When fish is no longer fresh, it can easily cause food poisoning. This may appear as a skin rash, which is caused by histamine present in decaying fish. This is a simple method enabling the visualization of histamine levels, allowing inspection by eye.
When red-fleshed fish, such as tuna and sardines, is no longer fresh, the histamine released can be a cause of food poisoning. Histamine extracted from fish is separated by electrophoresis, stained, and the amount present is measured by colour intensity.