Zn staining details for viva
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Here is a complete, viva-ready breakdown of Ziehl-Neelsen (ZN) staining:
Ziehl-Neelsen (ZN) Stain - Viva Details
What Is It?
A hot acid-fast stain used to detect acid-fast bacilli (AFB), primarily Mycobacterium tuberculosis and other mycobacteria. It is also used to identify M. leprae and some other organisms.
Principle (The KEY viva point)
Mycobacteria have a high lipid content cell wall (>60%), dominated by mycolic acids - long-chain fatty acids that:
- Resist penetration by conventional aniline dyes (e.g., crystal violet in Gram stain)
- Bind arylmethane dyes (carbol fuchsin) tightly in the presence of phenol + heat
- Resist decolorization by strong acid-alcohol once stained
This property is called acid-fastness. - Sherris & Ryan's Medical Microbiology, 8th Ed.
Procedure (Step-by-Step)
| Step | Action | Detail |
|---|---|---|
| 1 | Fix smear | Pass slide through flame 3 times (or flood with alcohol) |
| 2 | Primary stain | Cover with carbol fuchsin, steam gently 5 min over direct flame (or 20 min over water bath) - do NOT boil or let dry |
| 3 | Wash | Deionized water |
| 4 | Decolorize | 3% acid-alcohol (95% ethanol + 3% HCl) until only faint pink remains |
| 5 | Wash | Water |
| 6 | Counterstain | Loeffler's methylene blue for 1 minute |
| 7 | Wash and dry | Deionized water, air dry |
- Park's Textbook of Preventive and Social Medicine
Result
| Component | Colour |
|---|---|
| Acid-fast bacilli (AFB) | Bright red/pink |
| Background cells, non-AFB | Blue (methylene blue) |
Morphology of AFB on ZN Stain
- Slender, slightly bent or beaded rods
- May appear V-shaped (two organisms adhering at one end) in sputum
- Often lie parallel to each other
- Examined under 100x oil-immersion lens
- Quick Compendium of Clinical Pathology, 5th Ed.
Smear Reporting (WHO Standard - 1000x magnification)
| Finding | Result Reported |
|---|---|
| No AFB per 100 oil immersion fields | 0 (negative) |
| 1-9 AFB per 100 fields | Scanty (report exact number) |
| 10-99 AFB per 100 fields | 1+ |
| 1-10 AFB per oil immersion field | 2+ |
| >10 AFB per oil immersion field | 3+ |
- Smear positivity requires at least 10,000 organisms/mL of sputum
- One positive specimen out of two is sufficient for smear-positive TB diagnosis
- Park's Textbook of Preventive and Social Medicine
ZN Stain Variations - Comparison (High-Yield Viva Table)
| Feature | Ziehl-Neelsen | Kinyoun | Fite | Fluorochrome (Auramine-Rhodamine) |
|---|---|---|---|---|
| Heat | Yes (hot) | No (cold) | No (cold) | No |
| Primary stain | Carbol fuchsin | Carbol fuchsin (high concentration + detergent) | Carbol fuchsin | Auramine-O / Auramine-rhodamine |
| Decolorizer | 3% acid-alcohol (strong) | 3% acid-alcohol | Weak acid (sulfuric acid in ethanol) | Acid-alcohol |
| Counterstain | Methylene blue | Methylene blue | Methylene blue | Potassium permanganate |
| Microscopy | Light (100x oil) | Light (100x oil) | Light (100x oil) | Fluorescent (40x objective) |
| Organisms | True AFB | True AFB | Weakly AFB (M. leprae, Nocardia, Rhodococcus) | AFB appear golden/yellow fluorescent |
| Sensitivity | Baseline | Similar to ZN | Lower (weaker decolorizer by design) | ~20% MORE sensitive than carbol fuchsin stains |
- Quick Compendium of Clinical Pathology, 5th Ed.; Tietz Textbook of Laboratory Medicine, 7th Ed.
Why Mycobacteria Don't Stain with Gram Stain
- Their hydrophobic cell wall resists penetration of aniline dyes like crystal violet
- May appear as ghost bacilli (bacilli-shaped clear zones) on Gram stain
- Tietz Textbook of Laboratory Medicine, 7th Ed.
False Positives and Negatives
False positives:
- Red stain retained by scratches on slide
- Transfer of AFB from a positive slide to a negative one
- Environmental mycobacteria contaminating the specimen
- Acid-fast particles: food particles, precipitates
False negatives:
- Fewer than 10,000 organisms/mL
- Severe immunocompromise (HIV) - decreased inflammation means fewer bacilli expectorated
- Poor smear preparation or over-decolorization
Other Organisms Positive on ZN / Acid-Fast Stains
| Organism | Type of Acid-Fastness |
|---|---|
| Mycobacterium tuberculosis, M. leprae, NTM | True acid-fast |
| Nocardia spp. | Weakly acid-fast |
| Rhodococcus equi | Weakly acid-fast |
| Legionella micdadei | Weakly acid-fast |
| Cryptosporidium, Cyclospora, Cystoisospora (oocysts) | Modified acid-fast positive |
| Schistosoma egg spines, Echinococcus hooklets | Modified acid-fast positive |
Limitations / WHY Fluorochrome is Now Preferred by WHO
- Fluorochrome (auramine-rhodamine) is ~20% more sensitive than carbol fuchsin stains
- Allows reading at lower magnification (250-400x) - faster, less eye strain
- WHO has endorsed global phase-out of ZN in favour of auramine-rhodamine
- However, ZN remains widely used in resource-limited settings where fluorescent LED microscopes are unavailable (only 7% of global labs had fluorescent capability as of 2014)
- Tietz Textbook of Laboratory Medicine, 7th Ed.
Quick Recall Mnemonics
- ZN = Hot stain (uses heat; Kinyoun is the Cold version)
- AFB = Red on Blue (carbol fuchsin red on methylene blue background)
- Acid-fastness = Mycolic acids in cell wall resist acid-alcohol decolorization
- Smear positive = 1 positive out of 2 specimens is enough
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