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Revision of OECD 203 Test Guideline

OECD Guidelines are periodically reviewed to match regulatory need, and take into account scientific progress and animal welfare standards. The OECD 203 Guideline describes a method for acute fish exposure to a test item over 96 hours in order to provide knowledge of the relative toxicity of the test item, and use this to support the ecotoxicology risk assessment.

Twice updated since the original publication in 1981, this third revision to the Guideline, published on the 18th June 2019, includes efforts to reduce unnecessary vertebrate testing, suggests more fish species that better represent estuarine and marine exposure, and proposes improvements to the way that sub-lethal effects and any visible abnormalities are recorded during the test. The Guideline also incorporates recommendations of the OECD Fish Toxicity Testing Framework 2011 (OECD, 2012).

In order to reduce animal testing and use resources efficiently, the Guideline suggests alternative means of obtaining information on a test item’s toxicity to fish than the standard acute toxicity test, such as through a weight of evidence approach using methods such as QSAR, read-across, fish embryos (OECD 2013), fish cell lines, and others if reliable. Another alternative method suggested is the use of a threshold approach (OECD, 2010), or the limit test currently included in the Guideline. In the event that such methods are insufficient to fulfil regulatory needs, they can serve as range-finding assessments prior to definitive testing.

Amendments to the wording of the Guideline are also included in the revised version, with the stipulation that test concentrations causing 0 and 100 % mortality be reported being removed to avoid testing additional concentrations solely to demonstrate these mortality values. This should serve to reduce the number of animals involved in each test, in accordance with long-term OECD ambitions.

New guidance on when to use a dilution water control is also included, with this control now able to be omitted where appropriate based on the needs of the relevant regulatory authorities.

In order to better represent the environments in which exposure may occur, new fish species including the European sea bass (Dicentrarchus labrax) and Red sea bream (Pagrus major) among others are included in the Guideline as possible test organisms.

The assessment of sub-lethal endpoints in vertebrate testing is common practice in order to reduce terminal suffering of fish, however there is currently no international consensus as to which sub-lethal observations are indicative and predictive of morbidity and death. The revised OECD Guideline therefore provides tools and suggests methods for the collection and observation of signs leading up to fatal endpoints in order to allow for future consensus on the sub-lethal signs predictive of death.

Together these revisions keep the Test Guideline relevant to current scientific progress, potentially reduce the number of animals involved in testing, and look to the future of predictive assessments of fish health to humanely test on fish.

The revised Guideline can be downloaded from the OECD website:

For advice related to this article, please contact the team in Harrogate on +44 (0)1423 520245 or

Published 10th July 2019
Categories Agrochemicals, News
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