Question
Epsilometer OR E-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 plastic strip impregnated with a continuous, predefined gradient of antibiotic concentration (highest at one end, lowest at the other, with concentration values printed along the strip) is placed onto an agar plate evenly inoculated with a standardized (0.5 McFarland) suspension of the test organism, similar to disc diffusion. During incubation, the antibiotic diffuses from the strip, creating the same concentration gradient in the surrounding agar 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 intersecting the strip at a specific point — the concentration printed at that intersection point is read directly as the MIC.
Advantages: provides a precise, quantitative MIC (unlike categorical disc diffusion); technically simple, requiring no more specialized equipment than routine disc diffusion; particularly useful for fastidious/slow-growing organisms and where an exact MIC is clinically important (e.g., guiding vancomycin dosing).
Limitations: considerably more expensive per test than standard disc diffusion, limiting routine universal use to selected clinical situations rather than first-line testing of all isolates.