Q: A 70-year-old woman has been in long-standing poor health, with severe diabetes mellitus and rheumatoid arthritis. Her physician notes that she appears pale and orders a hematocrit, which shows a result of 35%. Examination of the blood smear reveals a microcytic anemia. The physician is considering a differential diagnosis of iron deficiency anemia versus anemia of chronic disease. Which of the following laboratory determinations would be most helpful in distinguishing these conditions?