Acid-Fast Stain
1. Principle
Mycobacteria (and some Nocardia) have waxy mycolic-acid–rich walls that retain carbol fuchsin after acid-alcohol decolorization — they are “acid-fast.”
2. Step-by-Step Procedure (Ziehl–Neelsen overview)
- Prepare and heat-fix smear (or use cold Kinyoun variant).
- Apply carbol fuchsin with heat (ZN) — drives stain into waxy wall.
- Decolorize with acid-alcohol.
- Counterstain (methylene blue / malachite green).
- Examine under oil immersion: acid-fast organisms = red/pink; background = blue/green.
- Fluorochrome stains (auramine) used in many labs for screening sensitivity.
3. Interpretation
- Positive: Acid-fast bacilli (AFB) present — suggestive of mycobacteria (correlate clinically; speciation needs culture/NAAT).
- Negative: No AFB seen — does not fully exclude TB (sensitivity limits).
- Pitfalls: Sparse bacilli; poor smear; Nocardia partially acid-fast; saprophytes.
4. Clinical Use Cases
- Gold standard microscopy for: Rapid AFB detection in sputum/tissue when mycobacteria suspected.
- Alternative / complement: Mycobacterial culture, GeneXpert-type NAAT, Gram Stain (mycobacteria stain poorly).
5. Comparison with Other Methods
| Method | Pros | Cons |
|---|---|---|
| AFB smear | Fast, cheap | Low sensitivity; not species-specific |
| Mycobacterial culture | Sensitive; ID + AST | Weeks |
| NAAT | Fast + specific | Cost; scope limited by panel |
6. Mnemonic / Visual Aid
Mycolic = acid-fast armor — Gram stain fails; acid-alcohol can’t wash the red out.