A 58-year-old male with bipolar I disorder has been maintained on lithium 1200 mg daily for 12 years with excellent mood stability. His current lithium level is 0.8 mEq/L. Routine laboratory monitoring reveals a serum creatinine of 1.6 mg/dL (baseline 1.0 mg/dL six years ago), eGFR of 48 mL/min/1.73m2 (previous eGFR 72 mL/min/1.73m2 three years ago), urine specific gravity of 1.005 on two consecutive samples, and urine osmolality of 250 mOsm/kg after 12-hour fluid restriction. His TSH is 5.8 mIU/L (normal: 0.4-4.0) and calcium is 10.2 mg/dL. He has no diabetes, takes lisinopril 10 mg for hypertension, and his blood pressure is 128/78 mmHg. He reports increased thirst and nocturia three times per night. The PMHNP is evaluating the renal findings and their implications. Which of the following best represents the appropriate evaluation?