Colorimetric method for the quantitative determination of acetaminophen in serum

A colorimetric method for quantitatively determining acetaminophen in serum is presented. The procedure involves preparing a protein-free filtrate by the addition of trichloroacetic acid, hydrolyzing the acetaminophen in the filtrate to p-aminophenol, and subsequent reaction of p-aminophenol with phenol and NH4OH to form an indophenol blue chromogen. The absorbance at 620 nm of this chromogen follows Beer's law up to acetaminophen concentrations of greater than 50 mg/1. The recovery of acetaminophen from serum is essentially 100% when compared with appropriately prepared standards. Serum acetaminophen concentrations as low as 1 mg/1 can be detected. Seventy drugs were tested for possible interference and none was found to interfere with the method. The coefficient of variation (day-to-day) is 4%. This new quantitative method provides the necessary sensitivity to quantitate therapeutic as well as toxic serum concentrations of acetaminophen.