Differentiate thalassemia trait from iron deficiency anemia.
Mentzer Index = MCV / RBC count. A value < 13 suggests thalassemia trait (high RBC count with low MCV), while > 13 suggests iron deficiency anemia (low RBC count with low MCV). Sensitivity is ~90%. Confirmatory testing (Hb electrophoresis, iron studies) is required. The index does not differentiate between alpha and beta thalassemia.