A 42-year-old woman with cystic fibrosis is hospitalized for acute exacerbation due to Pseudomonas aeruginosa respiratory infection. She is started on intravenous tobramycin 5 mg/kg once daily. On day 5 of therapy, she develops sudden-onset hearing loss and tinnitus. Serum creatinine is 0.9 mg/dL (baseline 0.8 mg/dL). What is the most appropriate next step?