A 45-year-old woman is undergoing chemotherapy for breast cancer. She presents 10 days after her last chemotherapy with fever (temperature >38.5), but no other symptoms except a sore throat and mouth. On examination, she looks well, there is oral mucositis, ears are normal, lungs are clear, and the central line site is clean. The CXR, urinalysis, and biochemistry are normal. Her WBC is 800/mL and the absolute neutrophil count is low (<500). Which of the following is the most appropriate next step in management?