Sodium hypochlorite centrifugation technique is used to concentrate the AFB present in sputum, as it increases the chances of detecting AFB in sputum smears. Sodium hypochlorite (NaOCl) is recommended for liquefying the sputum, as it kills M. tuberculosis, making the handling of specimens safe for laboratory technicians.
Procedure of sodium hypochlorite centrifugation technique
- Transfer 1-2 ml of the purulent part (i.e. containing any caseous materials) of the sputum to a screw cap universal bottle or other containers of 10-20 ml capacity.
- Add an equal volume of concentrated sodium hypochlorite (bleach) solution and mix well.
- Leave at room temperature for 10-15 minutes, shaking at intervals to break down the mucus in the sputum
- Add about 8 ml of distilled water and mix well.
- Centrifuge at 3000 g for 15 minutes. When centrifugation is not possible, leave NaOCl treated sputum to sediment overnight.
- Remove and discard supernatant fluid using a glass Pasteur pipette or plastic bulb pipette. Mix the sediment.
- Transfer a drop of well-mixed sediment to a clean scratch-free glass slide and spread the sediment to make a thin preparation and allow air-drying.
- Heat fix the smear and stain it using the Ziehl-Neelsen technique, and examine microscopically.