A false positive urobilinogen reaction on the reagent strip may occur when substances known to react with Ehrlich's reagent are present in the urine, including porphobilinogen*, sulfonamides and p-aminosalicylic acid. Drugs that contain azo dyes, such as Azo Gantrisin®, have a gold color that masks the reaction, causing a false-positive reaction. Atypical color reactions may be obtained in the presence of high concentrations of p-aminobenzoic acid.
Due to the instability of urobilinogen, a false-negative result may occur if the urine specimen remains at room temperature and exposed to light for an extended period of time. A false-negative result may also occur if formalin is present.
*The urobilinogen reagent strip test is not a reliable method for the detection of porphobilinogen.