The PYR test determines the activity of pyrrolidonyl aminopeptidase, an enzyme produced by Enterococcus faecalis and many other Enterococcus species.

L-pyrrolindonyl-β-naphthylamide impregnated into the strip serves as the substrate for the pyrolidonyl arylamidase. Catabolism of the substrate produces β-naphthylamide which reacts with the PYR reagent (0.01% cinnamaldehyde reagent) to form a bright red color.

Grup D enterococci (including Enterococcus faecalis and Enterococcus faecium) and Streptococcus pyogenes are examples of PYR positive bacteria.