- most commonly used to identify an active tuberculosis (TB) infection caused by the most medically important AFB, Mycobacterium tuberculosis
- 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).
AFB
Smears
- relatively quick way to determine if an infection may be due to one of the mycobacteria, such as M. tuberculosis.
- AFB smears can provide presumptive results within a few hours and are valuable in helping to make decisions about treatment while culture results are pending.
- this rapid test is less sensitive than culture to diagnosis a mycobacterial infection
nucleic
acid amplification test (NAAT)
- done in conjunction with an AFB smear
- are fairly sensitive and specific when they are performed on specimens where acid-fast bacteria were seen on the smear
AFB
Culture
- this test is more sensitive than an AFB smear
- takes longer for results to become available.
- Mycobacteria grow more slowly than other types of bacteria so positive identification may take days to several weeks, while negative results (no mycobacterial growth) can take up to 6 to 8 weeks to confirm
Positive
Results
- Positive AFB smears indicate a probable mycobacterial infection.
- However, a culture must be performed to confirm a diagnosis and identify the species of mycobacteria present
- May be due to other Mycobacterium species including M. ulcerans, and M. leprae and nontuberculous mycobacteria (NTM)
List of
Acid Fast organisms (Other than Mycobacteria)
- Nocardia spp: Partial Acid Fast
- Rhodococcus spp: Partial Acid Fast
- Legionella micdadei: Partially acid fast in tissue
- Cyst of Cryptosporidium: Acid Fast
- Cyst of Isospora: Acid Fast
References:
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