What this calculator does
Both thalassemia trait and iron-deficiency anemia produce small red blood cells, so a low MCV alone cannot tell them apart. The Mentzer index uses the fact that they differ in cell count: thalassemia trait typically produces many small cells, while iron deficiency produces fewer.
Dividing one by the other separates them. An index below 13 points toward thalassemia trait and above 13 toward iron deficiency, which is a useful first pass before ordering more expensive tests.
The formula
Divide the mean corpuscular volume in femtolitres by the red blood cell count in millions per microlitre. Both come from a standard full blood count.
| Term | Meaning |
|---|---|
| MCV | Mean corpuscular volume: average red cell size. |
| RBC count | Red blood cells per microlitre, which stays high in thalassemia trait. |
| Mentzer index | MCV divided by RBC count, with 13 as the dividing line. |
The inputs explained
| Field | What to enter |
|---|---|
| Mean corpuscular volume (MCV) (fL) | Mean corpuscular volume in femtolitres, from a full blood count. |
| Red blood cell count (million/µL) | Red blood cell count in millions per microlitre, from the same count. |
When to use it
Investigating a low MCV
The index narrows which of two common causes is more likely before further testing.
Deciding what to test next
It points toward either iron studies or haemoglobin electrophoresis.
Reading a full blood count
Both values are already on the report, so the index costs nothing to compute.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Where does the index cross 13?
The same cell count at three red cell volumes.
| MCV | Mentzer index | Reading |
|---|---|---|
| 60 fL | 12.0 | Below 13: thalassemia trait is more probable |
| 75 fL | 15.0 | Above 13: iron-deficiency anemia is more probable |
| 90 fL | 18.0 | Above 13: iron-deficiency anemia is more probable |
Questions
How reliable is the index?
It is a screening pointer with meaningful error rates in both directions, and studies report varying accuracy. It narrows the likelihood but never establishes a diagnosis, and a result near 13 is genuinely uninformative.
Why does thalassemia trait give a high cell count?
Because the bone marrow compensates by producing more red cells, each smaller than normal. Iron deficiency instead limits how many cells can be made, so the count falls along with the size.
What confirms the diagnosis?
Iron studies for deficiency and haemoglobin electrophoresis or genetic testing for thalassemia. Both conditions can also coexist, which the index cannot detect at all.
Are there other indices?
Several, including the England-Fraser, Srivastava and RDW index. They use the same blood count values in different combinations, and none has clearly displaced the others.
For iron status from an iron studies panel, see the transferrin saturation calculator. For protein balance in blood, see the albumin-to-globulin ratio calculator.