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Definiciones y puntos de corte del antibiograma

En este vídeo, el profesor Gunnar Kahlmeter nos da a conocer las definiciones y los puntos de corte del antibiograma.
Hello, everyone. I’m Gunnar Kahlmeter. I’m a clinical bacteriologist. I’ve spent a large part of my life with antimicrobial susceptibility testing and breakpoints. And, today, we’re going to talk about definitions and breakpoints.
Breakpoints distinguish between treatable and non-treatable organisms. Since the reference method is MIC by broth microdilution, the breakpoint is always a concentration at which a therapeutic success can be expected. The breakpoint will distinguish between treatable and non-treatable organism, as I said, and it will do that by categorising each organism as either susceptible, susceptible at increased exposure, the I, or resistant, the R, for an agent. A susceptibility test report may look something like this. In this case, Staphylococcus aureus is the infecting organism. So here is a very typical example. These are the MIC distribution– this is the MIC distribution of Streptococcus pneumoniae.
And you can see that the committee has decided to categorise all organisms with MICs of 0.06 and below as S, organisms between 0.12 and 2 as susceptible at increased exposure, and organisms at 4 and above as resistant, meaning that, whatever you do, you’re not going to successfully treat those organisms with, in this case, benzylpenicillin. Breakpoints are expressed as S less than or equal to a certain concentration in milligrammes per litre or R more than a certain concentration again in milligrammes per litre. Anything in between those two values is categorised as I, susceptible, increased exposure. And, of course, surrogate methods, methods which may measure activity in millimetres or some other measurement, will have corresponding breakpoints in that particular measurement.
To determine these breakpoints, the committee needs some tools. And these are some of the tools, the more important tools, that are needed. We need to know from the very beginning, which species are relevant for us. And we need to have MIC distributions and preferably ECOFFs of these relevant species. We need to know, are we going to treat urinary tract infections only or are we going to treat orthopaedic infections, pulmonary infections, because that will affect how we discuss and determine the breakpoints. We also need to know, is this an oral treatment only? Or is it only IV? Or is it maybe just topical?
We need to know something about the dose and the dosing intervals to be able to calculate a variety of indices in pharmacokinetics and pharmacodynamics. And also it’s important to know whether the maximum attainable concentration is the most important thing for the drug to be effective or whether it’s the time during which the concentration in the dosing interval is actually above the MIC. And then it’s good to have clinical evidence for effect. And, most of the time, we will be very happy to have good data to support that wild-type organisms, namely, organisms without resistance mechanisms, can be treated with the agent in question and that there is actually good evidence for that.
And, at times, we’re lucky, and we can find evidence that some non-wild-type organisms can be treated successfully with increased exposure or even standard exposure, as in ciprofloxacin. That will actually be able to treat some organisms with let’s say one mutation against fluoroquinolones. This is the EUCAST breakpoint table or a small section of it just as an example. And you can see down the columns that there are MIC breakpoints, S less than or equal to, R more than. In the left-hand column, there is an array of different beta-lactam antibiotics. And then you can see the zone diameter breakpoints that will correspond to those MIC breakpoints.
And, by going into the table, a clinical lab can say, well, you know, I have an MIC of 8 for ampicillin. And that renders my E. coli susceptible. So I will report it as S. Or I have a zone diameter of 14, or let’s say 18. And that again will render it S. So the table is the fundament for the clinical laboratory who wants to report on their susceptibility tests.

Los antibiogramas son extremadamente importantes para determinar qué antimicrobianos tienen más probabilidades de ser eficaces contra diferentes microorganismos. En este vídeo, el profesor Gunnar Kahlmeter nos da a conocer las definiciones y los puntos de corte del antibiograma.

Los puntos de corte se utilizan para distinguir entre microorganismos tratables y no tratables. Un punto de corte es una concentración en la que se puede esperar un logro terapéutico. Estos permiten clasificar los microorganismos en los siguientes grupos:

  • S de sensible

  • I por sensible cuando se incrementa la exposición

  • R de resistente

A continuación, se muestra un ejemplo de un informe de sensibilidad de Staphylococcus aureus: Meticilina R Ceftarolina S Ciprofloxacina I Vancomicina S

Los datos de distribución de CMI (mg/l) se utilizan para determinar los rangos de S, I y R. Como puntos de corte se expresan como: CMI: S ≤ X mg/l y R > Y mg/l, los valores de CMI entre X e Y se clasifican como I.

Para determinar los puntos de corte, hay algunas cosas que necesitamos saber, tales como:

  • Qué especies son relevantes

  • Distribuciones de CMI

  • Qué infecciones son relevantes

  • Modo de administración: i.v., oral o tópico junto con la dosis y los intervalos de esta

  • Farmacocinética y farmacodinamia del agente

  • Las pruebas clínicas de los efectos incluyen los datos de los organismos de tipo natural y no natural (a menudo resistentes) junto con el grado de eficacia se relaciona con diferentes valores de CMI.

EUCAST dispone de tablas de puntos de corte en las que se incluyen los puntos de corte de la CMI. Estos se utilizan como rangos de referencia que determinan cómo describir el antibiograma de microorganismos particulares que se pueden encontrar aquí.

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Introducción a la microbiología práctica

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