Question
Epsilometer test
Answer
The Epsilometer test (E-test) is a quantitative method of antimicrobial susceptibility testing that combines the simplicity of disc diffusion with a precise numerical Minimum Inhibitory Concentration (MIC) readout.
Principle: a thin, inert plastic strip is impregnated with a continuous, predefined gradient of antibiotic concentration along its length (highest at one end, lowest at the other, with the actual concentration values printed on the strip). The strip is placed onto an agar plate that has been evenly inoculated with a standardized suspension (0.5 McFarland) of the test organism, similar to disc diffusion technique. During incubation, the antibiotic diffuses from the strip into the surrounding agar, creating the same concentration gradient in the medium as exists along the strip.
Reading the result: bacterial growth is inhibited wherever the local antibiotic concentration exceeds the organism’s susceptibility threshold, producing a characteristic elliptical zone of inhibition that intersects the strip at a specific point. The concentration value printed on the strip at the point where the ellipse edge crosses it is read directly as the MIC for that organism-antibiotic combination.
Advantages: provides a precise, quantitative MIC value (unlike ordinary disc diffusion, which gives only a categorical Sensitive/Intermediate/Resistant result); technically simple, requiring no more specialized equipment than routine disc diffusion; particularly useful for testing fastidious or slow-growing organisms, and for antibiotics/organism combinations where an exact MIC is clinically important (e.g., guiding vancomycin dosing).
Limitations: considerably more expensive per test than standard disc diffusion, limiting its routine use to selected clinical situations rather than universal first-line testing.