biuret reaction n.
A test to determine the concentration in biological fluids of biuret, peptides that contain more than three amino acids or that contain amido groups in which such compounds react with copper sulfate in highly alkaline solutions to produce a violet color. Also called biuret test.
Inasmuch as most natural proteins contain several such groups, the biuret reaction is a very general test for proteins.
The absence of a biuret reaction would point in that direction and the question must await further study.
Amongst these is the biuret reaction: a pink colour obtained by adding a trace of copper sulphate and an excess of caustic soda.