A client with diabetes insipidus (DI) has an average urinary output of 500 ml. of dilute urine every hour for the last 4 hours. Which laboratory test is most important for the nurse to monitor?

Choice A rationale: White blood cell count monitors for infection or inflammation. While important for general health, it does not address the massive fluid loss and electrolyte imbalances specifically caused by diabetes insipidus. Choice B rationale: Capillary glucose monitors for diabetes mellitus. Diabetes insipidus involves antidiuretic hormone deficiency, not insulin issues; therefore, glucose levels are not the priority in managing the massive dilute polyuria seen here. Choice C rationale: Urine specific gravity will be consistently low in diabetes insipidus. While it confirms the diagnosis of dilute urine, it is less critical than monitoring the life-threatening systemic effects of dehydration. Choice D rationale: Serum sodium is the most important lab to monitor. Massive loss of dilute urine leads to severe hemoconcentration and hypernatremia, which can cause cerebral edema, seizures, and permanent neurological damage.