The Fish Embryo Acute Toxicity (FET) test with the zebrafish Danio rerio is designed to determine acute toxicity of chemicals on embryonic stages of fish.
The FET test takes a position between the acute fish test and the fish egg test. While the acute fish test only detects effects on adult fish and the fish egg test only on early egg development, the FET test evaluates the entire egg and embryonic development up to hatching.
For the FET tests newly fertilised zebrafish eggs are exposed to the test chemical for a period of 96 hrs. Every 24 hrs, up to four apical observations are recorded as indicators of lethality: (i) coagulation of fertilised eggs, (ii) lack of somite formation, (iii) lack of detachment of the tail-bud from the yolk sac, and (iv) lack of heartbeat. At the end of the exposure period, acute toxicity is determined based on a positive outcome in any of the four apical observations recorded, and the LC50 is calculated.