The Mentzer index, described in 1973 by Mentzer, is said to be helpful in differentiating iron deficiency anemia from beta thalassemia. If a CBC indicates microcytic anemia, the Mentzer index is said to be a method of distinguishing between them. In practice, the Mentzer index is not a reliable indicator and should not, by itself, be used to differentiate. Indeed, the relevance of the index may be questioned, as the presence of iron deficiency is easily determined by measuring the serum ferritin level, which should be obtained routinely in all cases of microcytic anemia. In addition, it would be possible for a patient with a microcytic anemia to have both iron deficiency and thalassemia, in which case the index would only suggest iron deficiency. (RWP 8-15-13)
The index is calculated from the results of a complete blood count. If the quotient of the mean corpuscular volume (MCV, in fL) divided by the red blood cell count (RBC, in Millions per microLiter) is less than 13, thalassemia is said to be more likely. If the result is greater than 13, then iron-deficiency anemia is said to be more likely.
The principle involved is as follows: In iron deficiency, the marrow cannot produce as many RBCs and they are small (microcytic), so the RBC count and the MCV will both be low, and as a result, the index will be greater than 13. Conversely, in thalassemia, which is a disorder of globin synthesis, the number of RBC's produced is normal, but the cells are smaller and more fragile. Therefore, the RBC count is normal, but the MCV is low, so the index will be less than 13. (In practice, the MCV has to be extremely low and/or the RBC extremely high to produce an index of less than 13, something rarely encountered, further calling into question the usefulness of the index. - RWP 8-15-13)