Many prescription medications, over-the-counter medications, vitamins, minerals, and herbal preparations can influence the results of laboratory tests. Mechanisms of drug effects are either pharmacologic (e.g., furosemide usually increases the excretion of potassium, resulting in a lower serum potassium level) or analytic (e.g., when a drug in a patient’s body fluid or tissue interferes with a chemical step in a laboratory test, resulting in an erroneous test result). The drug classes that cause the majority of analytic interferences include antibiotics, antihypertensives, anticonvulsants, hormones, and antidepressants.
Accurate and complete medication histories, including prescription medications, over-the-counter medications, vitamins, minerals, and herbal preparations, are essential to interpret laboratory test results that fall out of the normal range.
Table E.1 is not an exhaustive list, and whenever laboratory results are suspected of being spurious, further research is necessary. There are many references available, including drug monographs in American Hospital Formulary Service, Drug Information (published by the American Society of Health-System Pharmacists), drug package inserts, and the Physician’s Desk Reference, which contain official product information. Services that maintain this information include the Iowa Drug Information Service and DRUGDEX.
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