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Surrogate AST methods

In this video Gunnar Kahlmeter discusses surrogate AST methods
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Hello, everyone. Today, we’re going to talk about surrogate phenotypic methods.
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With surrogate AST methods, we mean methods which are different from the reference broth microdilution method, but which are calibrated to the reference broth microdilution method. The most commonly used surrogate methods are disc diffusion, gradient tests, and semi-automated devices such as Phoenix, Vitek 2, and MicroScan.
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This diffusion of the output is an inhibition zone diameter measured in millimetres. And that is, in turn, converted to an S, I, and R via a zone diameter breakpoint, which is part of the EUCAST breakpoint table. That breakpoint is calibrated to the MIC breakpoint in the same table. Gradient tests provide inhibitory concentrations, which lack the status of formal MIC values, but which are taken to the MIC breakpoint and interpreted into an S, an I, or an R. You need to remember that making the concentration scale of a gradient test work for a multitude of species and across the whole scale is very tricky.
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So you need to be a little bit suspicious of gradient tests, and you should ask the manufacturer exactly which species and for which concentrations on the scale the manufacturer is willing to guarantee the result. Semi-automated devices often have a very limited array of inhibitory concentrations in the plastic cards or trays, which are the basis for the method and in which the organism is placed. In one way or another, the inhibition of growth is measured as part of the method. And then, using breakpoints in the machine, these are converted to– the concentrations are converted to S, I, and R. Dilutions are never full scale simply because the manufacturer and the customer wants to save on material, number of cards.
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They want to put as many antibiotics on the card as is possible. And that means compromising. So a lot of the data you get out of these machines are MIC values of some sort listed as less than or equal to or more than a certain or a named value. These values lack the status of MIC values, and they are never accepted by organisations such as EMA, EUCAST, et cetera as true MIC values. Quality control of procedures is also very difficult. If you’re interested in setting up broth microdilution or disc diffusion and you’re a beginner and you don’t know exactly how to do it, you can go to the EUCAST web page.
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And, as you can see, there are numerous documents on media preparation and MIC determination and distribution methodology and how to implement this in your lab, et cetera, et cetera.

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.

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