Advantage, Disadvantage and limitations of Microscopy to detect Acid Fast Bacilli (AFB)

December 6, 2013 Acharya Tankeshwar Bacteriology 0

This post was most recently updated on January 26th, 2015

Mycobacteria are distinguished from other micro-organisms by thick lipid-containing cell-walls that retain biochemical stains despite decolourisation by acid-containing reagents (so-called ‘acid-fastness’).

Advantages:

  1. Microscopy of sputum smears is simple and inexpensive, quickly detecting infectious cases of pulmonary TB;
  2. Sputum specimens from patients with pulmonary TB – especially those with cavitary disease – often contain sufficiently large numbers of acid-fast bacilli to be readily detected by microscopy.

Disadvantages:

  1. Direct smear microscopy is relatively insensitive as at least 5,000 bacilli per millilitre of sputum are required for direct microscopy to be positive.
  2. Smear sensitivity is further reduced in patients with extra-pulmonary TB, those with HIV-co-infection, and thosewith disease due to nontuberculous mycobacteria (NTM).

Limitations: Microscopy for acid-fast bacilli (AFB) cannot distinguish

  1. Mycobacterium tuberculosis from NTM,
  2. Viable from non-viable organisms,
  3. Drug-susceptible from drug-resistant strains.
About Acharya Tankeshwar 422 Articles
Hello, thank you for visiting my blog. I am Tankeshwar Acharya. Blogging is my passion. I am working as an Asst. Professor and Microbiologist at Department of Microbiology and Immunology, Patan Academy of Health Sciences, Nepal. If you want me to write about any posts that you found confusing/difficult, please mention in the comments below.

Be the first to comment

Do you have any queries? Please leave me in the comments section below. I will be happy to read your comments and reply.

This site uses Akismet to reduce spam. Learn how your comment data is processed.