It is important that health care professionals be aware of the potential for medications to interfere with clinical laboratory tests. Medications can cause in vivo effects when the concentration or activity of the analyte is altered before the analysis and therefore the assay result is true and accurate. An in vitro effect occurs when the medication interferes with the assay, and the result is erroneous and cannot be interpreted. This report describes a recently identified case of interference of acetylcysteine with the urine test for ketones and demonstrates the importance of a thorough medication review in evaluating abnormal laboratory tests.
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