A 19-year-old previously healthy woman presents to the emergency department with nausea, vomiting, polyuria and abdominal pain. She appears dehydrated, respiratory rate is 28/min with Kussmaul respirations, blood pressure 100/60 mm Hg and heart rate 112/min. Capillary glucose is 28 mmol/L and serum ketones are positive. What is the most appropriate initial management step?