Epsilometer test

The Epsilometer test (usually abbreviated Etest) is a laboratory test used by microbiologists to determine whether or not a specific strain of bacterium or fungus is susceptible to the action of a specific antibiotic. This is most commonly used in the setting of medicine, where a particular organism has been found to infect a patient, and the doctor treating the patient is seeking guidance on what antibiotic is suitable.

History
The principle of the epsilometer test was first described in 1988 and was introduced commercially in 1991 by AB Biodisk.

Principle
The E-test is basically an agar diffusion method.

The E-test utilises a rectangular strip that has been impregnated with the drug to be studied. A lawn of bacteria is inoculated onto the surface an agar plate and the E-test strip is laid on top; the drug diffuses out into the agar, producing an exponential gradient of the drug to be tested. There is an exponential scale printed on the strip. After 24 hours of incubation, an elliptical zone of inhibition is produced and the point at which the ellipse meets the strip gives a reading for the inhibitory concentration of the drug.

Validation
The test has been validated for many organisms against the broth/agar dilution method and shown to have excellent correlation. This is a partial list of organisms and antibiotics for which the test has been validated.


 * Pseudomonas aeruginosa: amikacin, ceftazidime, gentamicin, piperacillin, ticarcillin, tobramycin,