Acetaminophen assay: the clinical consequences of a colorimetric vs a high-pressure liquid chromatography determination in the assessment of two potentially poisoned patients.

Following acetaminophen (APAP) overdosage, plasma half-life of APAP elimination appears to be a good prognostic indicator of potential hepatic toxicity. A half-life of greater than 4 hr is often associated with liver injury. In two patients who each allegedly ingested greater than 13 gm of APAP, serial assay procedures were performed using both a colorimetric (nitrous acid chromophore) method and HPLC for all samples. The data obtained were used to determine the plasma half life. A comparison of the results obtained from both methods revealed a significant variation. It appears that the colorimetric method, at lower plasma concentrations of APAP (less than 50 microgram/ml), overestimates the actual drug concentration, resulting in the calculation of a falsely prolonged plasma half-life. This may result in the unnecessary implementation of antidotal therapy. The HPLC method is considered to be more specific than the colorimetric analysis and to better reflect actual plasma APAP concentrations, especially at lower concentrations of APAP.