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Chapter 5.8 : Etest

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Abstract:

A standardized inoculum of bacteria is swabbed onto the surface of a Mueller-Hinton agar (MHA) plate. Etest strips containing a continuous gradient of antimicrobial concentrations are placed on the agar surface. After overnight incubation, an elliptical zone of inhibition forms as the antimicrobial agent inhibits growth. The MIC is read where growth intersects the Etest strip. MICs can be interpreted as susceptible, intermediate, or resistant based on the tables in the NCCLS MIC standard ( ).

Citation: Garcia L. 2010. Etest, p 69-76. In Clinical Microbiology Procedures Handbook, 3rd Edition. ASM Press, Washington, DC. doi: 10.1128/9781555817435.ch5.8
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Citation: Garcia L. 2010. Etest, p 69-76. In Clinical Microbiology Procedures Handbook, 3rd Edition. ASM Press, Washington, DC. doi: 10.1128/9781555817435.ch5.8
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References

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1.NCCLS. 2003. Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically, 6th ed. Approved standard M7-A6. NCCLS, Wayne, Pa.
2. Baker, C. N.,, S. A. Stocker,, D. H. Culver,, and C. Thornsberry. 1991. Comparison of the Etest to agar dilution, broth microdilution, and agar diffusion susceptibility testing techniques by using a special challenge set of bacteria. J. Clin. Microbiol. 29:533538.
3. Jorgensen, J. H.,, and J. D. Turnidge,. 2003. Antibacterial susceptibility tests: dilution and disk diffusion methods, p. 11081127. In P. R. Murray,, E. J. Baron,, J. H. Jorgensen,, M. A. Pfaller,, and R. H. Yolken (ed.), Manual of Clinical Microbiology, 8th ed. ASM Press, Washington, D.C.

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Citation: Garcia L. 2010. Etest, p 69-76. In Clinical Microbiology Procedures Handbook, 3rd Edition. ASM Press, Washington, DC. doi: 10.1128/9781555817435.ch5.8
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Quick reference list for performing Etest

This table is divided into two sections. The first part lists aerobic organisms and rapidly growing facultatively anaerobic organisms for which the FDA has cleared certain Etest strips for testing in the clinical laboratory. The second part lists organisms for which Etest studies have been performed but for which Etest strips have not yet been granted clearance by the FDA. Testing of Etest strip-organism combinations that are not yet FDA cleared should be done with caution. Always check the Etest package insert or contact the manufacturer to determine the FDA clearance status for testing an agent against a particular isolate from a patient. Not every drug is cleared for each organism group listed. Abbreviations: CMHA, chocolate, MHA; DCS, direct colony suspension standardized to a 0.5 McFarland turbidity standard; DCS/LPG, direct colony suspension or log-phase growth standardized to a 0.5 McFarland turbidity standard; GC, GC agar base with 1% defined supplement; HTM, test medium.

All incubation temperatures are 34 to 35°C.

Use a photometric device to adjust turbidity. This step is critical in avoiding erroneous results due to under-or overincoulation.

And other penicillinase-stable penicillins.

Use broth for inoculum suspension.

Generally, only a beta-lactamase test is warranted for

Use a 1.0 McFarland standard for inoculum preparation (manufacturer's recommendation).

Citation: Garcia L. 2010. Etest, p 69-76. In Clinical Microbiology Procedures Handbook, 3rd Edition. ASM Press, Washington, DC. doi: 10.1128/9781555817435.ch5.8

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