AFB Test: Specificity

 

• 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:

1. http://microbeonline.com/ziehl-neelsen-technique-principle-procedure-reporting/
2. https://labtestsonline.org/understanding/analytes/afb-culture/tab/test/