Principle of MGIT DST Reading

Principle of MGIT DST Reading:

Two Mycobacterial Growth Indicator Tubes (MGITs)  are inoculated with the test culture. Then, a known concentration of a test drug is added to one of the MGIT tubes (drug-containing tube), and growth is compared with the MGIT tube without the drug (growth control).

If the test drug is active against the isolated mycobacteria, it will inhibit the growth, and thus there will be suppression of fluorescence, while the growth control will grow uninhibited and will have increasing fluorescence.

The BACTEC MGIT 960 instrument continually monitors all tubes for increased fluorescence. Analysis of fluorescence in the drug-containing tubes compared to the fluorescence in the Growth Control tube is used to determine susceptibility results.

When the growth unit (GU) of the growth control reaches 400 within 4-13 days (SIRE) or 4-21 days (PZA), the GU values of the drug-containing vials are evaluated.

• S = Susceptible = the GU of the drug tube is less than 100 

• R = Resistant = the GU of the drug tube is 100 or more

Resources

1. Mycobacteriology Laboratory Manual

2. Revised National TB Control Programme Training Manual for Mycobacterium tuberculosis Culture & Drug susceptibility testing, Central TB Division, Ministry of Health and Family Welfare, GOI 

Assessment