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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. 

Resource

GLI Practical Guide to TB Laboratory Strengthening

 

Assessment

Question​

 

 

Answer 1​

 

 

Answer 2

 

 

Answer 3

 

 

Answer 4

 

 

Correct answer​

 

 

Correct explanation​

 

 

Page id​

 

 

Part of Pre-test​

 

 

Part of Post-test​

 

 

False/unexpected DST is due to which of the following reasons?

 

 

High inoculum

 

 

Mixed culture

 

 

Improper drug addition

 

 

All of the above

 

 

4

 

 

False/unexpected DST is caused by high inoculum, mixed culture, and improper drug addition.

 

 

 

YES

 

 

YES

 

 

 

 

 

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