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Detection of acid-fast bacilli (AFB) in stained and acid-washed smears examined microscopically may provide the initial bacteriologic evidence of the presence of mycobacteria in a clinical specimen.

 

There are two procedures commonly used for acid-fast staining:

Carbolfuchsin methods which use Ziehl-Neelsen and Kinyoun methods (direct microscopy)

Fluorochrome procedure using auramine-O or auramine- rhodamine dyes (fluorescent microscopy)

 

Smear examination is a quick procedure; results should be available within 24 hours of specimen collection.  However, smear examination permits only the presumptive diagnosis of TB because the AFB in a smear may be acid-fast organisms other than M. tuberculosis.

 

Difference Between Sputum Smears and Cultures

Acid-Fast bacilli stained in smear
Tubercle bacilli are shown in red

Fluorochrome staining: The use of any fluorescent dye (e.g., auramine, rhodamine) used to label or stain. Must be viewed using a fluorescence microscope.

Ziehl-Neelsen or Kinyoun:

Methods for staining acid-fast bacteria; acid-fast organisms appear red on a light blue or green background; carbol fuchsin is the primary stain for these methods. Ziehl-Neelsen requires heat during staining. The Kinyoun method (cold carbol fuchsin method) does not require heat, thus producing less toxic fumes.