American Board of Internal Medicine (ABIM) Certification Practice Exam

What test is used to diagnose lithium-induced nephrogenic diabetes insipidus?

1. 24-hour urine collection

2. Water restriction test

3. Serum electrolyte panel

4. Plasma osmolality measurement

The correct answer is: Water restriction test

The water restriction test is utilized to diagnose lithium-induced nephrogenic diabetes insipidus because it evaluates the kidney's ability to concentrate urine in response to restricted fluid intake. In patients with nephrogenic diabetes insipidus, such as that caused by lithium, the kidneys are unable to concentrate urine despite the presence of adequate levels of antidiuretic hormone (ADH). During the water restriction test, fluid intake is limited, and urine output is monitored. A failure to concentrate urine (i.e., producing large volumes of dilute urine) during this test indicates a dysfunction in the kidney's response, supporting a diagnosis of nephrogenic diabetes insipidus. While a 24-hour urine collection provides information about urine volume and concentration over a day, it does not specifically evaluate the kidneys' response to ADH after water deprivation. The serum electrolyte panel and plasma osmolality measurement can provide valuable information about a person's hydration status and electrolyte balance, but they are not definitive tests for diagnosing lithium-induced nephrogenic diabetes insipidus. The water restriction test distinguishes between the types of diabetes insipidus and confirms the diagnosis by demonstrating the kidneys' inability to concentrate urine when fluid intake is limited.