Colorimetry of serum acetaminophen (paracetamol) in uremia.

A popular colorimetric analysis for serum acetaminophen, based on ring-nitration of the drug, is demonstrated to yield both "false positives" and erroneously high results for serum from uremic patients. The interference appears to be anionic at physiological pH, and correlates significantly with serum creatine concentration and with the magnitude of the "anion gap." A modification of the analysis involving extraction of the acetaminophen with ether eliminates the interference. As little as 10 mg of drug per liter can be accurately measured in serum by the proposed procedure. Analytical recovery was 97% at a concentration of 1 g/L. Coefficients of variation for the analysis at respective concentrations of 100 and 500 mg/L were: within-run, 2.7% and 2.0%; between-run, 3.1% and 4.2%. I encountered no serious interferences from other drugs. The proposed method, rapid and reliable, is recommended for routine use in the clinical laboratory.