A 20-year-old woman complains of headache and discomfort in both sides of her jaw. Physical examination reveals enlarged parotid glands that are slightly tender on palpation. There is reddening of the orifice of Stensen's duct on intra oral examination; her temperature is 38.3degC, and the pulse rate is 80/min. Laboratory data show hemoglobin 14 g/dL; hematocrit 40%; WBC 11000/mL, with 33% segmented neutrophils, 7% monocytes, and 60% lymphocytes. Which of the following diagnostic tests will help to confirm the diagnosis of epidemic parotitis?