A 74-year-old man with type 2 diabetes presents with several days of lethargy and increasing confusion. His blood glucose concentration is 42 mmol/L, calculated serum osmolality is 360 mOsm/kg (reference range 275–295), blood ketones are 0.8 mmol/L and venous pH is 7.36. His blood pressure is 106/68 mmHg, serum potassium is 4.4 mmol/L (reference range 3.5–5.0), and there is no clinical evidence of pulmonary oedema. Which one of the following should be undertaken first?