Gaby-Hadley Reagent A is used for detection of oxidase activity of a bacterial culture along with Gaby-Hadley reagent B (R028).The oxidase test is a biochemical reaction that assays for the presence of cytochrome oxidase.When the reagent is oxidized by cytochrome c, it changes from colorless to a dark blue or purple compound, indophenol blue. Neisseria gonorrhoeae ATCC 19424 Deep purple to blue Positive ; Staphylococcus aureus ATCC 25923 No colour change Negative.