EC50 toxicity testing and percent inhibition describe different aspects of a biological response. Percent inhibition expresses the measured effect at a specified test concentration. EC50 identifies the concentration estimated to produce a defined 50% effect.

In bioluminescent assays using Aliivibrio fischeri, the relevant response is a reduction in bacterial light emission. Understanding the distinction helps laboratories interpret screening results, compare samples, and recognize when a concentration–response estimate is supported by the data.

For the broader method and its applications, see Bioluminescent Toxicity Testing: Methods and Applications.

What does percent inhibition measure?

Percent inhibition describes how much the measured light response decreases relative to the response expected under the assay’s control conditions.

The calculation must follow the selected method, including any correction for changes in control luminescence during exposure. Comparing the initial and final sample readings alone may therefore produce a different value from the method-corrected result.

A percent inhibition result belongs to a particular concentration and exposure time. Reporting “40% inhibition” without these details leaves an important part of the measurement undefined.

RIVM’s published luminescent bacteria protocol treats response calculation, exposure conditions, validity criteria, and reporting as connected elements of the assay.

An illustrative calculation

Assume that, after all corrections required by the method:

• The expected light response without inhibition is 1,000 units.
• The sample’s measured response is 600 units.

The response is 400 units below the expected value. This corresponds to 40% inhibition:

(1 − 600 ÷ 1,000) × 100 = 40%

This simplified example illustrates the meaning of the percentage. It is not an instrument operating procedure, a complete assay calculation, or a product performance claim.

The result describes the biological response at the concentration tested. It does not mean that the sample contains 40% contaminants.

What does EC50 mean in a bioluminescent assay?

For a luminescence inhibition assay reporting this endpoint, EC50 is the concentration estimated to reduce light emission by 50% under defined conditions.

A concentration series provides the relationship between sample concentration and response. The applicable calculation procedure uses these data to estimate the concentration associated with the specified effect.

Da Silva and colleagues used concentration–response curves when investigating individual stressors and mixtures with A. fischeri. Their experimental approach illustrates why an effect concentration depends on a series of responses rather than one isolated measurement.

Consider a hypothetical aqueous sample tested at several final concentrations. If the valid concentration–response analysis supports an EC50 near 20% sample by volume, that means a mixture containing approximately 20% of the original sample produces the defined 50% light inhibition under those conditions.

The two percentages have different meanings:

• 20% describes the concentration of the original sample in the test mixture.
• 50% describes the inhibition endpoint.

An EC50 expressed as a percentage of an environmental sample does not identify the concentration of an individual pollutant.

Percent inhibition and EC50 answer different questions

For routine reporting, keep the result and its conditions together. For example, a fixed-concentration screening result should identify the concentration tested, while an EC50 report should identify the concentration units and estimation procedure.

A lower EC50 can indicate greater potency

When samples are tested using comparable procedures and the same concentration basis, a lower EC50 indicates that less sample is required to produce the specified effect.

Suppose two hypothetical samples produce:

• Sample A: EC50 of 10% sample by volume.
• Sample B: EC50 of 40% sample by volume.

Under otherwise comparable conditions, Sample A reaches the endpoint at a lower sample concentration.

That comparison concerns potency in the selected assay. It does not establish a corresponding difference in human health risk, identify the responsible chemicals, or predict effects on every aquatic organism.

The reporting basis also matters. An EC50 in percent sample by volume cannot be directly compared with a value expressed in milligrams of a test substance per liter without understanding what each concentration represents.

EC50, IC50, and LC50 are not interchangeable

Endpoint terminology must follow the method.

Environment Canada’s solid-phase sediment reference method uses IC50 for the concentration estimated to inhibit light production by 50%. Its reporting basis includes a dry-weight concentration for sediment.

This is a continuously measured biological effect. It is not a count showing that half of the bacteria died.

LC50, by contrast, refers to a lethal concentration associated with death in 50% of a defined test population over a specified exposure period.

When reviewing a report, check the endpoint definition alongside its abbreviation.

What if the sample never reaches 50% inhibition?

A valid test may show inhibition below 50% throughout the tested concentration range. In that situation, the data may not support a numerical EC50 within that range.

The appropriate report should follow the method’s rules for qualified results, bounds, or endpoints that cannot be determined. A software-generated estimate outside the tested range requires scrutiny.

The CRED criteria developed by Moermond and colleagues emphasize the reliability of concentration–response data and distinguish interpolation within tested concentrations from extrapolation beyond them.

Three situations should remain distinct:

• A valid test with measurable inhibition below the endpoint.
• A valid test with no measurable inhibition under the tested conditions.
• An invalid test that failed required performance criteria.

None should be replaced by an invented EC50 of zero.

Conditions that matter when comparing results

Before comparing reports, confirm that they represent compatible measurements.

Temperature and salt concentration deserve attention. Correia Faria and Snook demonstrated that both affected bacterial luminescence and the kinetics of the response in their study of a kinetic toxicity assay. Their findings support careful control of assay conditions, rather than assuming that measurements made under different conditions are directly equivalent.

A useful review also checks:

• Sample preparation and concentration basis.
• Organism and reagent identification.
• Exposure duration.
• Control performance.
• Replication and uncertainty.
• Calculation method and deviations.

The CRED reporting framework identifies experimental design, exposure conditions, and statistical analysis as important information for evaluating ecotoxicity data.

Match the endpoint to the testing objective

A fixed-concentration inhibition result may answer a screening question. A concentration–response study addresses how the response changes with concentration and whether an effect concentration can be estimated.

Define the required output before selecting the workflow. This helps determine the sample preparation, number of test concentrations, documentation, and analysis needed.

For questions about BioLight Toxy, BioLight Reagents, and documentation relevant to your application, contact Aqua Science with your sample type, testing objective, and required reporting method.

References

• Verweij, W., et al. RIVM, 2010. Protocols belonging to “Toxicity measurements in concentrated water samples”. Report 607013011/2010.
• Da Silva, L. F. B. A., et al., 2018. Monitoring Aquaculture Water Quality: Design of an Early Warning Sensor with Aliivibrio fischeri and Predictive Models. Sensors, 18(9), 2848.
• Environment Canada. Sediment toxicity using luminescent bacteria: data analysis and interpretation.
• U.S. EPA. Health Effects Notebook Glossary.
• Moermond, C. T. A., et al., 2016. CRED: Criteria for reporting and evaluating ecotoxicity data. Environmental Toxicology and Chemistry, 35, 1297–1309.
• Correia Faria, E.; Snook, R. D., 2004. The effect of temperature and NaCl concentration on the kinetic method of toxicity determination using Vibrio fischeri. Journal of Environmental Monitoring, 6, 953–956.