Tuesday, January 26, 2016

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/

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