AFB Smear and Culture

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Also known as: TB culture and sensitivity; Mycobacterial smear and culture
Formal name: Acid-fast bacillus smear and culture and sensitivity
Related tests: TB Screening Tests; Bacterial Wound Culture; Susceptibility Testing; Mycobacteria tuberculosis nucleic acid amplification test; TB NAAT; Body Fluid Analysis; Sputum Culture; Adenosine Deaminase

At a Glance

Why Get Tested?

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

When to Get Tested?

When you have symptoms of a lung infection, such as a chronic cough, weight loss, fever, chills, and weakness, that may be due to TB or another mycobacterial infection; when your doctor suspects that you have an active TB infection; when you have a positive TB screening test and you are in a high-risk group for progressing to active disease; when you have a skin or other body site infection that may be due to mycobacteria; to monitor the effectiveness of TB treatment

Sample Required?

For suspected cases of tuberculosis lung infections, three sputum samples are collected early in the morning on different days. If the affected person is unable to produce sputum, a bronchoscope may be used to collect fluid during a procedure called a bronchoscopy. In children, gastric washings/aspirates may be collected. Depending on symptoms, urine, an aspirate from the site of suspected infection, cerebrospinal fluid (CSF), other body fluids, or biopsied tissue samples may be submitted for AFB smear and culture.

Test Preparation Needed?

None

The Test Sample

What is being tested?

Acid-fast bacilli (AFB) are a group of rod-shaped bacteria (bacilli). They get their name because they can be seen and counted under the microscope when smeared on a slide and treated with a special "acid-fast" staining procedure. There are a several types of bacteria that may be detected in this manner; however, the most common and medically important acid-fast bacilli (AFB) are members of the genus Mycobacterium.

Mycobacterium tuberculosis is one of the most prevalent and infectious species of mycobacteria. Most samples that are submitted for AFB smears and cultures are collected because the doctor suspects that a person has a lung infection caused by M. tuberculosis (TB). Another group of bacteria referred to as non-tuberculous mycobacteria (NTM), can also cause infections. However, only a few of the more than 60 species of mycobacteria that have been identified cause infections in humans. They include:

  • Mycobacterium africanum causes a disease similar to TB in certain parts of the world
  • Mycobacterium avium-intracellulare complex (MAC) can cause a lung infection in immunosuppressed patients, such as the elderly and those with AIDS; this infection is not contagious but can be difficult to treat as it tends to be highly 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 Mycobacterium bovis, can sometimes be transferred from animal to human.

An AFB smear, which can provide presumptive results in a few hours, is a valuable tool in helping to make decisions about treatment while culture results are pending. Typically, several AFB smears from different samples are screened for AFB since the number of bacilli may vary from sample to sample and 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 presumptive 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.

Patient samples are processed for AFB cultures at the same time as the smears. Mycobacteria grow more slowly than other types of bacteria so 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.

Tests that may be done in addition to an AFB smear and culture include:

  • Molecular tests for TB that detect the genetic components of mycobacteria have been developed to help decrease the amount of time necessary for a presumptive diagnose of tuberculosis. These include genetic probes and molecular TB testing. They amplify/replicate pieces of the microorganisms' genetic material to detect mycobacteria in body samples in less than 24 hours and can narrow the identification to a complex of mycobacteria (a combination, of which M. tuberculosis is the most common). They are fairly sensitive and specific when they are paired with positive AFB smears; when they are done on samples that are AFB negative by smear, they tend to be less accurate. These methods are approved for respiratory samples and must be confirmed with an AFB culture, but they do provide the doctor with a quick answer, allowing him to isolate potentially infectious patients and minimize the spread of the disease. Guidelines from the Centers for Disease Control and Prevention (CDC) recommend that people with signs and symptoms of TB have at least one sample tested using nucleic acid amplification to be used in conjunction with AFB smear and culture.
  • Susceptibility testing for cultures that are positive will determine the bacteria's susceptibility or resistance to drugs most commonly used to treat the infection.

Since TB is transmitted by airborne droplets from respiratory secretions, it is a public health risk. It can spread in confined populations, such as correctional facilities, nursing homes, and schools. Those who are very young, elderly, or have diseases and conditions, such as AIDS, that compromise their immune systems tend to be especially vulnerable. AFB smears and cultures can help track and minimize the spread of TB in these populations and help determine the effectiveness of treatment.

How is the sample collected for testing?

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

If a person is unable to produce sputum, the doctor may collect respiratory samples using a procedure called a bronchoscopy. Bronchoscopy allows the doctor to look at and collect samples from the bronchi and bronchioles. Once a local anesthetic has been sprayed onto the airway, the doctor can insert a tube into the bronchi and smaller bronchioles and aspirate fluid samples for testing. Sometimes, he will introduce a small amount of saline through the tubing and into the bronchi and then aspirate it to collect a bronchial washing.

Since young children cannot produce a sputum sample, gastric washings/aspirates may be collected. This involves introducing saline into the stomach through a tube, followed by fluid aspiration.

If the doctor suspects TB is present outside of the lungs (extrapulmonary), a condition fairly common in AIDS patients, he may test the body fluids and tissues most likely affected. For instance, one or more urine samples may be collected if he suspects TB has infected the kidneys. A needle may used to collect body fluid from joints or from other body cavities, such as the pericardium or abdomen. Occasionally, the doctor may collect a sample of cerebrospinal 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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Article Sources

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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.

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