AFB Smear -Three samples
Also Known As:
AFB Smear and Culture TB Culture and Sensitivity Mycobacteria Smear and Culture TB NAAT Acid-Fast Bacillus Smear and Culture and Sensitivity Mycobacterium tuberculosis Nucleic Acid Amplification Test
Most samples that are submitted for acid-fast bacilli (AFB) testing are collected because the healthcare practitioner suspects that a person has tuberculosis (TB), a lung infection caused by Mycobacterium tuberculosis. Mycobacteria are called acid-fast bacilli because they are a group of rod-shaped bacteria (bacilli) that can be seen under the microscope following a staining procedure where the bacteria retain the color of the stain after an acid wash (acid-fast). AFB laboratory tests detect the bacteria in a person’s sample and help identify an infection caused by AFB.
There are several types of AFB that may be detected with this testing; however, the most common and medically important ones are members of the genus Mycobacterium. Mycobacterium tuberculosis is one of the most prevalent and infectious species of mycobacteria.
Since TB is transmitted through the air when an infected person sneezes, coughs, speaks, or sings, it is a public health risk. It can spread in confined populations, such as in the home and schools, correctional facilities, and nursing homes. Those who are very young, elderly, or have preexisting diseases and conditions, such as AIDS, that compromise their immune systems tend to be especially vulnerable. AFB testing can help diagnose, track, and minimize the spread of TB in these populations and help determine the effectiveness of treatment.
Another group of mycobacteria referred to as nontuberculous 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. Some examples include Mycobacterium avium-intracellulare complex (MAC), which can cause lung infection and disseminated disease in people with weakened immune systems. (See the article on Nontuberculous Mycobacteria for more details on different types). In addition to TB, AFB testing can help identify infections caused by these nontuberculous mycobacteria.
See “How is it used?” under Common Questions below for details on AFB tests.
How is the sample collected for testing?
Sputum is the most commonly tested sample. Sputum is phlegm, thick mucus that is coughed up from the lungs. Preferably, three early morning samples obtained by deep cough are collected on consecutive days in individual sterile cups to increase the likelihood of detecting the bacteria.
If a person is unable to produce sputum, a healthcare practitioner may collect respiratory samples using a procedure called a bronchoscopy. Bronchoscopy allows the healthcare practitioner to look at and collect samples from the bronchi and bronchioles. Once a local anesthetic has been sprayed onto the patient’s upper airway, the practitioner can insert a tube into the bronchi and smaller bronchioles and aspirate fluid samples for testing. Sometimes, the healthcare practitioner 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 healthcare practitioners suspect TB is present outside of the lungs (extrapulmonary), they may test the body fluids and tissues most likely affected. For instance, one or more urine samples may be collected if the practitioner suspects TB has infected the kidneys. A needle may be used to collect fluid from joints or from other body cavities, such as the pericardium or abdomen. Occasionally, the practitioner may collect a sample of cerebrospinal 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, except to rinse the mouth with water before collecting the sputum sample.
How is it used?
AFB testing may be used to detect several different types of acid-fast bacilli, but it is most commonly used to identify an active tuberculosis (TB) infection.
Mycobacteria are called acid-fast bacilli because they are rod-shaped bacteria (bacilli) that can be seen under the microscope following a staining procedure in which the bacteria retain the color of the stain after an acid wash (acid-fast).
A few different tests may be used to help identify AFB as the cause of infection:
- AFB smear—a microscopic examination of a person’s sputum or other specimens that are stained to detect acid-fast bacteria. It is a rapid test used to provide presumptive results within one to two days. It is valuable in helping to make decisions about treatment while waiting for cultural results. However, AFB smears must be confirmed with AFB cultures.
- Molecular tests for TB (nucleic acid amplification test or NAAT) detect the genetic material of mycobacteria. These tests are often used when the AFB smear is positive or TB is highly suspected. Like AFB smears, they can provide a presumptive diagnosis, which can aid in the decision of whether to begin treatment and isolate potentially infectious people before culture results are available. Results of NAAT are typically available in one to three days after a sample is collected. Molecular methods are approved for use with respiratory samples but must be confirmed with an AFB culture. Guidelines from the Centers for Disease Control and Prevention recommend that people with signs and symptoms of TB have at least one sample tested using nucleic acid amplification with AFB smear and culture. The NAAT test currently available is done directly on sputum samples and can simultaneously detect TB and its resistance to rifampicin in less than two hours. Rifampicin is a common treatment for TB.
- AFB cultures are used to diagnose active M. tuberculosis infections as well as infections due to nontuberculous mycobacteria. AFB cultures can also be used to monitor the effectiveness of treatment and can help determine when a person is no longer infectious. Though culture is more sensitive than an AFB smear, positive results may take days to several weeks, while negative culture results (no mycobacterial growth) can take up to 6 to 8 weeks to confirm.
- Susceptibility testing is usually ordered in conjunction with an AFB culture to determine the most effective antibiotic to treat the mycobacterial infection. M. tuberculosis may be resistant to one or more drugs commonly used to treat TB.