The Ziehl-Neelsen (ZN) test, also known as acid-fast staining, is a microbiological technique used to detect and identify Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB).

Principle:

1. Mycobacteria have a unique cell wall that retains certain dyes even after acid treatment.
2. The ZN test uses a combination of carbol fuchsin (red dye) and acid to stain the bacteria.

Procedure:

1. Sputum or tissue sample collection
2. Smear preparation on a glass slide
3. Fixation with heat or methanol
4. Staining with carbol fuchsin
5. Acid treatment with hydrochloric acid or sulfuric acid
6. Counterstaining with methylene blue or malachite green
7. Microscopic examination

Results:

- Red-stained rods (acid-fast bacilli) indicate presence of Mycobacterium tuberculosis
- Blue or green counterstain highlights background material

Interpretation:

- 1-9 acid-fast bacilli per 100 fields: Scanty
- 1-9 acid-fast bacilli per 10 fields: 1+
- 1-9 acid-fast bacilli per field: 2+
- 10 or more acid-fast bacilli per field: 3+

Note: The ZN test is a simple, inexpensive, and widely used method for TB diagnosis, but it may not detect all cases, especially those with low bacterial loads.

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