A nurse suspects diabetes insipidus in a postoperative craniotomy patient. Which laboratory value should be obtained to assess for DI?

• A. Hematocrit
• B. Serum potassium
• C. Specific gravity
• D. Blood glucose

Answer: (C)

Diabetes insipidus disrupts the kidney’s ability to concentrate urine because of ADH deficiency or resistance, so the urine becomes very dilute even when the person is dehydrated. The quickest way to detect this is to look at how concentrated the urine is, which is measured by urine specific gravity. In DI, the specific gravity is low because the urine is not concentrated, reflecting a lack of ADH effect. A low urine specific gravity (often low end of the normal range or below 1.005–1.010) points toward DI and would prompt further testing like urine osmolality and serum osmolality/sodium.

Other labs listed don’t directly assess urine concentration: hematocrit can rise with dehydration but isn’t specific to DI; potassium and blood glucose don’t diagnose DI (potassium varies with many conditions, and glucose relates to diabetes mellitus rather than DI).