Surrogate AST methods

Surrogate AST methods are methods which are different from the reference broth micro dilution method which are calibrated to the reference micro broth dilution method.

The most commonly used methods are:

• Disk diffusion

• Gradient tests such as Etest and MTS

• Semi-automated devices such as Phoenix, Vitek2 and MicroScan

Disk diffusion

The output of this method is zone diameters (mm) which are converted to S, I and R via zone diameter breakpoints which are calibrated to the MIC breakpoints.

Gradient tests

These provide inhibitory concentrations that lack the status of formal MIC values which are taken to the MIC breakpoints and interpreted into S, I or R. It is important to remember, making the concentration scale of a gradient test work for a multitude of species and across the whole scale is difficult.

Semi-automated devices

Semi-automated devices often have a limited array of inhibitory concentrations in ‘plastic cards/trays’ which are the basis for the method and where organism is placed. Inhibition of growth is measured as part of the method and breakpoints within the machine converted these to S, I and R. However, dilutions are never full-scale as often the manufacturer/customer want to save on material and increase the number of antibiotics. This method lacks the status of MIC values and are not accepted by organisations such as EUCAST as true MIC values.

If you are interested in setting up MIC or disk diffusion and you are a beginner, the EUCAST webpages here have information and documents about media preparation, MIC determination and distribution terminology and how to set this up in your laboratory.

Kiestra is an automated system that tracks barcoded plates of medium to learn more about this, check out the video in the related links section below.