Plasma acetaminophen results are method dependent.

The management of patients who have taken large amounts of acetaminophen is often determined by the plasma level of the drug in relation to the time after ingestion. Nomograms or tables defining the risk of the hepatic damage for a given level at a particular time usually refer to the plasma-free acetaminophen concentration. It is important therefore that the assay used measures only free acetaminophen and not its inactive conjugates of glucuronide and sulfate. Two simple colorimetric methods were compared to assess the interference of acetaminophen conjugates. The method of Frings and Saloom measured both conjugates. Furthermore, these conjugates are not completely hydrolyzed, averaging about 22 and 85% hydrolysis for the glucuronide and sulfate conjugates, respectively. The method of Glynn and Kendal is little affected by acetaminophen conjugates and gives a better measure of the free drug. Interference by salicylates, although slight, can be corrected using modifications in the method.