A 35-year-old woman presents with several days of increasing fatigue and shortness of breath on exertion. She was recently diagnosed with Mycoplasma pneumoniae. Physical examination reveals BP 113/67, HR 114 beats/min, and respiratory rate 20 breaths/min. She appears icteric and in mild respiratory distress. Her hemoglobin is 9.0 g/dL and MCV is 110. Which of the following is the best next diagnostic test?