A 14-year-old girl with history of eating disorder presents to the ED with increased anxiety. She is accompanied by her parents who are concerned she may not have been compliant with the agreed treatment plan at home. She endorses some difficulty adhering to her diet but ensures you that she will start eating again and is willing to demonstrate this in front of you. Which of the following is the single most reliable laboratory finding to indicate this patient is at risk of refeeding syndrome and should be admitted for monitored treatment?