A 58-year-old woman with CKD stage 5 on hemodialysis presents with fatigue and dyspnea. Hemoglobin is 7.8 g/dL, MCV 78 fL, serum iron 45 µg/dL, ferritin 520 ng/mL, TIBC 210 µg/dL, reticulocyte count 0.8%. She is on erythropoiesis-stimulating agent (ESA) therapy. Which feature best distinguishes ESA-resistant anemia due to iron deficiency from ESA-resistant anemia due to chronic inflammation in CKD?