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’).
- Microscopy of sputum smears is simple and inexpensive, quickly detecting infectious cases of pulmonary TB;
- 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.
- Direct smear microscopy is relatively insensitive as at least 5,000 bacilli per millilitre of sputum are required for direct microscopy to be positive.
- 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
- Mycobacterium tuberculosis from NTM,
- Viable from non-viable organisms,
- Drug-susceptible from drug-resistant strains.