E-Test Strips

E-Test Strips: MIC Testing Guide for Children

The E-test strip, also called the epsilometer test, is a laboratory tool used to find the exact Minimum Inhibitory Concentration (MIC) — the lowest amount of an antibiotic needed to stop bacteria from growing. It combines a plastic strip carrying a gradient of drug concentrations with standard culture plate methods, giving laboratories a precise number rather than a general category when guiding infection treatment, including in children.

Introduction

Choosing the right antibiotic dose is not always straightforward. Some infections involve bacteria that are only partly sensitive to a drug, and a general "sensitive or resistant" label may not be enough to guide treatment safely.

The E-test strip solves this by measuring the actual concentration of a drug required to stop bacterial growth, expressed as a number in micrograms per milliliter (µg/mL). This number, the MIC, can then be compared with standard breakpoints to guide dosing decisions.

The test is performed entirely on a laboratory culture plate. It is non-invasive to the child at the point of testing, since no radiation or direct contact with the patient is involved — the only patient-related step is the original collection of a sample such as blood, urine, or a swab, done separately before the test.

History of the Device

Antibiotic susceptibility testing has been performed since the mid-20th century using disk diffusion methods, where paper disks soaked in antibiotics are placed on an agar plate and the size of the inhibition zone around each disk is measured.

Disk diffusion was useful for broad categorization but could not give an exact MIC value. Traditional MIC testing relied on broth microdilution, a labor-intensive method requiring many test tubes with decreasing drug concentrations.

The E-test was developed in the late 1980s by AB Biodisk, a Swedish diagnostics company, as a simpler alternative that combined the practicality of disk diffusion with the numeric precision of dilution testing. The technology was later acquired by bioMérieux, a diagnostics company that continues to manufacture and refine E-test products.

Over time, E-test strips were developed for a very wide range of antibiotics, antifungal agents, and even some agents used against mycobacteria. Pediatric microbiology laboratories adopted the method largely unchanged from adult laboratories, since the test is performed on the organism itself rather than directly on the patient, though sample volume needed for culture in infants and small children is often a practical consideration.

Today, E-test strips remain widely used in clinical and research laboratories worldwide, particularly where an exact MIC value is important, such as for a slow-growing organism or when treating a resistant infection in a child.

Purpose of the Device and Where It Is Used

The E-test strip measures how much of a specific antibiotic (or antifungal agent) is needed to inhibit the visible growth of a particular bacterium or fungus that has already been identified from a patient sample.

Determining precise antibiotic dosing for children with resistant or unusual infections
Testing bloodstream infections (bacteremia) where treatment margins are narrow
Testing slow-growing or fastidious organisms that are difficult to test with automated systems
Guiding treatment in infections affecting the central nervous system, bones, or joints, where drug penetration and dosing precision matter
Supporting antimicrobial stewardship programs by confirming resistance patterns
Research studies tracking antimicrobial resistance trends

These strips are typically used in hospital clinical microbiology laboratories, reference laboratories, and research centers. They are not used directly at home or at the bedside.

Key Point: The E-test is a measurement tool, not a diagnostic tool. It does not identify what organism is causing an infection — that step happens earlier, through culture and identification. The E-test only measures how sensitive an already-identified organism is to a specific drug.

Different Types of the Device

Standard Antibiotic E-test Strips

These cover common antibacterial agents such as penicillins, cephalosporins, and aminoglycosides, used for everyday bacterial infections.

Antifungal E-test Strips

Designed for antifungal agents, these help determine MIC values for fungal infections, which can be more difficult to test using disk diffusion alone.

Extended-Range or Combination Strips

Some strips cover a wider concentration gradient or combine two drugs at a fixed ratio, useful for studying drug combinations or resistant organisms.

Type	Common Use	Typical Setting
Standard antibacterial strip	Common bacterial infections	Hospital microbiology lab
Antifungal strip	Fungal infections	Hospital or reference lab
Extended-range/combination strip	Resistant organisms, research	Reference or research lab

Parts and Components of the Device

Plastic Carrier Strip

A thin, inert plastic strip that holds the dried antibiotic gradient and provides a stable surface for handling and reading.

Antibiotic Concentration Gradient

A continuous range of drug concentrations printed along the strip, from very low to very high, allowing the exact inhibition point to be read as a number.

Reference Scale

Printed numeric markings along the strip corresponding to MIC values in µg/mL, used to read the result where the inhibition zone crosses the strip.

Agar Culture Plate

The plate on which the organism is grown and the strip is applied; plate type varies depending on the organism being tested.

Component	Function	Replacement Interval
Plastic carrier strip	Holds drug gradient, single use	Used once, then discarded
Antibiotic gradient	Creates measurable concentration range	Fixed at manufacture
Agar plate	Growth medium for the organism	Prepared fresh for each test

How the Device Works

The organism to be tested is spread evenly across the surface of an agar plate, forming what is called a bacterial lawn. The E-test strip is then placed on top, gradient side down.

The drug diffuses out of the strip into the agar, creating a concentration gradient that mirrors the printed scale. During incubation, usually overnight, the organism grows everywhere except where the drug concentration is high enough to stop it.

This creates an elliptical, teardrop-shaped zone of no growth around the strip. The point where the edge of this ellipse crosses the strip corresponds to the exact **MIC** value, read directly from the printed scale.

Step-by-Step User Guide

Prepare the inoculum: A standardized suspension of the identified organism is prepared to match a set turbidity standard.
Inoculate the agar plate: The suspension is spread evenly over the entire surface of the plate using a sterile swab.
Apply the E-test strip: Once the plate surface has dried slightly, the strip is placed gradient-side down using sterile forceps or an applicator.
Incubate the plate: The plate is placed in an incubator at the appropriate temperature, typically for 16 to 24 hours, depending on the organism.
Read the inhibition ellipse: After incubation, the point where the elliptical zone of no growth intersects the strip is located.
Record the MIC value: The number at the intersection point is read from the printed scale and recorded as the MIC in µg/mL.
Interpret against breakpoints: The MIC value is compared with standard reference breakpoints to classify the organism as sensitive, intermediate, or resistant to that drug.

Correct reading requires proper training, adequate lighting, and familiarity with normal versus irregular growth patterns. Manufacturer instructions for each specific strip and organism combination should always be followed closely.

Precautions and Possible Dangers

Incorrect inoculum density can produce an inaccurate or unreadable MIC result
Some organisms produce irregular or hazy growth patterns that can make the ellipse difficult to read
Strips must be stored and used within their labeled shelf life and temperature range
Contamination of the plate can invalidate results and delay treatment decisions
Results must always be interpreted alongside clinical judgment, not used in isolation

A delayed or misread MIC result in a seriously ill child, such as one with bloodstream infection or meningitis, can delay appropriate antibiotic adjustment. Laboratories should flag any unusual, borderline, or unexpected results promptly to the treating clinical team.

How to Keep the Device Safe and Well Maintained

Store strips at the manufacturer-recommended temperature, often refrigerated, and check expiry dates before use
Keep strips protected from moisture and light in their original packaging until use
Follow calibration and quality control procedures using known reference organisms on a regular schedule
Maintain incubators at correct, verified temperatures with routine servicing
Record and back up all MIC results in the laboratory information system
Keep software used for result tracking and reporting updated as recommended by the laboratory system provider

Interactive Tool

MIC Result Interpretation Checker — Enter the MIC value obtained and the standard breakpoint for the drug and organism being tested.

MIC value (µg/mL):

Susceptible breakpoint (µg/mL):

This tool gives a simplified illustration only and does not replace professional laboratory interpretation or clinical guidance.

Interactive FAQ

Is the E-test safe for babies and children?

Yes. The test is performed entirely in a laboratory on a sample already collected, such as blood, urine, or a swab. It does not involve any direct contact with the child during the test itself.

How long does the E-test take to give a result?

Most E-test results are available within 16 to 24 hours, which is the time needed for the bacteria to grow visibly on the agar plate after the strip is applied.

What are the different types of E-test strips?

E-test strips are made for many different antibiotics, antifungal agents, and even some antituberculosis drugs, each with a gradient specific to that drug's expected concentration range.

Does the E-test involve radiation or invasive procedures on the child?

No. The strip is applied to a laboratory culture plate, not to the child. The only invasive step, if any, is the original sample collection, which is a separate procedure.

Can the E-test diagnose what infection a child has?

No. The E-test does not identify what is causing an infection. It is used after a specific bacterium or fungus has already been identified, to measure how sensitive that organism is to a chosen drug.

What does the child feel during the E-test itself?

Nothing, because the E-test is performed on a sample in a lab. Any discomfort a child may experience relates only to the earlier collection of that sample, not to the strip test.

How is the E-test different from simpler susceptibility methods?

Older disk diffusion methods usually classify bacteria as sensitive, intermediate, or resistant, while the E-test provides an exact numeric concentration value, giving more detailed information for dosing decisions.

Who typically performs and reads the E-test?

Trained clinical microbiology laboratory technologists or technicians prepare the culture plates, apply the strips, and read the results under the supervision of a laboratory physician or microbiologist.

How accurate is the E-test compared to other methods?

The E-test is generally considered highly reliable and correlates well with reference broth microdilution methods, though accuracy can vary somewhat depending on the organism and drug being tested.

Can the E-test be used for unusual or slow-growing organisms?

Yes, one of its advantages is that it can be adapted for fastidious or slow-growing organisms that are harder to test using some automated systems.

How often is the E-test performed?

It is performed as needed, whenever a laboratory or treating clinician requires a precise concentration value rather than a general category, such as in complicated or resistant infections.

What happens if a child is anxious about having a sample collected for this test?

Since the E-test itself happens after collection, any anxiety relates to the sample collection step, where comfort measures and distraction techniques are commonly used to help a child stay calm.

Other Methods and Alternatives

Method	Basic Principle	Common Use
Disk diffusion	Paper disk with fixed drug amount; inhibition zone size measured	General screening, categorical results
Broth microdilution	Serial dilutions of drug in liquid culture	Reference standard MIC method
Automated susceptibility systems	Machine-read growth curves in liquid media	High-throughput hospital laboratories
E-test strip	Gradient diffusion from a strip on agar	Precise MIC for individual isolates

Frequently Overlooked Points Worth Knowing

A single MIC result reflects one point in time; repeated testing may be needed if an infection does not respond as expected
MIC breakpoints can differ between reference guidelines and may be updated periodically
Accuracy can be affected by inoculum density, plate depth, and incubation conditions
An MIC value alone does not account for how well a drug penetrates a particular site, such as the brain or bone
Results should always be reviewed alongside the child's clinical response, not treated as a stand-alone answer

How to Read and Understand the Results

Result Parameter	What It Means
MIC value (µg/mL)	The lowest drug concentration that visibly stops the organism from growing
Susceptible	The organism is likely to respond to standard dosing of that drug
Intermediate	The organism may respond only with higher dosing or at certain body sites
Resistant	The organism is unlikely to respond to that drug at standard doses

Reference breakpoints used to interpret MIC values may vary by organism, drug, and the guideline body used by the laboratory. These are general descriptions, not fixed clinical cutoffs for every situation.

Advantages and Limitations

Advantages

Provides an exact numeric MIC value rather than only a broad category
Relatively simple to perform compared with manual dilution methods
Adaptable to a wide range of organisms, including fastidious or slow-growing ones
Useful for confirming or clarifying results from automated systems

Limitations

More expensive per test than basic disk diffusion
Requires careful technique to avoid inaccurate readings
Turnaround time still depends on organism growth rate, which can take a day or more
Some organisms produce irregular growth patterns that complicate interpretation

Troubleshooting Common Problems

Problem	Possible Cause	Suggested Solution
No visible inhibition ellipse	Inoculum too light, or organism resistant to entire strip range	Repeat with corrected inoculum; confirm with alternate method
Hazy or irregular ellipse edge	Uneven plate surface or organism growth pattern	Repeat test on a fresh, evenly poured plate
Strip lifting off the agar	Plate surface too wet when strip applied	Allow plate surface to dry briefly before applying strip
Inconsistent results between repeats	Variation in inoculum density or incubation temperature	Standardize inoculum preparation and verify incubator calibration

When to Contact the Manufacturer or Service Provider

Strips consistently fail to produce readable results despite correct technique
Suspected defect in strip packaging, printing, or gradient
Questions about interpretation for an unusual organism-drug combination
Needing updated technical or safety data sheets for the product

Tip: Keep a record of lot numbers, expiry dates, and any related quality control results. Having these on hand speeds up any technical support conversation with the manufacturer.

Checked and reviewed by a pediatrician

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