A patient who is taking digoxin is admitted to the hospital for treatment of congestive heart failure. The prescriber has ordered furosemide [Lasix]. The nurse notes an irregular heart rate of 86 beats/min, a respiratory rate of 22 breaths per minute, and a blood pressure of 130/82 mm Hg. The nurse auscultates crackles in both lungs. Which laboratory value causes the nurse the most concern?