An IgM Paraprotein Causing a Falsely Low Result in an Enzymatic Assay for Acetaminophen

Since the development of enzymatic assays for acetaminophen in the early 1980s, their use has rapidly become widespread (1). Their advantages of speed and ready automation have ensured that >95% of laboratories submitting to a external quality assessment scheme now use this method (M.A. Thomas, personal communication). Given this fact and the absolute dependence on the laboratory result for clinical decision making, it is important to be aware of any possible discrepancies associated with this technique. I therefore report here a case of a man with an IgM monoclonal gammopathy who ingested acetaminophen and whose subsequent serum acetaminophen concentration was falsely low on enzymatic assay.When we were investigating a 77-year-old man who was admitted after a cerebrovascular accident in 1990, we noted that he had an IgMκ monoclonal component of 5 g/L. In the absence of any other clinical or laboratory evidence of a hematological dyscrasia, a diagnosis of monoclonal gammopathy of unknown significance (MGUS) was made. On a subsequent occasion, he was admitted to the Accident and Emergency …