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What is an Acid-fast Bacilli Culture test?

An acid-fast bacilli (AFB) culture test detects AFB that retain the colour of a stain after an acid wash. This test is done to diagnose diseases caused due to Mycobacterium infection such as tuberculosis, leprosy, HIV-related infections, lung, skin and soft tissue diseases. AFB test also helps monitor the efficiency of treatment of these diseases. 

A sample of sputum or tissue is obtained from the patient for culture. It is placed in a container with nutrients for the bacilli to grow. A positive bacterial growth over a period of time in the container indicates a positive test and mycobacterial infection.

An AFB smear is usually done before the culture for a presumptive diagnosis. Positive smears signify a possible infection, and culture will confirm the diagnosis. If the smear is negative but clinical suspicion is high, a culture may still show microbial growth that was undetectable through microscopy. AFB cultures also help recognise specific mycobacterium species and identify whether the organism is resistant to antibiotics.

  1. Why is AFB Culture test performed?
  2. How do you prepare for AFB Culture test?
  3. How is AFB Culture test performed?
  4. What do AFB test results indicate?

AFB culture test is performed when symptoms of Mycobacterium infection are present. It is generally recommended in the following situations: 

  • Pulmonary tuberculosis infection with symptoms such as cough, fever, chills, malaise and weight loss
  • Extra-pulmonary tuberculosis in the spinal cord, kidneys or bones, with common symptoms of infections such as back pain, paralysis, anaemia, headache, coma and joint or abdominal pain
  • High-risk groups, such as persons with HIV/AIDS and exposure to tuberculosis cases
  • People who have stayed in hospitals and jails
  • To monitor the progress of treatment in people undergoing treatment for tuberculosis

Since fasting samples are needed, preparation should be done to take the test early in the morning before breakfast. Mouth should be rinsed thoroughly before coughing up phlegm.

Samples for the AFB test are collected as mentioned below:

  • Sputum: Expectorated early morning, deep cough sputum should be collected with minimal saliva content; 5 to 10 mL of sample is collected on three consecutive days for correct diagnosis. It is advised to rinse the mouth with water before collecting sputum. This will ensure that no food particles, mouthwash or oral drugs contaminate the specimen, which tends to inhibit the growth of Mycobacteria.
  • Induced sputum: If there is difficulty in coughing sputum, an aerosol inhalation of sterile 5-10% sodium chloride in water is given. This will trigger a cough that will generate sputum.
  • Bronchoscopy: A bronchoscope is inserted in the nose or mouth to gain access to lungs for obtaining a sputum or tissue sample. This procedure is done in case of poor sputum production. (Read more: Preparing for bronchoscopy)
  • Urine, cerebrospinal fluid and tissue samples are taken if tuberculosis is suspected outside the lung.

Sputum collection is done for a minimum of two days consecutively. Additional time will be needed if a bronchoscope is needed to collect sputum. There are no side effects when a phlegm or urine sample is taken.

Bleeding, infection, pain and local discolouration may occur if a blood sample is taken. AFB culture is monitored for a few weeks, and the sample may be collected again to monitor treatment.

AFB culture can be positive or negative, which indicates the presence or absence of an infection. Results are reported when growth occurs or when no growth is seen at six weeks.

Normal results: A negative result indicates no AFB in the sample taken. This means that there is no infection.

Abnormal results: A positive result indicates the presence of AFB in the sample. The commonest AFB  is Mycobacterium tuberculosis. An additional antibiotic susceptibility test will guide the physician towards the appropriate drugs for effective treatment. If there is a positive culture even after several weeks of continuing treatment, it is an indication of the inefficiency of the treatment. 

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. This information is purely from an educational perspective and is in no way a substitute for medical advice from a qualified doctor.

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References

  1. Lewinsohn, D, et. al. (2017 January 03). Official American Thoracic Society/Infectious Diseases Society of America/Centers for Disease Control and Prevention Clinical Practice Guidelines: Diagnosis of Tuberculosis in Adults and Children. Clinical Infectious Diseases, Volume 64,
  2. MedlinePlus Medical Encyclopedia: US National Library of Medicine; Sputum stain for mycobacteria
  3. National Institute of Allergies and Infectious diseases [internet]: National Institute of Health. US Department of Health and Human Services; Mycobacterium
  4. Griffith DE, Askamit T, Brown-Elliot BA, et al. An Official ATS/IDSA Statement: Diagnosis, Treatment, and Prevention of Nontuberculous Mycobacterial Diseases. American Journal of Respiratory and Critical Care Medicine. 2007;175:367-416.
  5. University of Rochester Medical Center [Internet]. Rochester (NY): University of Rochester Medical Center; Acid-Fast Bacteria Culture
  6. NSW goverment [internet]: New South Wales Ministry. Australia; Guideline: Laboratory Diagnostic Tests and Interpretation
  7. Missouri Department of Health and Senior Services [internet]. US; Acid-Fast Bacilli (AFB) Smear and Culture
  8. Johnson MM, Odell JA. Nontuberculous mycobacterial pulmonary infections.. Journal of Thoracic Disease. 2014; 6(3):210-20. PMID: 24624285
  9. Delaware Department of Health and Social Services (DHSS) [internt]. Delaware. US; SPECIMEN COLLECTION PROCEDURES FOR TB (MYCOBACTERIOLOGY)