Procedure for Reading MGIT DST Results

Procedure for Reading MGIT DST Results

Once the Antimicrobial Susceptibility Testing (AST) sets are loaded/entered, the BACTEC MGIT 960 instrument continually monitors all tubes for increased fluorescence. When the growth unit (GU) of the Growth Control (GC) tube reaches ≥400 within the timed protocol, the instrument marks the DST as complete and interprets the results. GU values of the drug-containing vials are evaluated, and final results are reported as Susceptible or Resistant as follows:

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

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

 The steps involved in reading the MGIT DST results are listed below: 

1. Completed AST sets (indicated by a red “+” on the drawer) are removed from the respective drawers, and an “Unloaded AST Set” report is printed.

2. All the unloaded AST sets are matched with the printed report and any discrepancies observed are resolved. 

3. Before reporting, all ‘resistant’ tubes are observed visually for evidence of contamination. ZN stain is performed on any suspicious tube and subculture to a Blood Agar or Brain Heart Infusion agar Plate. Additionally, when drug resistance is observed and the patient’s isolate was not previously resistant to the drug, tube(s) are tested with ZN and Blood Agar or Brain Heart Infusion Agar Plate to ensure that growth is not due to contaminants or MOTT.

4. DST reports are recorded as  “Susceptible” or “Resistant” on the internal lab worksheet/book and the laboratory register. 

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