False positive results may occur when patients are on large doses of chloropromazine, and may occur in the presence of metabolites of phenazopyridine. When these compounds are present, the urine becomes red. Metabolites of etodolac may cause false-positive or atypical results.
False-negative bilirubin reagent strip results are often due to testing a specimen that is not fresh. Bilirubin breaks down when exposed to light. Indoxyl sulfate (indican) can produce a yellow orange-to-red color response, which may interfere with the interpretation of a positive or negative reaction. Positive nitrite due to a urinary tract infection may also cause a false-negative result.