Gold(III) bromide can be used as a testing reagent for the presence of ketamine.
0.25%  0.1M NaOH is prepared to give a brownish-yellow solution. Two drops of this are added to a spotting plate and a small amount of ketamine is added. The mixture gives a deep purple color within approximately one minute, which turns to a dark, blackish-purple color within approximately two minutes.
Acetaminophen, ascorbic acid, heroin, lactose, mannitol, morphine, and sucrose all cause an instant colour change to purple, as do other compounds with phenol and hydroxyl groups.
Nothing commonly found in conjunction with ketamine gave the same colour change in the same time.
"The initial purple color may be due to the formation of a complex between the gold and the ketamine. The cause for the change of color from purple to dark blackish-purple is unknown; however, it may be due to a redox reaction that produces a small amount of colloidal gold."