AFB Smear and Culture

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Also known as: TB culture and sensitivity
Formal name: Acid-fast bacillus smear; culture; and sensitivity
Related tests: TB Skin Test

At a Glance

Why Get Tested?

To help identify a mycobacterial infection; to diagnose tuberculosis (TB) or to monitor the effectiveness of treatment

When to Get Tested?

If you have symptoms, such as a long lasting cough, weight loss, fever, chills, and weakness, that your doctor thinks may be due to TB or another mycobacterial infection. If your doctor suspects that you have active TB or wants to monitor the effectiveness of TB treatment.

Sample Required?

Usually, three specially collected spit (sputum) samples are collected early in the morning on different days. If you are unable to produce a sputum sample, a bronchoscope may be used to collect fluid during a procedure called a bronchoscopy. In children, stomach washings/aspirates may be collected. Depending on symptoms, urine, cerebral spinal fluid (CSF), other body fluids, small samples of tissue samples may be biopsied and used to help indentify an infection.

Test Preparation Needed?

None

The Test Sample

What is being tested?

The test looks for the presence of bacteria called acid-fast bacilli (AFB) which are rod shaped bacteria that can be seen and counted under the microscope in a specially stained sample on a glass slide, called an AFB smear. The most common AFB are of a type called mycobacteria. 

Mycobacterium tuberculosis is the most common species of mycobacteria, and is the most infectious. Most AFB smears and cultures are collected because the doctor suspects that the patient has TB. Only a few mycobacteria that have been identified cause infections in humans. They include:

  • M. africanum, causes a disease similar to TB in certain parts of the world
  • Mycobacteria avium-intracellulare complex (MAC), can cause a lung infection in immunosuppressed patients, such as the elderly and those with AIDS; this infection is not easily spread to other people, but can be difficult to treat because it is very resistant to antibiotics
  • Other mycobacterial species, such as Mycobacterium marinum, grow in water, such as fish tanks, and can cause skin infections, while other rapidly growing mycobacteria can infect wounds and prosthetic devices 
  • A few mycobacteria, such as M. bovis, can sometimes be transferred from animal to human

Several smears from different samples should be checked for AFB since the number of bacilli may vary day to day. If acid-fast bacilli are present on any of the smears, a mycobacterial infection is likely. Since M. tuberculosis is the most common cause of respiratory infections with mycobacteria, a provisional diagnosis of TB can be made but other follow-up testing must be done to positively identify the acid-fast bacilli as either M. tuberculosis, or another mycobacteria species.

Body fluids such as urine or CSF are used for AFB cultures at the same time as the smears. The cultures are used to grow AFB in the laboratory.  Since mycobacteria grow slowly, positive identification of the species that is/are present may take days to several weeks, while negative results (no mycobacterial growth) can take up to 6 to 8 weeks to confirm.

How is the sample collected for testing?

Since M. tuberculosis and M. aviummost frequently infect the lungs, sputum is the most commonly tested sample. Sputum is phlegm, thick mucous that is coughed up from the lungs. Usually, three to five early morning samples are collected (on consecutive days) in individual sterile cups.

 

If you are unable to produce sputum, your doctor may collect respiratory samples using a procedure called a bronchoscopy. Bronchoscopy allows your doctor to look at, and collect samples from, your lungs by inserting a tube through your throat after giving a local anaesthetic.

Since young children will not be able to produce a sputum sample, stomach washings/aspirates may be collected. This involves putting a little salt solution (saline) into the stomach through a tube, and sucking the fluid back up the tube.

If your doctor thinks you may have TB outside of the lungs (which is fairly common in patients with AIDS), the body fluids and tissues which are most likely to be affected will be sampled and tested. For instance, if it is suspected that TB has infected your kidneys then one or more urine samples may be collected for testing.  A needle may be used to collect fluid from your joints or from other body cavities, such as the lining around the heart (pericardium) or abdomen.  Occasionally, your doctor may need to use a needle to collect a sample of cerebral spinal fluid (CSF) or perform a minor surgical procedure to obtain a tissue biopsy.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.