Troubleshooting False Susceptible DST Results

Reason

Corrective action

Inoculum too light

• 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

Ensure that the starting suspension is uniformly homogenized.

Culture too old

Ensure that the cultures selected for DST are in the proper growth phase.

Growth control grew too fast

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

Proper amount of supplement the not added to the tubes

Ensure that correct amount of supplement (as per the standard protocol) is added to the tubes.

Supplement not stored properly 

Ensure that the supplement is stored as per the manufacturer’s instructions.

Drugs  not reconstituted properly

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

Too much drug added to tubes

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

Incorrect media and/or supplement used for PZA

Ensure that PZA medium and PZA supplement is used for PZA DST.

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​

In the case of solid media, the initial suspension to be used for DST must satisfy which of the following conditions?

> 1.0 McFarland

> 2.0 McFarland

<1.0 McFarland

None of the above

1

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

YES

YES