According to the OECD test guidelines, a range finding test needs to be performed prior the definitive test or a limit test (100mg/l for acute and 10mg/l for chronic test) based on whether there is any effect or not.
The test guidelines also suggests that we can decide on the dose concentrations fro the acute tests. However, I have not found any further information on that. There are some instructions for the human health toxicity studies by ECHA but nothing on the ecotox studies.
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The dose concentration selected for the chronic tests should be based on the results of the acute test. The concentration chosen should have a noticeable effect on the test organisms but not cause lethal effects. The selection of the appropriate concentration will depend on the toxicology of the substance being tested and the species used in the test.
Suppose the acute test results indicate that the substance has high toxicity. If the substance has low toxicity, the dose concentration selected for the chronic test can be higher. In that case, the chosen dose concentration for the chronic test should be at or below the no observed effect concentration (NOEC) identified in the acute test.
It is important to consider the purpose of the chronic test and the endpoints being evaluated when selecting the dose concentration. For example, if the test is focused on reproductive effects, a concentration that results in a noticeable impact on reproduction but does not cause lethal effects should be selected.
In general, choosing a range of concentrations that cover the range from the NOEC identified in the acute test to a concentration that causes noticeable effects is a general practice. This will allow for a more thorough evaluation of the chronic toxicity of the substance.
It is also important to consider the test species used in the acute and chronic tests. Suppose the species used in the acute test differ from those in the chronic test. In that case, the acute test results may not be directly applicable to the chronic test, and the dose concentration may need to be adjusted accordingly.
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