Green & King Index Calculator

Prioritise a microcytic anaemia toward thalassaemia trait or iron deficiency using MCV, RDW and haemoglobin. A screening aid for deciding who gets electrophoresis — never a diagnosis.

Last updated 9 Sep 2026

Green & King Index

MCV, RDW, Hb → index

MCV (fL)Mean corpuscular volume. The index is only meaningful in a microcytic anaemia — applied to a normal or high MCV it means nothing.

RDW-CV (%)The coefficient-of-variation form of the RDW, which is what almost every report prints. If your analyser reports RDW-SD in fL instead, this index cannot be calculated from it — the two are not interchangeable.

Haemoglobin (g/dL)In g/dL. If your report is in g/L, divide by 10 first — 115 g/L is 11.5 g/dL.

47.8G&KExample

MCV 65 fL, RDW-CV 13.0%, haemoglobin 11.5 g/dL

Formula

Green & King = MCV² × RDW ÷ (Hb × 100)
Below 65 → beta-thalassaemia trait more likely · 65 or above → iron deficiency more likely

MCV: mean corpuscular volume in fL, squared — the index weights cell size heavily, which is why it is only applied to an anaemia that is already microcytic
RDW: the RDW-CV in per cent. This is the index's distinguishing input: iron deficiency widens the size distribution and thalassaemia trait usually does not, so the RDW pulls the index up in one condition and not the other
Hb: haemoglobin in g/dL, in the denominator. In thalassaemia trait the haemoglobin is typically only mildly reduced for the degree of microcytosis, which drives the index down
65: the published cut-off. Values below it favour thalassaemia trait. In the Hoffmann 2015 meta-analysis of 99 studies the index reached a diagnostic odds ratio of 29.8 (95% CI 18.5–47.8) with an area under the curve of 0.898 — seventh of the twelve indices compared, and close enough to the others that no index in the family deserves to be called the best

Worked example

MCV 65 fL, RDW-CV 13.0%, haemoglobin 11.5 g/dL
65² = 4,225
4,225 × 13.0 = 54,925
11.5 × 100 = 1,150
54,925 ÷ 1,150 = 47.8
Below 65, so beta-thalassaemia trait is the more likely of the two — send haemoglobin electrophoresis or HPLC, and ferritin
Widen the RDW to 18% and the same patient scores 66.1, above the cut-off and pointing the other way. That single input is what the index is built on, and it is also where it is most fragile

Three discriminant indices on the same patient

The three use overlapping but different inputs, so a report missing one of them can usually still be assessed. When they disagree, that disagreement is information — it usually means the picture is mixed — and the answer is to send both confirmatory tests rather than to pick whichever index gave the expected answer.
Index	Formula	Inputs needed	Favours thalassaemia trait
Mentzer	MCV ÷ RBC	MCV, red cell count	Below 13
Green & King	MCV² × RDW ÷ (Hb × 100)	MCV, RDW-CV, haemoglobin	Below 65
England & Fraser	MCV − RBC − (5 × Hb) − 3.4	MCV, red cell count, haemoglobin	A negative result

What the index is for, and what it is not for

Roughly a hundred published studies have compared these indices and none has found one accurate enough to stand alone. Their value is in triage, where a modestly accurate test applied cheaply to everyone still saves a great many expensive confirmatory ones.
Question	Answer
Can it diagnose thalassaemia trait?	No. It was derived to prioritise who is sent for haemoglobin electrophoresis in screening programmes, not to replace it
Can it exclude iron deficiency?	No. Ferritin does that, read against CRP; a low index in an iron-deficient patient is common
Does it work where both conditions are present?	No, and this is its most important failure. Coexisting iron deficiency is common wherever thalassaemia trait is common, and it drags every index toward the middle
Does its accuracy travel between populations?	Poorly. Performance varies substantially with the prevalence of each condition and with the mix of thalassaemia genotypes, so a cut-off validated in one region under-performs in another
Does it apply to a normal MCV?	No. Every index in this family assumes an established microcytosis and is meaningless without one

Which of two conditions to test for first

Beta-thalassaemia trait and iron deficiency both produce a microcytic hypochromic anaemia, and telling them apart matters: one needs iron and an investigation for the source of blood loss, the other is a lifelong carrier state with consequences for the patient’s relatives and for any pregnancy. Confirming either takes a specific test — ferritin for one, haemoglobin electrophoresis or HPLC for the other. The discriminant indices exist because in a screening programme it is not affordable to send both to everyone, and they were derived to decide which to send first.

Green and King’s index uses the RDW, which is what distinguishes it from the older formulae. The reasoning is physiological. In iron deficiency the supply of iron falls progressively, so the marrow produces cells of steadily differing size and the size distribution widens. In thalassaemia trait the defect is constitutional and present from the start, so every cell is affected equally and the distribution stays narrow. The MCV is squared in the numerator and the haemoglobin sits in the denominator, capturing the further observation that thalassaemia trait produces a haemoglobin that is higher than the degree of microcytosis alone would predict. Below 65 favours thalassaemia trait; 65 or above favours iron deficiency.

None of these indices is diagnostic, and that is not a caveat added for safety — it is the finding of the literature. A meta-analysis of 99 studies covering more than 135,000 patients compared twelve of them and concluded that their sensitivity and specificity are not high enough for a definitive diagnosis. Green and King’s ranked seventh, Mentzer’s eighth and England and Fraser’s fifth, with areas under the curve clustered between about 0.88 and 0.90 — close enough that arguing about which is best is arguing inside the noise. Performance also varies substantially between populations, because it depends on how common each condition is locally and on which thalassaemia genotypes are present.

The failure that matters most in practice is the one the arithmetic cannot see: iron deficiency and thalassaemia trait frequently coexist, and they are commonest together in exactly the populations where screening is done. When both are present every index is pulled toward the middle and every index misleads. Worse, iron deficiency lowers HbA₂ toward the normal range, so electrophoresis performed on an iron-deficient patient can miss the trait as well — which is why the safest sequence is to measure ferritin, correct any deficiency, and then perform electrophoresis. Ferritin and electrophoresis settle this question. Arithmetic decides only the order in which they are sent.

Frequently asked questions

What is the Green & King index?

A red cell discriminant that helps decide whether a microcytic anaemia is more likely to be beta-thalassaemia trait or iron deficiency. It is calculated as MCV squared times the RDW, divided by a hundred times the haemoglobin. Below 65 favours thalassaemia trait and 65 or above favours iron deficiency.

Does the Green & King index diagnose thalassaemia trait?

No. It was derived to prioritise who is sent for haemoglobin electrophoresis in a screening programme, not to replace it. In meta-analysis its area under the curve is about 0.90, which is useful for triage and not adequate for diagnosis. Ferritin and electrophoresis are what settle the question.

Which discriminant index is the most accurate?

None convincingly. A meta-analysis of 99 studies and more than 135,000 results found the twelve indices clustered closely — England and Fraser’s ranked fifth, Green and King’s seventh, Mentzer’s eighth — and concluded that none reached diagnostic accuracy. Performance also shifts with the population being screened.

What happens if iron deficiency and thalassaemia trait are both present?

Every index misleads. The two conditions pull the red cell parameters in opposite directions and the result lands in the middle, where the cut-off is least reliable. Coexistence is common wherever thalassaemia trait is common, so an index that disagrees with the clinical picture should be ignored rather than explained away.

Can I use RDW-SD instead of RDW-CV in this index?

No. The index was derived on the RDW-CV, the percentage form, and RDW-SD is a different measurement in femtolitres taken at a different point on the red cell histogram. The two are not interchangeable and substituting one for the other gives a meaningless number.

Related calculators

References

Green R, King R. A new red cell discriminant incorporating volume dispersion for differentiating iron deficiency anemia from thalassemia minor. Blood Cells. 1989;15(3):481–491.
Hoffmann JJML, Urrechaga E, Aguirre U. Discriminant indices for distinguishing thalassemia and iron deficiency in patients with microcytic anemia: a meta-analysis. Clin Chem Lab Med. 2015;53(12):1883–1894.
Ntaios G, Chatzinikolaou A, Saouli Z, et al. Discrimination indices as screening tests for β-thalassemic trait. Ann Hematol. 2007;86(7):487–491.

Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.