Troubleshooting Unexpected/False Profile of DST Isolates

The cause for the unexpected/false profile of DST isolate can be due to various reasons that reflect quality aspects of the laboratory as well as the characteristic of the MGIT growth. The following need to be looked into as cause for unexpected/false profiles of DST isolate:

1. Laboratory procedures:

• Organism inoculum too light ​
• Inoculum not pure
• Suspension not homogeneous
• Culture too old ​
• Growth control dilution too heavy ​
• Supplement not properly added to tubes and stored
• Drugs not reconstituted properly ​
• Too much drug added to tubes
• Pipetting errors
• Tubes not capped tight during incubation such that oxygen can leak into the tube ​

2. Characteristics of MGIT growth
 

• Organism could be borderline-resistant ​
• MTBC mixed with NTM

Quality laboratory procedures for standard operating procedures, reagents and calibrated instruments should be followed to prevent unexpected/false DST profiles. Regarding unexpected results due to growth characteristics, perform definitive identification​ for the growth, perform ZN smear on positive MGIT tubes, identify MTBC, and identify NTMs that show resistance to anti-mycobacterial drugs. 

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