Troubleshooting False Resistant DST Results

Reason

Corrective action

Culture .contaminated or mixed culture

Ensure the purity of the culture before DST

Organism inoculum too heavy

• Ensure that precise pipettes are used for dilutions and addition to tubes.

• Ensure that the starting suspension is > 1.0 McFarland (in case of solid media).

Suspension not homogeneous or had clumps

Ensure that the starting suspension is uniformly homogenized.

Growth control dilution  too light

Ensure that proper dilutions are used for inoculating the growth control tube.

Drug not reconstituted properly

Ensure that the lyophilized drugs are reconstituted as per the manufacturer’s instructions.

Drug not stored properly

Ensure that the reconstituted drugs are stored as per the manufacturer’s recommendations.

Drug not added to the tube

Ensure that the exact volume ( 100 µl) of the drug is added to the tube.

MGIT tube not mixed properly after the addition of the organism suspension

Ensure that MGIT tubes are mixed properly before loading the AST carrier into the MGIT instrument.

Organism could be borderline-resistant

Check the Growth units and repeat DST if GU is >95 but <100. 

Question​

Answer 1​

Answer 2

Answer 3

Answer 4

Correct answer​

Correct explanation​

Page id​

Part of Pre-test​

Part of Post-test​

During DST, what is the volume of drug which must be added to the MGIT tubes?

100 µl

1000 µl

2.5 ml

4.0 ml

1

Ensure that the exact volume ( 100 µl) of the drug is added to the tube.

YES

YES