Specimen Processing for TB Cultures

Sputum specimens are viscous materials contaminated with normal flora. Therefore, processing involves pre-treatment of the sputum specimens via:​

• Digestion: To free the TB bacilli from the mucus in which they may be embedded.
• Decontamination: To eradicate normal flora that grows more rapidly than TB bacilli, and would interfere with the ability to recover TB bacilli.
• Homogenization: Of the digested materials.
• Concentration: Of the TB bacilli by centrifugation before smear preparation and media inoculation.

Figure: Factors Affecting the Efficacy of Processing Methods

Methods of Culture Specimen Processing

Various processing methods are used for TB specimens; amongst them, the most common methods are:

1. N-acetyl-L-cysteine - sodium hydroxide (NALC-NaOH) method: It is the mildest decontamination method which can kill about 33% of mycobacteria in a clinical specimen. It can be used with both liquid and solid media.
2. Petroff’s sodium hydroxide method: It is a harsher method – it can kill up to 70% of mycobacteria in specimens. Although useful with highly contaminated specimens, it is not recommended for use with liquid MGIT media.

Resources

• MGIT Procedure Manual, Mycobacteria Growth Indicator Tube (MGIT) Culture and Drug Susceptibility Demonstration Projects, FIND Training Manual.
• GLI Training Module on Specimen Processing, STOP TB Partnership.

Kindly provide your valuable feedback on the page to the link provided HERE