A 28-year-old woman with type 1 diabetes presents to the emergency department with polyuria, abdominal pain and vomiting. Capillary glucose is 26 mmol/L, venous pH 7.12, and ketones are markedly elevated, confirming diabetic ketoacidosis. After starting intravenous 0.9% sodium chloride and a fixed-rate insulin infusion, her blood glucose falls rapidly to 13.5 mmol/L while she remains acidotic. According to Australian adult DKA protocols, what is the next best step?