Anaemia Indices and Discrimination

1. The One Clue Behind Every Index

In iron deficiency the marrow cannot make haemoglobin, so it makes fewer red cells and they are small: the count falls with the volume. In beta-thalassaemia trait the marrow makes plenty of red cells and each one is small, so the count is preserved or high while the volume is low. Every formula here asks how small the cells are relative to how many there are. That is what makes the red cell count the most informative single number on the report: a microcytic anaemia with a count above about 5.0 × 10¹²/L should make you think of a trait before iron. The Mentzer index is that thought written as a division.

2. The Eight Indices This Tool Computes

Matos and Carvalho runs the opposite way to the other seven: a high value favours the trait. It is also the only one that uses the mean corpuscular haemoglobin concentration, and the only one whose original paper was read directly when this tool was built.

3. Two Indices This Tool Deliberately Does Not Compute

Both are omitted rather than shipped with a guessed constant. The output of a calculator running on a constant nobody can pin down looks exactly like the output of the seven that are right.

• England and Fraser: MCV minus RBC minus five times the haemoglobin, minus a constant, with a value below zero favouring the trait. Published secondary tables give that constant as 3.4 in some places and 8.4 in others, and on a borderline count those give opposite answers. The 1973 Lancet paper could not be read directly to settle it, and a 1973 follow-up letter from the same authors may be the origin of the divergence.
• Ricerca: RDW divided by the red cell count. The published cut-off appears as 3.3 in some sources and 4.4 in others, with a locally derived Indian value of 3.8 in a third.

4. The Cut-offs Travel Badly, and They Travel Worst to Here

The largest analysis of these indices, 99 studies and 135,409 results, found performance markedly worse in South-East Asian populations than in European ones, a pooled diagnostic odds ratio of 9.3 against 53.1. A threshold validated in Milan or Manchester should not be applied to an Indian report without knowing what it does here. Kumar and colleagues at Manipal ran the classical indices against HPLC-confirmed cases and found that every published cut-off performed better once it was re-derived locally.

• The result panel prints both columns wherever a local value exists. Where the two disagree on a given patient, that disagreement is the honest state of the evidence and not a defect in the tool.
• A separate North Indian series from Ambala found Shine and Lal to have a specificity of 3.3 per cent, calling almost every iron-deficient patient a thalassaemia trait. That is the strongest argument available for never acting on one index alone.

5. The Patient Who Has Both

This is the failure mode that matters most in India, and the one no index handles.

• In a Delhi series of 463 people with beta-thalassaemia trait, 27.2 per cent also had iron deficiency, with haemoglobin, mean corpuscular volume and mean corpuscular haemoglobin all significantly lower than in iron-replete carriers.
• The added iron deficiency drives the volume down further and pushes the Mentzer index further below 13, so the tool becomes more confident of the trait at exactly the moment the patient also needs iron.
• Haemoglobin A₂ stayed elevated in all but one of the iron-deficient carriers in that series, so HPLC still found the trait.
• The evidence on whether iron deficiency masks a trait is genuinely mixed: a Turkish paediatric series found 45 per cent of iron-deficient carriers fell below a 3.5 per cent A₂ threshold, a Sicilian series found no significant difference. Either way the safe practice is the same: if HPLC is borderline and the patient is iron deficient, correct the iron and repeat the HPLC.

6. What Actually Settles It

• Haemoglobin HPLC. The Indian national guideline on haemoglobinopathies sets haemoglobin A₂ at 2.3 to 3.5 per cent as normal, 3.5 to 3.9 per cent as equivocal and requiring further evaluation, and 4.0 to 8.0 per cent as diagnostic of beta-thalassaemia trait.
• The equivocal band is not a formality. A Mumbai study from the national haematology institute found a beta-globin mutation in 73 per cent of people whose A₂ sat in the borderline zone. Send a borderline result for molecular testing rather than filing it as normal.
• Ferritin. WHO sets the threshold at below 15 µg/L in anyone aged 5 and over, and below 70 µg/L where infection or inflammation is present. Ferritin is an acute phase protein: a value of 40 with an infection is entirely compatible with iron deficiency. WHO recommends measuring C-reactive protein and alpha-1-acid glycoprotein alongside it wherever inflammation is common, which in an Indian ward is most of the time.

7. The Reticulocyte Production Index

A reticulocyte percentage is a proportion of a red cell count that is itself abnormal, so in anaemia it flatters the marrow twice over. Two corrections fix it: multiplying by the patient's haematocrit over a normal 45 divides out the anaemia, and the maturation factor divides out early release, because as anaemia deepens the marrow pushes reticulocytes out earlier and they survive longer in the blood.

• The maturation factor table is not agreed, and this tool names which one it uses. Three mutually incompatible versions circulate. The peer-reviewed one used here: 1.0 at a haematocrit of 40 and above, 1.5 from 30 to below 40, 2.0 from 20 to below 30, and 2.5 below 20. A widely reproduced alternative shifts every band down by about four points, which changes the reported index by up to half at a haematocrit in the thirties.
• Below 2 the marrow is not responding. Above 3 it is responding appropriately, which points at haemolysis or blood loss rather than a production failure. Between 2 and 3 is genuinely indeterminate, although one widely used reference collapses that zone and calls 2 adequate.
• The absolute reticulocyte count sidesteps the whole argument and many haematologists prefer it. It is the percentage multiplied by the red cell count, and this tool prints it alongside.

8. Thresholds, Altitude and the Indian Argument About Them

WHO issued a new guideline on haemoglobin cut-offs on 5 March 2024. Almost everything was retained: the only threshold that moved was for children aged 6 to 23 months, from 110 to 105 g/L.

• Adult thresholds remain 12.0 g/dL for non-pregnant women and 13.0 g/dL for men, and pregnancy is now stated by trimester: 11.0, 10.5 and 11.0 g/dL.
• The altitude adjustment starts at 500 metres, not at 2500. A resident of Shimla, Gangtok or Ooty needs the correction, and it reaches 1.1 g/dL by 1500 metres.
• The adult rows are labelled 15 to 65 years, with no row above 65. Applying the adult cut-off to an 80 year old is an extrapolation the guideline does not make.
• The Indian argument. Sachdev and colleagues took venous samples from healthy Indian children and adolescents in the Comprehensive National Nutrition Survey and derived thresholds 1 to 2 g/dL below the WHO values; on their own cut-offs anaemia prevalence fell from 30.0 to 10.8 per cent. That work is contested, partly because most of the original sample was excluded from the healthy reference group, and India has not adopted a different cut-off. This tool uses the WHO thresholds, and says so.
• The national figures. Anaemia was dropped entirely from the National Family Health Survey 6, whose fact sheets were released on 29 May 2026 with no haemoglobin indicator at all, and the estimate has moved to the ICMR Diet and Biomarkers Survey. The most recent national figures therefore remain those of NFHS-5: 57.0 per cent of women aged 15 to 49, 25.0 per cent of men, 67.1 per cent of children aged 6 to 59 months.
• Anemia Mukt Bharat, launched in 2018 under the National Health Mission, runs a 6x6x6 strategy: six target beneficiary groups, six interventions and six institutional mechanisms, across the life cycle. It applies the same WHO cut-offs and the same severity bands to decide who receives iron and folic acid, who is dewormed and who is referred. It also carries the supplementation and deworming schedule for each group; this page does not. Read its operational guidelines beside this result rather than treating from the number alone.

9. Screening, and Why This Matters Beyond One Patient

• The average beta-thalassaemia carrier frequency in India is 3 to 4 per cent, around 42 million carriers, and 10,000 to 15,000 children with thalassaemia major are born each year. A systematic review of 69 studies put the general population figure at 3.74 per cent and tribal groups at 4.6 per cent. Carrier rates are higher in Sindhi, Punjabi, Gujarati and Bengali communities and in several tribal groups.
• The National Sickle Cell Anaemia Elimination Mission, launched on 1 July 2023, is screening 7 crore people aged 0 to 40 across 278 districts in 17 states, with an elimination target of 2047. By July 2025 it had screened 6 crore and identified 16.7 lakh carriers. It is sickle cell specific: thalassaemia prevention still sits under the 2016 national haemoglobinopathies guidelines and state programmes rather than a comparable national mission.
• Finding a trait is never only about the patient in front of you. It is an indication to test the partner before a pregnancy, and to counsel the family.

10. What This Tool Deliberately Does Not Do

• Diagnose. What it produces is a direction for the next two investigations.
• See alpha-thalassaemia trait, in which the same indices behave unpredictably and haemoglobin A₂ is normal.
• See haemoglobin E trait, which matters in the North East and in Bengal.
• Compute a probability. The published indices give a classification and not a likelihood, and manufacturing one from their agreement would be inventing precision.
• Interpret a count taken during or within three months of a transfusion.
• Stage or manage the anaemia it finds.