A 69-year-old woman, with poor dietary habits and alcoholism, is found to have a macrocytic anemia with hyper segmented neutrophils. Which of the following is the most appropriate diagnostic test?

Folate deficiency can be secondary to small bowel disease, alcoholism, inadequate intake, disease states with high cell turnover (hemolytic anemia), drugs (methotrexate), and pregnancy. The concentration of folate in plasma changes rapidly with changes in food intake, so the diagnosis of anemia secondary to folate deficiency is made more reliable by measuring red blood cell folate.