A 12-year-old girl has had increasing sluggishness and poorer school performance over the past year. There is no history of increased numbers of infections. The child now complains of headaches. There is no hepatosplenomegaly or lymphadenopathy. A complete blood count shows: Hgb 11.8 g/dL, Hct 32.8%, MCV 70 fL, platelet count 290,000/microliter, and total WBC count 8000/microliter. Examination of her peripheral blood smear shows basophilic stippling of erythrocytes. The serum haptoglobin is 5 mg/dL.

Which of the following laboratory test findings is most likely to be present in this girl?