How to Read and Interpret an Etest Strip for MIC Determination

The Etest (also known as gradient diffusion method) is a quantitative antimicrobial susceptibility testing technique that uses a predefined, continuous concentration gradient of an antimicrobial agent immobilized on a plastic strip to determine the minimum inhibitory concentration (MIC) of a bacterial isolate. The MIC is read directly from the strip at the point where the elliptical zone of inhibition intersects the strip's scale. This method is particularly useful when a precise MIC value is needed for clinical decision-making, epidemiological surveillance, or when testing fastidious organisms that may not grow reliably in broth microdilution systems. Unlike disk diffusion, which provides only qualitative susceptible/intermediate/resistant (SIR) categorization, the Etest yields a continuous MIC value that can be compared directly to clinical breakpoints from organizations such as CLSI or EUCAST.

At a Glance

Scientific Principle of Gradient Diffusion

The Etest strip contains a predefined, continuous concentration gradient of an antimicrobial agent immobilized on one side of an inert plastic strip. The concentration range typically spans 15 log₂ dilutions (e.g., 0.002–32 µg/mL or 0.016–256 µg/mL, depending on the drug). When the strip is placed on an inoculated agar plate, the antimicrobial agent immediately begins to diffuse into the agar medium, establishing a stable concentration gradient perpendicular to the strip's long axis. After incubation, bacterial growth appears as a lawn on the agar surface, except in a teardrop-shaped or elliptical zone surrounding the strip where the antimicrobial concentration exceeds the MIC for that isolate. The MIC is read at the precise point where the edge of the inhibition zone intersects the numerical scale printed on the strip.

The gradient diffusion method differs fundamentally from disk diffusion (Kirby-Bauer) in that the concentration gradient is continuous rather than logarithmic from a single disk. This allows direct MIC reading rather than requiring a zone diameter–MIC regression curve. The method also differs from broth microdilution, which uses discrete twofold dilution steps; Etest can theoretically detect MIC values between standard doubling dilutions, though in practice results are rounded to the next standard dilution for interpretation.

Materials and Instrumentation Choices

Agar Medium Selection

The choice of agar medium is critical for Etest performance. Mueller-Hinton agar (MHA) is the standard medium for non-fastidious bacteria, as recommended by CLSI and EUCAST. For fastidious organisms, supplemented media are required:

• Streptococcus pneumoniae: MHA with 5% defibrinated sheep blood (MH-F) or cation-adjusted MHA with 5% sheep blood
• Haemophilus spp.: Haemophilus test medium (HTM) or chocolate MHA
• Neisseria gonorrhoeae: GC agar base with 1% defined growth supplement
• Anaerobic bacteria: Brucella blood agar supplemented with hemin and vitamin K₁

The depth of the agar should be 4.0 ± 0.5 mm (approximately 25 mL in a 100 mm plate). Agar that is too thin or too thick can alter the diffusion gradient and produce inaccurate MIC values. The agar surface must be dry before inoculation; excess moisture can cause the strip to float or produce a distorted inhibition zone.

Inoculum Preparation

The inoculum should be prepared using the direct colony suspension method from an 18–24 hour pure culture. The turbidity should be adjusted to a 0.5 McFarland standard (approximately 1–2 × 10⁸ CFU/mL for most bacteria). Inoculum density significantly affects MIC results: an overly heavy inoculum may produce falsely elevated MICs, while a light inoculum may produce falsely low MICs. For fastidious organisms, the inoculum may need to be adjusted to 1.0 McFarland, as specified by the manufacturer's instructions.

Strip Handling and Storage

Etest strips must be stored at −20°C in their original sealed packaging until use. Once opened, strips should be used within the manufacturer's specified timeframe (typically 1–2 weeks when stored at 2–8°C in a desiccated container). Before opening the package, allow it to reach room temperature (15–30 minutes) to prevent condensation on the strips. Handle strips only by the upper end (the end with the highest concentration) using sterile forceps or gloved hands. Do not touch the gradient area of the strip.

Incubation Conditions

Standard incubation is at 35 ± 2°C in ambient air for 16–20 hours. Some organism–drug combinations require specific conditions:

• Methicillin-resistant staphylococci: Incubate at 33–35°C for 24 hours (oxacillin, cefoxitin)
• Vancomycin for staphylococci: Full 24-hour incubation
• Anaerobic bacteria: Incubate in anaerobic conditions for 24–48 hours
• Yeast: Incubate at 35°C for 24–48 hours

Controls

Quality control (QC) strains must be tested with each batch of Etest strips or at least weekly, as required by laboratory policy. The following QC strains are commonly used:

The QC MIC must fall within the acceptable range published by CLSI or EUCAST for the specific antimicrobial agent. If the QC result is out of range, the test is invalid and must be repeated. Common causes of QC failure include incorrect inoculum density, expired strips, contaminated medium, or incorrect incubation conditions.

Conceptual Workflow

Step 1: Inoculation of the Agar Plate

Dip a sterile cotton swab into the standardized inoculum suspension. Rotate the swab against the tube wall above the liquid level to remove excess inoculum. Streak the swab evenly across the entire agar surface in three directions (rotating the plate approximately 60° between each streaking) to ensure confluent growth. Allow the inoculated plate to dry for 5–15 minutes with the lid slightly ajar. The surface should appear uniformly moist but without visible droplets.

Step 2: Application of Etest Strips

Using sterile forceps, remove the Etest strip from its packaging. Place the strip onto the dried agar surface with the MIC scale facing upward and the highest concentration end toward the edge of the plate. Press gently on the strip with the forceps to ensure complete contact with the agar surface. Avoid trapping air bubbles beneath the strip. For multiple strips on one plate, place them radially with the high-concentration ends toward the plate center, maintaining at least 20–25 mm between strip centers to prevent overlapping inhibition zones.

Step 3: Incubation

Invert the plate and incubate under appropriate conditions (see above). Do not stack plates more than four high to ensure uniform temperature and air circulation.

Step 4: Reading the MIC

After incubation, examine the plate for a clear, elliptical inhibition zone surrounding the strip. The MIC is read at the point where the edge of the inhibition zone intersects the numerical scale on the strip. The reading point is determined by the lowest concentration that completely inhibits visible growth.

Quality Checks Before Reading

Before recording the MIC, verify the following:

1. Confluent growth: The agar surface should show a uniform lawn of growth. Sparse growth or colonies within the inhibition zone indicate problems with inoculum or medium.
2. Elliptical zone shape: The inhibition zone should be symmetrical and teardrop-shaped. Asymmetrical zones may indicate uneven strip contact or agar surface irregularities.
3. No contamination: Check for colonies with different morphology that could indicate mixed culture.
4. QC within range: Confirm that the QC strain MIC falls within the acceptable range for the batch.

Result Interpretation

Reading the MIC Value

The MIC is read at the intersection of the inhibition zone edge with the MIC scale. For most strips, the scale is printed in µg/mL. The reading point is determined as follows:

• Clear endpoint: Read at the point where growth stops abruptly. The MIC is the value at the intersection.
• Ghosting or microcolonies: If faint growth (ghosting) or microcolonies appear within the inhibition zone, read at the point where the majority of growth is inhibited. Some guidelines recommend reading at the point where there is an 80% reduction in growth compared to the lawn.
• Double endpoints: When two distinct inhibition zones are present (e.g., due to heteroresistance or mixed population), read the MIC at the outer (more susceptible) zone. Document the inner zone separately in comments.

Handling Specific Reading Scenarios

Comparison to Breakpoints

Once the MIC is recorded, compare it to the clinical breakpoints established by CLSI or EUCAST for the specific organism–drug combination. The MIC value is interpreted as:

• Susceptible (S): MIC ≤ susceptible breakpoint
• Intermediate (I): MIC between susceptible and resistant breakpoints (or within the intermediate range)
• Resistant (R): MIC ≥ resistant breakpoint

Note that breakpoints may differ between CLSI and EUCAST, and laboratories must follow the standard adopted by their institution.

Troubleshooting

Limitations

The Etest method has several important limitations that users must understand:

1. Cost: Etest strips are significantly more expensive per test than disk diffusion or broth microdilution, making them impractical for high-volume screening.

2. Variable agreement with reference methods: Essential agreement (EA, MIC within ±1 doubling dilution) and categorical agreement (CA, SIR classification) vary widely depending on the organism–drug combination. For example, a multicenter study of Streptococcus pneumoniae found EA ≥90% for most Etest–drug combinations but CA as low as 74% for penicillin on certain agar brands [3]. Similarly, fosfomycin Etest showed only 45.7–80% CA compared to agar dilution, depending on the resistance profile of the isolates [2].

3. Reading subjectivity: Endpoint interpretation can be subjective, particularly with bacteriostatic agents, trailing endpoints, or heteroresistant populations. Different readers may record different MIC values for the same test.

4. Not suitable for all antimicrobials: Some antimicrobials (e.g., fosfomycin, daptomycin) require supplementation of the medium (glucose-6-phosphate for fosfomycin; calcium for daptomycin) that may not be compatible with the Etest format.

5. Limited automation: Unlike broth microdilution, which can be automated with plate readers, Etest reading remains a manual process requiring trained personnel.

6. Agar brand effects: The composition of Mueller-Hinton agar can vary between manufacturers, affecting diffusion and MIC results. A study of aztreonam–ceftazidime-avibactam synergy testing found that the side-by-side Etest method had 72% CA with broth microdilution and 28% major errors, while the strip cross method showed 98% CA with only 2.6% major errors [1]. This highlights the importance of method validation for specific applications.

Documentation

Each Etest result should be documented with the following information:

• Patient/sample identifier
• Organism identification (species level)
• Antimicrobial agent tested
• MIC value (in µg/mL)
• Interpretation (S/I/R based on applicable breakpoints)
• QC strain and result (must be in range)
• Date of test
• Technician initials
• Any comments (e.g., "heteroresistance noted," "repeat testing recommended")

Results should be recorded in the laboratory information system (LIS) or on standardized worksheets. Out-of-range QC results must be documented with corrective actions taken.

Biosafety Considerations

Etest procedures involve handling live bacterial cultures and should be performed in a Biosafety Level 2 (BSL-2) laboratory when working with human pathogens. For routine teaching laboratories using BSL-1 organisms (e.g., Escherichia coli K-12, Micrococcus luteus), standard microbiological practices apply:

• Perform all work in a certified biological safety cabinet (BSC) when aerosol-generating procedures are involved
• Wear appropriate personal protective equipment (lab coat, gloves, eye protection)
• Decontaminate work surfaces before and after procedures with an appropriate disinfectant (e.g., 10% bleach or 70% ethanol)
• Dispose of all contaminated materials (plates, swabs, strips) in biohazard waste containers
• Wash hands thoroughly after handling cultures

For clinical isolates or organisms with known pathogenicity, follow institutional biosafety protocols and the guidelines in the CDC/NIH publication Biosafety in Microbiological and Biomedical Laboratories (BMBL), 6th Edition [6]. Laboratories working with recombinant or synthetic nucleic acid molecules must also comply with the NIH Guidelines [7].

Frequently Asked Questions

Q1: What should I do if the inhibition zone edge falls exactly between two markings on the Etest strip?

A: When the inhibition zone edge falls between two concentration markings, round up to the next higher concentration. For example, if the edge is between 0.5 and 1.0 µg/mL, record the MIC as 1.0 µg/mL. This conservative approach ensures that the reported MIC is not lower than the true value, which could lead to a false susceptible interpretation.

Q2: Can I use Etest strips on blood agar or chocolate agar instead of Mueller-Hinton agar?

A: Etest strips are validated for use on specific media, typically Mueller-Hinton agar or its supplemented variants. Using non-standard media may alter the diffusion gradient and produce inaccurate MIC values. For fastidious organisms, use the manufacturer-recommended supplemented medium (e.g., MH-F for Streptococcus pneumoniae, HTM for Haemophilus). Always verify with QC strains when using alternative media.

Q3: How do I interpret an Etest when there are microcolonies growing within the inhibition zone?

A: Microcolonies within the inhibition zone can indicate heteroresistance (a subpopulation of resistant cells within a susceptible population) or contamination. First, subculture the microcolonies to confirm they are the same organism. If heteroresistance is confirmed, read the MIC at the point where the majority of growth is inhibited (the outer zone edge). Document the presence of microcolonies and the inner zone MIC in the comments. For clinical decision-making, the higher MIC (inner zone) may be more relevant, as the resistant subpopulation could emerge during therapy.

Q4: Why does my Etest result sometimes differ from broth microdilution for the same isolate?

A: Discrepancies between Etest and broth microdilution (BMD) are well-documented and can arise from several factors: (1) Etest uses a continuous gradient while BMD uses discrete twofold dilutions, so the MIC may fall between BMD concentrations; (2) the agar environment differs from broth, affecting drug diffusion and bacterial growth; (3) some antimicrobials bind to plastic or agar components; (4) reading subjectivity, especially with trailing endpoints. Essential agreement (MIC within ±1 dilution) is typically expected in ≥90% of tests, but categorical agreement can be lower for certain organism–drug combinations [2][3]. When discrepancies occur, repeat both methods and consider using a reference method (BMD) for confirmation.

References and Further Reading

1. Khan A, Manuel C, Maynard R, Humphries RM. Evaluation of two gradient diffusion tests to determine susceptibility to aztreonam and ceftazidime-avibactam in combination. 2025. PubMed ID: 39918316. https://pubmed.ncbi.nlm.nih.gov/39918316/

2. Závora J, Kroneislová G, Kroneisl M, Adámková V. Fosfomycin-Overcoming Problematic In Vitro Susceptibility Testing and Tricky Result Interpretation: Comparison of Three Fosfomycin Susceptibility Testing Methods. 2024. PubMed ID: 39596744. https://pubmed.ncbi.nlm.nih.gov/39596744/

3. Martens S, Cuypers L, Bélik F, et al. Multicenter comparison of Etest, Vitek2 and BD Phoenix to broth microdilution for beta-lactam susceptibility testing of Streptococcus pneumoniae. 2024. PubMed ID: 38801483. https://pubmed.ncbi.nlm.nih.gov/38801483/

4. Salam MA, Al-Amin MY, Pawar JS, Akhter N, Lucy IB. Conventional methods and future trends in antimicrobial susceptibility testing. 2023. PubMed ID: 36852413. https://pubmed.ncbi.nlm.nih.gov/36852413/

5. Bender JK, Fleige C, Funk F, Moretó-Castellsagué C, Fischer MA, Werner G. Linezolid Resistance Genes and Mutations among Linezolid-Susceptible Enterococcus spp.-A Loose Cannon? 2024. PubMed ID: 38275330. https://pubmed.ncbi.nlm.nih.gov/38275330/

6. CDC and NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL), 6th Edition. U.S. Department of Health and Human Services, 2020. https://www.cdc.gov/labs/bmbl/index.html

7. National Institutes of Health. NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules. https://osp.od.nih.gov/policies/biosafety-and-biosecurity-policy/nih-guidelines-for-research-involving-recombinant-or-synthetic-nucleic-acid-molecules/

8. National Center for Biotechnology Information. NCBI Bookshelf: Molecular Biology and Laboratory Methods. https://www.ncbi.nlm.nih.gov/books/

Related Articles

• Etest for MIC Determination: How to Read and Interpret Results Accurately
• How to Perform an Etest for MIC Determination: Protocol and Interpretation
• Understanding CLSI Breakpoints for Disk Diffusion and MIC Interpretation
• Minimum Inhibitory Concentration (MIC) Determination by Broth Microdilution: A Practical Protocol
• How to Perform an Antibiotic Susceptibility Test Using the Etest Method
• Disk Diffusion Method for Antimicrobial Susceptibility Testing: Protocol and Interpretation