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ID Learning Unit — The Etest

Posted Jun 12 2012 9:00pm

Every year I attend on the general medical service, so it gives me a chance to work directly with the medical residents — and to brush up on my non-ID related Internal Medicine.

In exchange for what they teach me, each day on rounds I try to tell them about at least one ID-related thing that they may not know. Since I do this in the Spring, they’re awfully sharp. Fortunately I’ve been doing this ID stuff a long time, so can usually find something.

Today’s learning unit was the “Epsilometer Test, or “Etest” — that brilliantly simple way of estimating the MIC of an antibiotic to an organism by using a drug-impregnated strip that has a gradient of concentration, from high to low. Find where the “elipse” of inhibition crosses the strip, and presto! There’s your MIC.

We take for granted that this method is readily available and, for the most part, clinically valid. It’s worth remembering, however, that when it debuted in the late 1980s/early 1990s it was considered pretty slick and not entirely trustworthy.

And there’s still some debate about how reliable it is, especially with MRSA .

If you want to learn how to do it, enjoy the movie.

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