In June 2013, the Joint Meeting agreed that where possible, a more consistent use of the term “test chemical” describing what is being tested should now be applied in new and updated Test Guidelines. However, it is important to note that previously adopted OECD Test Guidelines will still use the terms "test item", "test compound", "test substance" or other similar term to describe what is being tested.”
The intention of this proposal is not to provide a new definition of the term “chemical(s)”, but rather to be consistent with the UN definition of it when applicable, i.e. in Test Guidelines that make reference to the UN GHS for Classification and Labelling where “chemical” means “substance and mixture”.
OECD Test Guidelines are recommendations. In OECD Test Guidelines, the term “should” is used to mean an action that is recommended and should be done in the way described.
“Should” is not reflecting an option that can be taken or not without incidence on the outcome of the assay. Any deviation from the Guideline should be reported in the study report.