Review Question - QID 216378

A 35-year-old woman presents to the emergency room with fatigue, dyspnea, and palpitations. A week earlier, the patient had received a dose of prophylactic cefotetan before an uncomplicated laparoscopic appendectomy. She has no other significant past medical history. She denies any family history of hereditary conditions. Her temperature is 98.1°F (36.7°C), blood pressure is 130/85 mmHg, pulse is 110/min, and respirations are 20/min. Jaundice and splenomegaly are noted on physical exam. Her hemoglobin is 7.1 g/dL, down from 13.2 g/dL 1 week ago. Additional laboratory tests are as follows:

Leukocyte count: 7,000/mm^3
Platelet count: 300,000/mm^3
Mean corpuscular volume: 90 µm^3
Reticulocyte count: 9%
Lactate dehydrogenase: 480 U/L (Normal: 140-280 U/L)
Haptoglobin: 10 mg/dL (Normal: 41-165 mg/dL)
Indirect bilirubin: 2.7 mg/dL
D-dimer < 250 ng/mL (Normal: < 250 ng/mL)

The peripheral blood smear shows reticulocytosis and microspherocytes. After appropriate therapeutic interventions, which of the following is the most appropriate next diagnostic test?