Indophenol method for acetaminophen in serum examined

We show that an indophenol-based colorimetric method for acetaminophen (Clin Toxicol 15: 67-73, 1979) has less than maximal sensitivity because of (a) incomplete hydrolysis, related to suboptimal acid concentration; (b) suboptimal hydrolysis time; and (c) possible deterioration of the ammonium hydroxide reagent after 2.5 months. We have modified the method by optimizing duration of hydrolysis and color development. Results by the method correlate linearly with acetaminophen concentrations to 250 mg/L (1.66 mmol/L). Moreover, the modified method is free from interference by compounds in serum from uremic patients.