To the Editor: In Case Record 46–1988 (Nov. 17 issue),1 there was a concern that the patient had an adrenal mass that may have been a pheochromocytoma. Dr. Kantor noted that certain drugs can interfere with the diagnosis of this tumor on the basis of the levels of urinary catecholamines or their metabolites. He then noted that the patient was not taking any such drugs. Actually, he was, since studies from our laboratory have shown that a metabolite of propranolol can interfere with the metanephrine assay. As a result of this observation, we routinely discontinue propranolol or metoprolol, which may….