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Z-N staining (Ziehl-Neelsen staining) is the acid-fast stain used to find acid-fast bacilli (AFB), chiefly mycobacteria, in a smear under a microscope. Heated carbol fuchsin drives red dye into the waxy cell wall, and a counterstain turns everything else blue or green. Reagent quality, quality-control slides and a defined reading scale decide how reliable it is.
Definition and scope
Ziehl-Neelsen (ZN) staining is a differential stain for acid-fast bacilli. It is the classic technique for smear microscopy in tuberculosis diagnosis: CDC authors describe microscopy as still the primary diagnostic tool for tuberculosis, with an estimated 83 million smears examined each year, most of them stained by the ZN method.1 This guide covers the principle, reagents, reading and quality side of the method; a laboratory should follow its own validated SOP at the bench.
“AFB staining” is the same test family under a different label: AFB stands for acid-fast bacilli, the organisms the stain is designed to reveal. Fluorochrome methods such as auramine-phenol also detect acid-fast organisms, but they are a different stain read under a fluorescence microscope.
How Z-N staining works in practice
Why mycobacteria need a hot carbol fuchsin stain
A national laboratory SOP from the Oman Ministry of Health explains the principle: the mycobacterial cell wall is rich in mycolic acid, which makes it difficult to stain with the Gram stain, and heat is used to help drive the primary stain into the waxy cell walls.2 That is why a Gram-stained smear is a poor way to look for mycobacteria (see the Gram stain guide for what that method does show). Once the primary stain is in, the cells keep it when treated with acid-alcohol, which is the “acid-fast” property.
Reagents and what you see
The SOP lists three reagent groups: carbol fuchsin as the primary stain, 3% acid-alcohol as the decolorizer, and methylene blue or malachite green as the counterstain. Smears are examined with the 100x oil-immersion objective. A positive result is red-stained rods against a blue or green background, depending on the counterstain used.2
Reading the smear and AFB grading
Grading scales differ between programmes, so treat any scale as the convention of the laboratory that issued it. The Oman Ministry of Health SOP uses the following:2
| Finding in the SOP | Report as |
|---|---|
| No AFB in at least 100 fields | No AFB seen |
| 1-9 AFB per 100 fields | Report the exact number |
| 10-99 AFB per 100 fields | + |
| 1-10 AFB per field in at least 50 fields | ++ |
| More than 10 AFB per field in at least 20 fields | +++ |
The same SOP treats positive results as critical results to be communicated promptly.
Reagent concentration and quality: why 1% carbol fuchsin
This is the part most study notes get out of date. A 2016 letter by CDC and partner authors reports that the WHO, The Union and partners revised the recommended carbol fuchsin concentration to 1%, replacing the previously recommended 0.3%, because 1% has been shown to be more reliable and in particular improves sensitivity. The same authors note that the original stain concentration was given as 1% carbol fuchsin, and state that visualisation of AFB by the ZN method depends primarily on the quality of the basic fuchsin. They add that the consensus GLI manual, based on WHO and Union agreement, replaces the older manuals.1
Phenol matters too. A 2003 Journal of Clinical Microbiology letter states that the role of phenol is critical in the staining process, and notes that diluting 1% carbol fuchsin with water to 0.3% also lowered the phenol to about 1.7%.3 If a laboratory’s older manual still says 0.3%, that is a reason to check the manual against the current handbook, not a reason to change a validated method unilaterally.
Where it matters for labs and clinics: a QC and safety checklist
The Oman Ministry of Health SOP is a useful model of what a laboratory writes down around the stain itself:2
- Controls: known positive and negative control slides with each batch and with each new reagent preparation.
- Biosafety: all specimens are treated as potentially infectious, and suspected mycobacteria are handled in a Class II cabinet (see biosafety cabinet classes and the biosafety cabinet definition). Heat fixing does not kill Mycobacterium species.
- Result handling: positive results are critical results, and a random 5% of negative smears are sent to a reference laboratory for review.
- Known variability: accuracy depends on specimen quality, heating, staff competency and microscope maintenance.
The SOP accepts sputum (early morning preferred), bronchoalveolar lavage, endotracheal secretions, gastric aspirate, cerebrospinal fluid, urine and tissue as specimens.
Common misunderstandings
“A negative Z-N smear rules out infection.”
No. The Oman SOP states that Ziehl-Neelsen staining is less sensitive than auramine-phenol staining and molecular methods.2 A negative smear is one result, interpreted alongside other tests.
“Heat fixing makes the slide safe.”
No. The SOP is explicit that heat fixing does not kill Mycobacterium species, so slides stay potentially infectious.2
“0.3% carbol fuchsin is still the standard.”
That is the older recommendation. The current WHO/Union guidance cited above is 1%.1
“Z-N staining and H&E or Giemsa staining do the same job.”
They answer different questions. Tissue stains such as H&E show morphology; the ZN stain is chosen specifically to highlight acid-fast organisms. For broader context on the organisms involved, see what bacteriology is.
FAQ
What does Z-N staining stand for?
Ziehl-Neelsen staining, named for the acid-fast technique that uses heated carbol fuchsin; the original stain concentration was described as 1% carbol fuchsin.1
Is AFB staining the same as Z-N staining?
AFB staining is the general name for staining acid-fast bacilli. Z-N is one AFB stain; fluorochrome methods such as auramine-phenol are another.
What colour are the bacilli?
Red rods on a blue or green background, depending on whether methylene blue or malachite green is the counterstain.2
Why is heat used?
Heat helps drive the primary stain into the waxy, mycolic-acid-rich cell wall.2
Sources
- Angra P, Ridderhof J, Tahseen S, Van Deun A. “Read the new microscopy handbook: even the Ziehl-Neelsen technique has changed.” Int J Tuberc Lung Dis 2016;20(4):567. CDC author manuscript, stacks.cdc.gov/view/cdc/45952. Checked 2026-10-29.
- Ministry of Health, Oman. “Ziehl-Neelsen (ZN) stain for AFB microscopy SOP”, MoH/DGSMC/SOP/060/Vers.01, May 2023, moh.gov.om. Checked 2026-10-29.
- Angra P, Ridderhof J, Smithwick R. Letter, J Clin Microbiol 2003;41(7):3459, PMC165351. Checked 2026-10-29.
