A 55-year-old male with bipolar I disorder (stable on lithium 1200 mg) presents to the ED with acute confusion, slurred speech, and coarse tremor. His wife reports he started ibuprofen 3 days ago for knee pain. Lithium level returns at 2.1 mEq/L. Creatinine is 1.8 mg/dL (baseline 1.0). What is the MOST critical assessment and action?