A 52-year-old man with a 10-year history of alcohol use disorder presents with macrocytic anemia (MCV 108 fL). His reticulocyte count is 0.8% (normal 0.5–2.5%). A 48-year-old woman with recently diagnosed HIV (CD4 count 150 cells/μL) also presents with macrocytic anemia (MCV 110 fL) and reticulocyte count 0.6%. Both have low B12 levels. Which laboratory finding best distinguishes megaloblastic anemia (B12 deficiency) from non-megaloblastic macrocytic anemia in this cohort?