False positive acetaminophen concentrations in patients with liver injury

False positive acetaminophen tests may result when enzymatic-colorimetric assays are used, most commonly with bilirubin concentrations > 10 mg/dl, leading to potential clinical errors in this setting. Bilirubin (or possibly other substances in acute liver failure sera) appears to affect the reliable measurement of acetaminophen, particularly with enzymatic-colorimetric assays.