A client is admitted with diabetic ketoacidosis (DKA) and is receiving an IV infusion of 0.9% sodium chloride and insulin. Two hours later, the client's serum laboratory results show a decrease in the serum blood glucose from 580 to 430 mg/dL (32.2 to 23.86 mmol/L). It is most important for the nurse to monitor which additional laboratory result? Reference Range