizing solution is used to determine acid-fastness of the bacilli.
  • The sputum is then placed on the appropriate culture medium and allowed to incubate. Final reports for tuberculosis (AFB culture) may take 1 to 6 weeks.

Posttest

  • Label the specimen container and transport it to the laboratory as soon as possible. Note
  • any current antimicrobial therapy on the label.
  • Gloves should be worn when handling the specimen.
  • Report positive results to the primary care provider.

Clinical Alerts

  • A positive AFB smears indicates a likely mycobacterial infection. The AFB culture
  • is then used to identify the specific mycobacteria
  • If the AFB smear/culture is positive after several weeks of drug treatment, this indicates ineffective treatment and that the patient is still infectious. A change of treatment regimen would be warranted.

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Dr Lobby | DrLobby.com: Acid-Fast Bacilli (AFB) Test
Acid-Fast Bacilli (AFB) Test
The acid-fast method is a special staining technique that is particularly useful when identifying mycobacteria in sputum specimens, which often contain a variety of organisms.
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