False-positive results may occur when patients are on large doses of chlorpromazine 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 results are often due to testing an old specimen exposed to light, leading to the breakdown of bilirubin. High concentrations of ascorbic acid and nitrite may also cause a false-negative result.
Indoxyl sulfate (indican) can produce a pigment that interferes with interpreting a positive or negative reaction.
