Solid and Liquid Culture in TB

Culturing TB Bacilli is well known and historic method for detection/ confirmation of Tuberculosis. It is a highly sensitive and specific phenotypic test; it can detect even a few viable bacilli in the sample (Upto 10 Colony Forming Units- CFUs). TB bacilli multiply in the culture and form colonies of TB bacilli which can are easily be identified.

Based on the growth media used Culture is divided in to two types, Solid and Liquid Culture methods. Types Culture:

• Solid Culture on Lowenstein Jensen media : Historic gold standard culture test. Results take usually upto 2 months (60 days).
• Modern Liquid culture systems: (e.g. BACTEC MGIT 960, BacT Alert or Versatrek etc.) Results take usually up to 42 days. 

Uses

1. Solid culture is the gold standard diagnostic test for TB. But it is not used for the purpose of TB diagnosis due to the long turn around time of 2 months. It is largely used for research purposes where it is used as the baseline test on which the sensitivity and specificity of other tests are calculated.
2. Liquid Culture is being used for follow-up monitoring of patients on drug resistant TB treatment to detect treatment failure. Liquid culture is also used for long term follow up patients who have successfully completed treatment to detect recurrence.
3. Liquid culture is used as a previous step to grow bacilli and obtain isolates prior to Drug Susceptibility Testing.
4. Liquid cultures are also used in TB prevalence surveys for its high sensitivity and specificity

Resources

• Guidelines for Programmatic Management of Drug Resistant Tuberculosis in India 2021

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