Mentzer index — MCV ÷ RBC
Mentzer index = MCV (fL) ÷ RBC (×1012/L)
Cutoff 13 (Mentzer, Lancet 1973): index < 13 → beta-thalassemia trait more likely; > 13 → iron deficiency anemia more likely; = 13 → non-discriminatory. Take MCV and RBC directly from the CBC report.
fL (femtoliters)
×1012/L (millions/µL)
Try: MCV 70 fL, RBC 5.5 → index = 70 ÷ 5.5 = 12.73 (thalassemia-leaning)
Try: MCV 75 fL, RBC 4.0 → index = 75 ÷ 4.0 = 18.75 (iron-deficiency-leaning)
Supporting discriminant features
Context, not a substitute: two more CBC features sharpen the screen.
RDW normal (11.5–14.5%) fits thalassemia trait; high RDW (> 14.5%) fits iron deficiency (anisocytosis). RBC count ≥ 5.0 ×1012/L (normal/high) fits thalassemia trait; low RBC fits iron deficiency. All three agreeing = strongest screening suggestion.fL
×1012/L
% (red cell distribution width)
Try: MCV 70, RBC 5.5, RDW 12.3% → concordant thalassemia pattern
Try: MCV 75, RBC 4.0, RDW 16.8% → concordant iron-deficiency pattern
Interpretation & next steps
Enter the CBC values to get a combined pattern and the confirmatory tests to discuss with a clinician. This mode adds no new measurement — it interprets the same features and tells you what should come next.
fL
×1012/L
%
Try: MCV 68, RBC 5.2, no RDW → next steps for a thalassemia-leaning pattern
What this index cannot do
This tool is a screening aid for microcytic anemia only. It does not diagnose thalassemia or iron deficiency; it cannot distinguish either from anemia of chronic inflammation or other causes; and a person can have both iron deficiency and thalassemia at once (in which case the index typically points only at iron deficiency). Roughly 1 in 6 to 1 in 10 cases can be misclassified. The two conditions are confirmed by different tests — serum ferritin/iron studies for iron deficiency, hemoglobin electrophoresis for beta-thalassemia trait — so a screening suggestion is a reason to test, never a conclusion.
Frequently asked questions
Answers match the structured data below, byte for byte.
What is the Mentzer index?
The Mentzer index is a screening ratio calculated from a complete blood count (CBC): MCV (mean corpuscular volume, in fL) divided by the RBC count (in millions per microliter, i.e. x10^12/L). It was described by William C. Mentzer in The Lancet in 1973 as a quick way to suggest whether microcytic anemia is more likely due to beta-thalassemia trait or iron deficiency anemia.
How do you calculate the Mentzer index?
Divide the MCV (fL) by the RBC count (x10^12/L). Example: MCV 70 fL with an RBC count of 5.5 x10^12/L gives 70 / 5.5 = 12.73. Example: MCV 75 fL with RBC 4.0 gives 75 / 4.0 = 18.75. No other measurements are needed for the index itself.
What does a Mentzer index below 13 mean?
An index below 13 suggests beta-thalassemia trait is more likely. The principle: in thalassemia, globin synthesis is defective so red cells are small but the marrow still produces a normal number of them — low MCV with a normal RBC count gives a low ratio. This is a screening suggestion only; it must be confirmed with hemoglobin electrophoresis.
What does a Mentzer index above 13 mean?
An index above 13 suggests iron deficiency anemia is more likely. The principle: in iron deficiency the marrow cannot produce as many red cells, and the cells it makes are small — both the RBC count and MCV are low, which pushes the ratio up. This is a screening suggestion only; it must be confirmed with serum ferritin and iron studies.
What if my Mentzer index is exactly 13?
An index of exactly 13 sits on the cutoff and is non-discriminatory — the index alone suggests neither condition. In that case the supporting features (RDW, RBC count) and confirmatory tests (ferritin/iron studies, Hb electrophoresis) carry the full weight.
How do RDW and RBC count help interpret the index?
Two CBC features support the interpretation. RDW (red cell distribution width): normal (11.5-14.5%) fits thalassemia trait, while a high RDW fits iron deficiency (IDA causes anisocytosis). RBC count: normal or high (>= 5.0 x10^12/L) fits thalassemia trait, while a low count fits iron deficiency. When all three features agree, the screening suggestion is strongest; when they disagree, neither direction should be assumed.
How accurate is the Mentzer index?
Accuracy is modest and varies between studies. One study of 120 patients with microcytic anemia reported a sensitivity of about 72.8% and specificity of about 84.6% for distinguishing iron deficiency anemia from beta-thalassemia trait; other reviews place overall performance around 85-90%. That means roughly 1 in 6 to 1 in 10 cases can be misclassified — so the index is a screening aid, never a substitute for ferritin/iron studies or hemoglobin electrophoresis.
Can someone have both iron deficiency and thalassemia?
Yes. A person with microcytic anemia can have both iron deficiency and thalassemia trait at the same time, and in that case the Mentzer index typically points only toward iron deficiency, masking the thalassemia. This is why mixed or discordant features should trigger both workups rather than a single assumed diagnosis.
What tests confirm thalassemia trait?
The confirmatory test for beta-thalassemia trait is hemoglobin electrophoresis (or HPLC), which quantifies HbA2 and HbF. An elevated HbA2 fraction is characteristic of beta-thalassemia trait. Family history and ancestry (Mediterranean, Middle Eastern, South or Southeast Asian) also raise the prior probability.
What tests confirm iron deficiency anemia?
The key confirmatory test is serum ferritin, usually together with iron studies (serum iron, TIBC and transferrin saturation). Low ferritin confirms iron deficiency. One caution: ferritin is an acute-phase reactant and can be falsely normal during inflammation, in which case CRP can help interpret the result. If iron deficiency is confirmed, the cause (diet, menstrual or GI blood loss, malabsorption) still needs to be found.
Is the Mentzer index only for adults?
The index was described and validated in general populations with microcytic anemia, and the same MCV/RBC arithmetic applies wherever a CBC is available. However, pediatric reference ranges for MCV and RBC differ by age, and conditions common in children (such as anemia of chronic inflammation or lead exposure) are not distinguished by this index. Results in children should always be interpreted by a clinician.
Does this calculator diagnose anemia?
No. This tool is an educational screening aid: it computes the Mentzer index and supporting discriminant features and suggests which confirmatory tests to discuss with a clinician. It cannot diagnose thalassemia, iron deficiency, or any other condition, and it is not medical advice. Always confirm with laboratory testing and a qualified healthcare professional.