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; culture and sensitivity
Related tests: TB screening test; Bacterial Wound Culture; Susceptibility Testing; Mycobacteria tuberculosis nucleic amplification test; TB NAAT; Body fluid analysis; Sputum culture

At a Glance

Why Get Tested?

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

When to Get Tested?

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

When your doctor suspects another mycobacterial infection such as when you have a skin or other body site infection that is not responding to routine antibiotics.

 

Sample Required?

For suspected cases of mycobacterial 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 rod shaped bacteria. 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 that differs from the routine stain. The most common and medically important acid-fast bacilli 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 the patient has a lung infection caused by M. tuberculosis (TB). Another group of bacteria referred to as non-tuberculous mycobacteria (NTM), can also cause infections. These organisms are common in the environment (including water and soil) however only a few of them cause infections in humans. They include:

  • Mycobacteria avium-intracellulare complex (MAC) can cause a lung infection or a disseminated infection in immunosuppressed patients, such as the elderly and those with AIDS; this infection is not contagious, but it can be difficult to treat as it tends to be highly resistant to antibiotics.
  • Cervical lymphadenitis been seen in young children as a swollen lymph gland in the neck. It is most commonly caused by MAC, but also by M. tuberculosis or other NTM. Surgical excision is often required for treatment.
  • Rapid growing mycobacterium (RGM) (M. abscessus, M. chelonae, M. fortuitum) may cause lung or non-pulmonary disease such as wound infection of prosthetic device infection.
  • Mycobacterium ulcerans causes sporadic cases of non-healing ulcers (some names include Bairnsdale ulcer, Buruli ulcer, Daintree ulcer) that often require surgical treatment.
  • Mycobacterium marinum grows in water, such as fish tanks, and can cause skin infections.
  • Mycobacterium leprae is the causative agent of leprosy.
  • Some 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. A presumptive diagnosis of TB can be made if a patient has risk factors for disease, 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 a positive result will be available more rapidly allowing the potentially infectious patient to be isolated to minimise the spread of the disease.
  • Antibiotic susceptibility testing may not be routinely performed, but depends on the mycobacterial species cultured. Genotypic resistance testing is increasingly used for certain drugs such as isoniazid and M. tuberculosis. Susceptiblity testing on cultures is complex and takes a long-time to return a result but is used for some drug/bug combinations. The third option is to treat the infection with empiric antibiotics that would normally treat an infection of the sort that has occurred in the patient. This option may be used as the organisms are difficult to grow and the susceptibility result may be predictable.

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 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 lungs (pulmonary disease), 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 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/she 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), he/she may test the body fluids and tissues most likely affected. For instance, one or more urine samples may be collected if he/she suspects TB has infected the kidneys. A needle may used to collect fluid from joints or from other body cavities, such as the pericardium or abdomen. Occasionally, the doctor may collect a sample of cerebral spinal fluid (CSF) or perform a minor surgical procedure to obtain a tissue biopsy. 

 

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.

Article: (2007) Griffith et al. An official ATS/IDSA statement: diagnosis, treatment, and prevention of nontuberculous mycobacterial diseases.  Am J Respir Crit Care Med 175(4):367-416.  Available online at http://ajrccm.atsjournals.org/content/175/4/367.long.  Accessed August 2012.