A 35-year-old female farmer with no significant past medical history presents with pallor, dizziness, and fatigue for 1 year. Laboratory tests: RBC count 1.9×10<sup>12</sup>/L, hemoglobin 45g/L, WBC count 5.4×10<sup>9</sup>/L (neutrophils 70%, lymphocytes 16%, eosinophils 14%). Peripheral smear shows red blood cells with increased central pallor (target cells). Serum ferritin is 16μg/L. To determine the cause of anemia, which test should be prioritized?