When an ACE inhibitor or diuretic is initiated for hypertension, how soon and how often should electrolytes be checked?
—Kiron Malhotra, MD, Palm Bay, Fla.

This is an important question for which there is no dogmatic answer. Following initiation of a diuretic, the usual recommendation is to check electrolytes within a month. I tend to check the electrolytes sooner if I have prescribed a relatively large dose or if the patient is elderly or of small size, has a past history of hypokalemia on a diuretic, or eats a diet low in potassium (few fruits or vegetables). I would also check the electrolytes sooner if the diuretic is chlorthalidone, which, because of its longer half-life, causes hypokalemia more frequently than does hydrochlorothiazide.

When it comes to ACE inhibitors or angiotensin receptor blockers, hyperkalemia is very uncommon in patients with normal renal function, and waiting a month or more to check electrolytes is not generally a problem. I would check them sooner, perhaps even within two weeks, if the patient has some degree of renal insufficiency or is taking other medications that could increase the serum potassium level, such as nonsteroidal anti-inflammatory drugs (NSAIDs) or spironolactone. In the elderly, it is important to inquire about the use of OTC NSAIDs, because many older patients take these agents for arthritis.
—Samuel J. Mann, MD (116-4)

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