AFB

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 an infection:

• AFB smear—a microscopic examination of a person’s sputum or other specimen that is 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 culture 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.