Meera Nair has been taking iron tablets for eight months.
She has been consistent, mostly. One tablet, after lunch. On the diet side, she has eaten the dates, drunk the beetroot juice her aunt keeps recommending, and cooked in the iron kadhai her mother posted to her from Nashik specifically for this purpose. Because according to her doctor, even the vessels you cook in matter.
Meera’s reports are in. This is her third since January – yes, she is a little on the health junkie side, and that is not a bad place to be. It just means she is tracking closely.
Her haemoglobin has moved. It was 9.6 g/dL in January. It is 10.4 now, which everybody agrees is progress.
But one line has not moved at all.
MCV: 68.9 fL. Almost exactly where it was eight months ago.
| Result | Meera’s value | Reference range |
| Haemoglobin | 10.4 g/dL ↓ Low | 12.0 – 15.0 |
| RBC count | 5.12 million/µL ↑ High | 3.8 – 4.8 |
| MCV | 68.9 fL ↓ Low | 80 – 100 |
| MCH | 20.3 pg ↓ Low | 27 – 32 |
| MCHC | 29.5 g/dL ↓ Low | 32 – 36 |
| Haematocrit | 35.3% ↓ Low | 36 – 46 |
| RDW-CV | 14.1% | 11.5 – 14.5 |
| Ferritin | 42 ng/mL | 15 – 150 |
Six arrows on the page.
And her ferritin, one of the main markers used to assess iron stores, is no longer low. It was 8 ng/mL in January. Now it is 42.
Whatever else is happening, her iron status looks very different from where it started.
Her red blood cells are still small.
Which raises a question nobody had asked her in eight months. She thought of it herself, because she reads.
Hence the nuance of the question.
Yes, I made that word up.
If iron is no longer the missing piece, why are the cells still small?
The answer she was given
When Meera first showed her report to a doctor friend, the advice took about four seconds.
You’re anaemic. Take iron.
Two things about that.
The friend was not wrong. Her haemoglobin was low, iron deficiency is the most common single cause of anaemia in India, and a ferritin of 8 confirmed it at the time.
But Meera needed to hear something more than take iron, and she could not quite prod for it. Four seconds of advice answered the haemoglobin. It left the MCV completely alone.
What does a low MCV mean?
MCV stands for mean corpuscular volume: the average size of the red blood cells in your sample.
It sorts red cells into three broad groups:
- Microcytic – smaller than expected on average
- Normocytic – average size within the reference range
- Macrocytic – larger than expected on average
Meera’s 68.9 fL is clearly microcytic. Her laboratory’s range begins at 80.
But a low MCV is a description, not a diagnosis.
Why are these cells small?
Two possibilities come up quickly, particularly in India.
Iron deficiency, where the body does not have enough available iron to make haemoglobin normally.
And thalassaemia trait, where the problem is not a shortage of iron at all.
Haemoglobin is the oxygen-carrying protein inside red blood cells. Part of it is built from protein pieces called beta chains. Your body has instructions for making those chains.
In beta-thalassaemia trait, one of the two copies of that instruction is altered. The body can still make haemoglobin, usually enough for the person to remain well, but it makes less beta chain than usual. One result is red blood cells that tend to be smaller.
That is all we need “globin change” to mean here.
Iron deficiency: not enough raw material.
Beta-thalassaemia trait: an inherited change in the instructions.
Both can leave you with small, pale red blood cells.
Very similar result.
Very different reason.
Iron deficiency needs the deficiency corrected and, importantly, an explanation for why the iron became low.
Beta-thalassaemia trait itself usually does not need treatment. Iron will not correct the small cells caused by the trait if iron deficiency is not also present.
Other causes of microcytosis exist too, including some forms of anaemia associated with chronic inflammation, sideroblastic anaemia and lead exposure.
So MCV tells us what the cells look like.
It cannot tell us why.
Which is when the other numbers start earning their place.
Where RDW comes in, and why it is not enough
RDW tells us how much the red cells differ in size from one another.
In developing iron deficiency, newer cells may become smaller while older cells made under better iron conditions are still circulating.
Now the sample contains a wider mix of sizes.
RDW often rises.
With beta-thalassaemia trait, the small-cell pattern can be more uniform, so RDW may stay within range or be less elevated than it is in iron deficiency.
But that is a tendency.
Not a rule.
A Mangalore study of 350 people with microcytic, hypochromic anaemia makes the problem obvious. Average RDW was 19.3% in iron-deficiency anaemia and 16.9% in beta-thalassaemia trait. Both groups could have elevated RDW, and there was enough overlap that RDW alone could not tell you which condition an individual had.
Now look at Meera.
| January | September | |
| Haemoglobin | 9.6 g/dL ↓ | 10.4 g/dL ↓ |
| MCV | 66.2 fL ↓ | 68.9 fL ↓ |
| RDW-CV | 17.4% ↑ High | 14.1% |
| Ferritin | 8 ng/mL ↓ Low | 42 ng/mL |
In January, she had iron deficiency on top of whatever was already keeping her cells small.
Her RDW was high.
Eight months later, her ferritin has recovered into range and her RDW has narrowed.
The MCV remains stubbornly low.
For Meera, RDW is a weather report, not a climate record.
The inherited trait is the climate. Her iron status changed the weather.
That is also why the internet rule:
high RDW = iron deficiency
normal RDW = thalassaemia
works just often enough to become dangerous.
What about the Mentzer index?
So researchers built formulas combining several results.
The best known is the Mentzer Index, and it rests on another difference: both conditions make small cells, but the number of cells often behaves differently.
Think of the bone marrow as a bakery.
In iron deficiency, the bakery is short of flour. Production becomes harder, and the loaves it manages to make are small.
In beta-thalassaemia trait, if iron is adequate, flour is not the main problem. The recipe is altered. The bakery often keeps turning out a relatively large number of loaves, but they tend to be small.
The Mangalore study showed that pattern. Average RBC count was 3.98 million/µL in iron deficiency and 5.22 million/µL in beta-thalassaemia trait.
The Mentzer Index turns that into maths:
MCV ÷ RBC count
Traditionally:
below 13 → leans towards beta-thalassaemia trait
above 13 → leans towards iron deficiency
Meera gets:
68.9 ÷ 5.12 = 13.5
By the classic cut-off, that lands just on the iron-deficiency side.
But here is the part Google calculators rarely tell you.
In the Mangalore study itself, the cut-off that performed best in that population was closer to 14.1, not 13. Meera’s exact same 13.5 would then fall on the thalassaemia side.
Different cut-off, different answer.
Which is why Mentzer is a screening clue, not a diagnosis. The blood count has now told us almost everything it can.
It still has not settled the question.
So what actually settles it?
Not the CBC. It only tells you which question to ask.
If iron deficiency is the question, clinicians look at iron status directly.
- Ferritin is the most useful single test. A low result strongly supports depleted stores. A normal one does not rule it out, because ferritin climbs with inflammation – which is when transferrin saturation and inflammatory markers get added.
If thalassaemia trait is the question, the next step is a haemoglobin study, using a method such as HPLC or electrophoresis.
- It separates out the different types of haemoglobin you carry. Beta-thalassaemia trait usually raises one of them, HbA2, above roughly 3.5%. The laboratory and the wider pattern both matter.
- If the blood count still looks thalassaemic and the haemoglobin report comes back normal, alpha thalassaemia is the other possibility. It can look essentially normal on that test, so establishing it usually needs genetic testing.
Alongside either, two things: a peripheral blood film, where someone looks at the cells directly instead of the averages an analyser produces, and your family history and older reports.
Two complications get missed.
Timing. Iron deficiency can lower HbA2 in some people, which muddies a borderline result. But the science is not as tidy as iron deficiency hides thalassaemia – other work has found that carriers usually still show HbA2 in the diagnostic range even when iron deficient. Where a result does sit on the line, repeating it after iron is corrected, or moving to genetic testing, is reasonable.
Coexistence. Among blood donors found to have a low MCV, roughly half turn out to have iron deficiency alone, about a third an inherited haemoglobin condition alone, and the rest both. Finding one does not close the file on the other.
Which is what Meera’s two reports have been saying all along.
The part nobody mentioned
Meera is 29. She and her husband have started talking about having a child.
Nobody had asked her about that, which is the most consequential omission in this story.
Thalassaemia trait is something you are born with. You inherit one set of haemoglobin instructions from your mother and one from your father. Meera has one working set and one faulty one. The working set is enough to keep her well, which is how she spent twenty-nine years not knowing.
Meera’s child would inherit one copy of those haemoglobin instructions from her and one from her husband.
If her husband is not a beta-thalassaemia carrier, their child cannot inherit beta-thalassaemia disease from this pairing. Each child would have a 50% chance of inheriting Meera’s trait.
If her husband is also a carrier, each pregnancy has:
25% chance of a child inheriting neither altered copy
50% chance of a child being a carrier like the parents
25% chance of a child inheriting an altered copy from both parents and having beta-thalassaemia
How severe the condition is depends on the particular variants inherited. Some forms require regular blood transfusions; others are less severe.
So Meera’s result on its own settles very little. The next useful question is whether her husband carries a beta-thalassaemia variant too.
When should you speak to a doctor about a low MCV?
Take the report to a clinician, with any older ones you can find. It warrants a proper look if:
- Haemoglobin is also below the reference range
- MCH, MCHC or haematocrit are low, or the RBC count is high
- You are pregnant or planning a pregnancy
- There is a family history of thalassaemia, or a relative told to take iron indefinitely without improvement
- The same pattern has appeared across several reports
- You have been taking iron and the MCV has not shifted
- There is any ongoing blood loss, including heavy periods or gastrointestinal symptoms
Seek prompt medical attention for severe breathlessness, chest pain, fainting, confusion, heavy bleeding or rapidly worsening symptoms, whatever the blood count says.
Meera’s report, read properly
Her haemoglobin study came back with an HbA2 of 4.8%, against a threshold of 3.5%.
Thalassaemia trait. Inherited from one of her parents, carried for twenty-nine years, found because one number on a blood count refused to move.
Nothing about her health has changed. She feels exactly as she did last week. Trait needs no treatment and will not shorten her life.
Two things have changed.
She has stopped taking iron she no longer needs, because her stores are full and the small cells were never about iron.
And her husband has a blood test booked, to find out whether he carries it too.
Her report has not changed. The sentence attached to it has.
It no longer says: my haemoglobin is low and the iron is not working.
It says: my red blood cells are small because of something I was born with. Now I know, and now it is information rather than a mystery.
Six arrows on a page. It took eight months and one more test to find out what they were actually saying.
FAQs
What does a low MCV mean?
The red blood cells in your sample are smaller than expected on average. This is called microcytosis. It describes the cells. It does not identify the cause.
Is a low MCV always iron deficiency?
No. Iron deficiency is the most common cause in India and thalassaemia trait is another common one. Anaemia associated with chronic inflammation, lead exposure and sideroblastic anaemia can also produce small red cells.
Can RDW tell iron deficiency from thalassaemia trait?
Not reliably. Iron deficiency tends to raise RDW more and trait tends to keep the spread narrower, but the two overlap enough that RDW alone cannot decide an individual case.
What is the Mentzer index?
MCV divided by RBC count. Below 13 favours thalassaemia trait, above 13 favours iron deficiency. It performs well across groups, and results close to the cut-off are common, so it is a screening aid rather than a diagnosis.
Which test confirms thalassaemia trait?
A haemoglobin study, by HPLC or electrophoresis, is the usual route for beta-thalassaemia trait, which typically shows HbA2 above 3.5%. Alpha thalassaemia trait generally requires genetic testing.
Can you have iron deficiency and thalassaemia trait together?
Yes, and it is not unusual. Confirming one does not exclude the other. Iron deficiency can also make trait harder to detect, which is why testing may be repeated once iron stores are corrected.
Will iron fix a low MCV?
Only if iron deficiency is the cause. In thalassaemia trait, MCV does not respond to iron, because iron was not the limiting factor.
My haemoglobin improved on iron but my MCV did not. What does that mean?
Worth taking to a clinician. It can happen when iron deficiency was present alongside another cause of small red cells, and it is one of the patterns that prompts a haemoglobin study.
Why does thalassaemia trait matter if it causes no symptoms?
Because it is inherited. Where both partners carry it, each pregnancy carries a one in four chance of thalassaemia major, which is serious and transfusion-dependent.
Does a normal RDW mean my low MCV is nothing to worry about?
No. A normal RDW means the cells are similar to one another in size. It says nothing about whether that size is normal. Meera’s report is exactly this case.
