A diagnostic test kit for detecting the presence or absence of an enzyme (e.g., leukocyte esterase) within a test sample is provided. The test kit comprises a substrate that is capable of being cleaved in the presence of the enzyme to release a nucleophilic aromatic compound. The kit also comprises a lateral flow device that comprises a chromatographic medium. The chromatographic medium defines a detection zone within which is contained a first reagent (e.g., diazonium ion) that is capable of reacting with the nucleophilic aromatic compound to form a second reagent (e.g., aromatic azo compound). The second reagent exhibits a color that is different than the color of the first reagent. The lateral flow device also includes an absorbent material located adjacent to the chromatographic medium, the absorbent material receiving the test sample after flowing through the chromatographic medium.