Mentzer Index

• MCV/RBC count < 13 favors thalassemia over iron deficiency.
• This test has a high sensitivity but low specificity.
• Basically in iron deficiency, the marrow can’t produce RBCs and they’re small so the RBC count will be low along with the MCV.
• In thalassemia, RBC production is preserved, though the cells are small and fragile. So the RBC count is normal with a low MCV. This concept may be more helpful than remembering the Mentzer index.