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

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.