A 37-year-old woman is evaluated for generalized weakness. She has been hospitalized twice over the last year with similar complaints and her serum potassium concentration was found to be

Question

A 37-year-old woman is evaluated for generalized weakness. She has been hospitalized twice over the last year with similar complaints and her serum potassium concentration was found to be < 3.0 mEq/L. Her potassium concentration returned to normal with aggressive potassium replacement. She eats a normal diet and denies vomiting, purging, and cathartic or diuretic use. Her blood pressure is within normal limits. Her only medication is an oral contraceptive. Her review of systems is otherwise unremarkable. Her sister suffers from Crohn's disease and her mother has congestive heart failure, diabetes mellitus, and bipolar disorder.
Her blood pressure is 112/68 mm Hg and her pulse is 90/min. Her BMI is 22 kg/m². Physical examination is normal.
Laboratory studies are shown below:

Spot urinary chloride concentrations on two separate occasions were 15 and 65 mEq/L.
Which of the following is most likely to establish diagnosis in this patient?

Quizplus · Accepted Answer

This patient presents with unexplained hypokalemia and metabolic alkalosis. The most common causes are vomiting/purging and diuretic use. Surreptitious vomiting or purging can be difficult to diagnose, but it usually presents with significant metabolic alkalosis and a low urinary chloride. This patient also lacks signs of self-induced vomiting such as ulcers on the dorsum of the hand, dental erosions from chronic acid exposure, or puffy cheeks from hypertrophied salivary glands. Diuretics cause a high urine chloride and metabolic alkalosis, but the urine chloride returns to low levels once the drug's effect wears off. As a result, surreptitious diuretic use is often associated with intermittent hypokalemia and variable urinary chloride excretion. In patients without physical signs of self-induced vomiting, a positive urinary screen for diuretics (usually on several occasions) is required to confirm the diagnosis of diuretic abuse. (Choice A) CT angiography of the renal arteries diagnoses renal artery stenosis and fibromuscular dysplasia, which both cause hypertension usually with a urine chloride > 40 mEq/L. This patient's normal blood pressure and variable urinary chloride make this less likely. (Choice B) Colonoscopy would be indicated for gastrointestinal causes of hypokalemia (e.g., diarrhea, surreptitious cathartic use, or a secretory villous adenomas), but those patients usually have metabolic acidosis from bicarbonate loss rather than alkalosis. (Choice C) Elevated plasma aldosterone with suppressed renin suggests hyperaldosteronism, which usually presents with persistent hypokalemia and hypertension. This patient's normal blood pressure and episodic hypokalemia make this less likely. (Choice D) Elevated C-peptide and insulin levels would suggest exogenous oral hypoglycemic agent abuse or insulinoma. Elevated insulin levels with low C-peptide levels suggest exogenous insulin abuse. However, this patient's lack of hypoglycemia makes this less likely. Educational objective: Intermittent hypokalemia with metabolic alkalosis in the absence of vomiting suggests a diagnosis of either surreptitious purging or diuretic use. A variable urinary chloride favors diuretic abuse and can be confirmed with a urinary screen for diuretics. __________ References: The clinical features of covert diuretic use. (http://www.ncbi.nlm.nih.gov/pubmed/7179952)

A 37-Year-Old Woman Is Evaluated for Generalized Weakness