Overview
E-test (and similar gradient strips) place a predefined antimicrobial gradient on agar; the inhibition ellipse intersects the strip at an MIC value.
Clinical Importance
Obtain MIC results for drugs or organisms not well served by routine panels
Confirm or refine automated AST results
Specimen Requirements
- Specimen: pure culture isolate of the organism to be tested
- Inoculum: prepare to the McFarland standard specified by the method (commonly 0.5 McFarland for many disk/MIC methods)
- Media: use CLSI/EUCAST-recommended medium (e.g., Mueller-Hinton agar ± blood for fastidious organisms)
- Controls: include QC organisms on the validated schedule
Analytical Method
A standardized lawn is prepared on appropriate agar; the gradient strip is applied; after incubation the MIC is read where the ellipse edge meets the strip scale, following manufacturer reading rules (including trailing growth guidance).
Instruments Used
Automated analyzers
- Automated strip image readers in selected laboratories
Manual methods
- Manual Etest/gradient strip application and visual MIC reading
Reference Values
Microbiology results are interpreted against organism identification, specimen quality, clinical context, and current CLSI and/or EUCAST interpretive criteria (including CLSI M100 for susceptibility where adopted). Reporting conventions and critical-notification lists are institution-defined; verify against the performing laboratory SOP and current guideline editions.
MIC interpretive criteria follow CLSI/EUCAST for the organism–drug pair. Manufacturer instructions define reading of ellipses, colonies in zones, and trailing.
Interpretation
Common Causes of Increased Results
- Higher strip MIC suggests reduced susceptibility; apply official breakpoints
Common Causes of Decreased Results
- Lower strip MIC suggests greater in vitro susceptibility; still categorize with current tables
Clinical Significance
- Selected fastidious organism AST
- Supplemental testing for critical agents (e.g., when panel drugs are limited)
Factors Affecting Results
Pre-analytical
- Wrong medium or atmosphere for the organism
Analytical
- Misreading ellipse intersections
- Ignoring manufacturer rules for colonies within the ellipse
Post-analytical
- Reporting colonizers as pathogens without clinical/site correlation
- Wrong interpretive criteria (CLSI vs EUCAST; outdated breakpoints)
- Failure to notify critical positive cultures per institutional policy
Advantages and Limitations
Advantages
- Flexible single-drug MIC on agar
- Useful problem-solving tool
Limitations
- Higher consumable cost per drug
- Reading subjectivity for some agents
- Not identical to broth microdilution for every drug–bug pair
Clinical Pearls
If the ellipse is ragged, do not invent a precise MIC; follow the reading rule or repeat.
References
Authoritative textbooks, guidelines, and reviews supporting this laboratory test reference. Verify reference intervals, critical limits, and method details against institutional protocols and current guideline editions.
Textbooks
- Carroll KC, Pfaller MA, Landry ML, et al., eds. Manual of Clinical Microbiology. ASM Press (verify current edition).
- Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 15th ed. Elsevier; 2020.
- Mahon CR, Lehman DC, Manuselis G. Textbook of Diagnostic Microbiology. 6th ed. Elsevier; 2018.
- Carroll KC, Hobden JA, Miller S, et al. Jawetz, Melnick, & Adelberg's Medical Microbiology. 28th ed. McGraw-Hill; 2019.
- Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2021.
CLSI and Professional Guidelines
- Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing. CLSI document M100 (verify current edition and institutional adoption).
- Centers for Disease Control and Prevention / NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL). Current edition (verify institutional adoption).
- Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing. CLSI document M100 (verify current edition and institutional adoption).
- Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Disk Susceptibility Tests. CLSI document M02 (verify current edition).
- Clinical and Laboratory Standards Institute. Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically. CLSI document M07 (verify current edition).
- European Committee on Antimicrobial Susceptibility Testing (EUCAST). Breakpoint tables and documents (verify current version and institutional adoption).
WHO Publications
- World Health Organization. Laboratory and diagnostic guidance relevant to clinical microbiology practice (verify current WHO publication for the method).
Frequently Asked Questions
Common questions about using this E-test laboratory test Medical Laboratory Library reference in Medical Laboratory Science education.
What is the E-test laboratory test reference used for?
This Medical Laboratory Library page summarizes the clinical importance of E-test. Review the Clinical Importance section for why the test is ordered in laboratory medicine practice.
Where can I find E-test specimen requirements?
See the Specimen Requirements section on this page for specimen information related to E-test. Confirm collection details against your laboratory protocol.
Where are E-test reference values listed?
See the Reference Values section on this Laboratory Library page. Reference ranges can vary by method and population, so verify intervals with your laboratory.
How do I interpret E-test results?
Use the Interpretation section for laboratory interpretation framing, including increased and decreased result patterns when provided. Do not treat educational text as a patient-specific diagnosis.
What analytical method is used for E-test?
See the Analytical Method section for principle and method notes. Method details may differ by instrument and institutional SOP.
Are related laboratory tests linked from E-test?
Yes. The Related Laboratory Tests section lists connected references for continued lookup in the Medical Laboratory Library.
How can I continue learning after reading E-test?
Use Continue learning links on this page for related Academy lessons, medical laboratory quizzes, or laboratory case studies when available for this Medical Microbiology topic.
What are the clinical pearls for E-test?
See the Clinical Pearls section for concise laboratory practice notes associated with E-test. Apply pearls alongside institutional protocols.
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