The growth of contaminant bacteria results in positive fluorescence. Therefore, it is important to observe all fluorescent positive MGIT tubes visually for turbidity and to make an AFB smear. If an MGIT tube broth is heavily turbid, contamination is suspected even if the AFB smear is positive.
Liquid Culture Contamination and Sources of Contamination
MGIT results are interpreted as described in Flowcharts 1-3:
The different steps of tube removal are as follows:
BACTEC MGIT 960 System Growth Detection
Principle:
Breaks in Liquid culture protocol can happen due to deviations from standard operating procedures. Some common breaks in the protocol are shown below:
In Cartridge-based Nucleic Acid Amplification Test (CBNAAT) assay, the basis for detection of rifampicin resistance is the difference between the first or early Cycle threshold (Ct) and the last or late Ct value, of M. tuberculosis-specific probes. Or the difference between probes with the maximum Ct value and minimum Ct value is greater than four.