Interactions of Drugs and Laboratory Tests

The interference of drugs on laboratory test results is discussed. Drug interference can be due to the pharmacological properties of a drug the toxic effects of the drug on the patient or an incompatibility of the drug with the test procedure. Formulations containing estrogens will cause falsely elevated values in thyroid funcion tests such as the protein-bound iodine and thyroxine tests. The progestational component in oral contraceptives has been known to increase serum cholesterol levels. Drug toxicity can interfere with liver function tests the determination of serum amylase levels and creatine phosphokinase tests. Ascorbic acid nalidixic acid and parenterally administered tetracyclines are examples of drugs that interfere with testing procedures especially tests of urinary glucose. Phenothiaxines have been found to produce false negative results in immunologic pregnancy tests. It is recommended that the physician be aware of the patients complete medication history (including over-the-counter drugs) be knowledgeable enough of test procedures to rule out drug interference and be certain that a diagnostic or therapeutic decision is based on the most specific laboratory determination possible.