A 24-year-old woman with type 1 diabetes presents with abdominal pain, nausea and polyuria. She has tachycardia, Kussmaul respirations and ketotic breath. Capillary glucose is 25 mmol/L, venous pH is 7.15 and serum ketones are high. According to standard DKA protocols, what is the single most important initial management step after ABC assessment and establishing IV access?