A 48-year-old woman with bipolar I disorder has been stable on lithium 600 mg twice daily for 5 years. At her routine outpatient visit, serum lithium level is 1.1 mEq/L (therapeutic), but her serum creatinine has risen from baseline 0.9 mg/dL to 1.3 mg/dL over the past 18 months. Urinalysis shows no proteinuria. She denies polyuria or polydipsia. What is the most appropriate next step in management?