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Etest (previously known as the Epsilometer test) is a way of determining antimicrobial sensitivity by placing a strip impregnated with antimicrobials onto an agar plate. A strain of bacterium or fungus will not grow near a concentration of antibiotic or antifungal if it is sensitive. For some microbial and antimicrobial combinations, the results can be used to determine a minimum inhibitory concentration (MIC). Etest is a proprietary system manufactured by bioMérieux. It is a laboratory test used in healthcare settings to help guide physicians by indicating what concentration of antimicrobial could successfully be used to treat patients' infections.
Etest is a pre-prepared non-porous plastic reagent strip with a predefined gradient of antibiotic, covering a continuous concentration range. It is applied to the surface of an agar plate inoculated with the test strain, where there is release of the antimicrobial gradient from the plastic carrier to the agar to form a stable and continuous gradient beneath and in nearby to the strip.
The time taken for a plate to be ready depends on the m that is being tested, and the conditions of the agar plate. The predefined Etest gradient remains stable for at least 18 to 24 hours; that is, a period that covers the critical times of many species of fastidious and non-fastidious organisms.
After the test, the bacterial growth becomes visible after incubation and a symmetrical inhibition ellipse centered along the strip is seen. The MIC value is read from the scale in terms of μg/mL where the ellipse edge intersects the strip. After the required incubation period, the minimum inhibitory value is read where the edge of the inhibition ellipse intersects the side of the strip. The plate should not be read if the culture appears mixed or if the lawn of growth is too light or too heavy.
Etest MIC endpoints are usually clear-cut although different growth/inhibition patterns may be seen depending on the antifungal or antibiotic used.
These media may require supplemental nutrients to obtain enhanced growth of nutritionally fastidious organisms such as pneumococci, streptococci, Abiotrophia, Haemophilus, gonococci, meningococci and Campylobacter. In general, media recommendations from the Clinical and Laboratory Standards Institute (CLSI) and European Committee on Antimicrobial Susceptibility Testing (EUCAST) are considered appropriate for Etest.
During the 1950s, Hans Ericsson (Professor of microbiology at the Karolinska Hospital and Karolinska Institute, Stockholm), the scientific founder of AB BIODISK, developed a method to standardize the disk diffusion test and to improve its reproducibility and reliability for clinical susceptibility predictions. The inhibition zone sizes from disk test results were compared to MIC values based on the reference agar dilution procedure. The correlation between zone sizes and MIC values was then assessed using regression analysis and regression lines were used for extrapolating zone interpretive limits that corresponded to the MIC breakpoint values that defined susceptible, intermediate and resistant categorical results.
Etest was first presented at the Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) in Los Angeles in 1988 as a novel gradient concept for MIC determinations. In September 1991, Etest was launched globally as a MIC product after receiving the USA Food and Drug Administration (FDA) clearance.
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