A 12-year-old boy is hospitalized with recurrent fever and dark yellow urine for 4 years. Physical exam: mild icterus of the sclera, spleen palpable 3 cm below the costal margin. Laboratory tests: hemoglobin (Hb) 75g/L, reticulocytes 9%, spherocytes with loss of central pallor, urine bilirubin (-), urine urobilinogen strongly positive, normal RBC osmotic fragility test, Rous test (-). Which lab test is most appropriate for definitive diagnosis?