A 42-year-old man with bipolar disorder has been on lithium 1200 mg daily for 2 years. He presents to the emergency department with confusion, coarse tremor, hyperreflexia, and ataxia. Serum lithium level is 2.2 mEq/L (therapeutic range 0.6–1.2 mEq/L). Serum creatinine is 1.8 mg/dL (baseline 1.0 mg/dL). What is the most appropriate immediate next step?