An **AFB (Acid-Fast Bacilli) Detection by Smear Examination using ZN (Ziehl-Neelsen) Stain for Body Fluids** is a rapid, fundamental diagnostic test used to detect the presence of mycobacteria (such as *Mycobacterium tuberculosis*) in various non-blood bodily fluids.

It is one of the oldest, fastest, and most cost-effective methods available to immediately screen for tuberculosis outside of the lungs (extrapulmonary TB).

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### How the Test Works

* **The Fluids Tested:** This test can be performed on various sterile body fluids, including **Pleural fluid** (around the lungs), **Cerebrospinal fluid (CSF)** (around the brain/spinal cord), **Ascitic/Peritoneal fluid** (abdominal cavity), **Synovial fluid** (joints), or **Pericardial fluid** (around the heart).
* **Concentration:** Because body fluids often contain a very low concentration of bacteria compared to sputum, the laboratory first spins the fluid in a high-speed centrifuge to concentrate any bacterial cells into a pellet at the bottom.
* **The ZN Staining Process:**
1. A smear of the concentrated fluid is fixed onto a glass slide and stained with a bright red dye called **Carbol Fuchsin**.
2. The slide is gently heated, which allows the dye to penetrate the thick, waxy, lipid-rich cell walls (mycolic acid) characteristic of mycobacteria.
3. The slide is washed with an acid-alcohol solution. Standard bacteria lose their color (decolorize), but mycobacteria resist the acid wash and retain the bright red dye—hence the name **"Acid-Fast"**.
4. A blue counterstain (Methylene Blue) is applied to color the background cells and non-acid-fast debris.