MCH: what it is, what it tells you and what a low level means

MCH shows how much haemoglobin an average red blood cell contains. It almost always moves with MCV, which shows how big your red cells are. If it is low, iron is usually short. Sometimes it means carrying thalassaemia trait, which is inherited and common in Italy.

Why we test it

Haemoglobin is the protein that carries oxygen. MCH shows how much of it sits inside each red blood cell, on average.

When iron is short, your body makes smaller red cells with less haemoglobin, and MCH drops. Of all the blood count numbers, it is one of the most useful signs of low iron.

You find it in the full blood count, the most common blood test, usually next to MCV and MCHC.

What it tells you, and what it does not

If it is low, iron is often short. But it is low in people with thalassaemia trait too, and then iron usually does not help.

If it is high, your red cells are usually bigger than normal. You may be short of vitamin B12 or folate, another B vitamin, or alcohol may be behind it.

It is not read on its own: it is calculated from haemoglobin and the red cell count. But read with the others, it is a good sign of low iron.

That is why it is read together with other tests:

• MCV: How big your red cells are. It usually moves with MCH
• Haemoglobin: Whether there is anaemia
• Ferritin: Whether iron is short
• MCHC: How concentrated the haemoglobin in your red cells is

Normal MCH levels

The lab decides what is normal. It is usually 27 to 32 pg, the same for women and men. Your own report may print different limits.

A pg, or picogram, is a tiny amount: it is used to measure what is inside a single cell.

If your MCH is just outside the limits and your haemoglobin, MCV and ferritin are normal, it usually means little.

In the Rosa report you also see your optimal range, read together with MCV, haemoglobin and ferritin.

What your result means

Find the row for your result, in pg. Which values are dangerous? On its own, MCH usually is not: what counts is whether haemoglobin, MCV or ferritin are out of range too.

• Below your report’s limit, Usually below 27: Low. Each red cell carries less haemoglobin. Usually iron is short, sometimes an inherited trait is behind it. Check your MCV, haemoglobin and ferritin too. If any of them is out of range, talk to your doctor.
• Within your report’s limits, Usually 27 to 32: Normal. Usually fine. But your iron store can drop before MCH changes. In the Rosa report you also see whether you are in your optimal range.
• Above your report’s limit, Usually above 32: High. Your red cells are usually bigger than normal, from too little vitamin B12 or folate, or alcohol. If it is just above and everything else is normal, it usually matters little. Check your MCV, haemoglobin and vitamin B12 too. If any of them is out of range, talk to your doctor.

If your MCH is low or high

• Low MCH: Iron is usually short, sometimes an inherited trait is behind it. The causes are below.
• High MCH: It usually means bigger red cells. More on this below.

Low MCH: the causes

The first is the most common.

• Too little iron: Without enough iron your body makes smaller red cells with less haemoglobin. Heavy periods, pregnancy or too little iron in your diet are common reasons.
• Hidden blood loss: Small losses from the gut that you cannot see. In men and after menopause, they are the first thing doctors look for.
• Thalassaemia trait: An inherited trait that is common in Italy. Red cells are small and low in haemoglobin, but iron is usually not short.
• Long-lasting inflammation: An illness that lasts a long time can stop your body using its iron well, even when there is iron in store.

High MCH: what it means

It usually comes with a high MCV: bigger red cells hold more haemoglobin. It does not mean your blood carries more oxygen.

The most common causes are too little vitamin B12 or folate, alcohol and some medicines. More rarely the liver or the thyroid is involved.

If MCH is high and MCV is not, the first suspect is a problem with the sample: your doctor usually has the test repeated.

When to worry

Talk to your doctor if:

• It is low and your haemoglobin is low too.
• It is low but your ferritin is normal: your doctor may check whether you carry thalassaemia trait.
• It is high and your MCV is high too, or your vitamin B12 is low.
• You are a man, or a woman after menopause, and iron is short.
• You see black stools or blood in your stools: call 112 or go to the emergency department.
• You are short of breath at rest, have chest pain or feel faint: call 112 or go to the emergency department.

Frequently asked questions

Low MCH and low MCV: what does it mean? Your red cells are small and low in haemoglobin. Iron is usually short, and ferritin helps tell. If your ferritin is normal, your doctor may check whether you carry thalassaemia trait.

Slightly low MCH: should I worry? Usually not, if your haemoglobin, MCV and ferritin are normal. Show the report to your doctor at your next check-up.

What is the difference between MCH and MCHC? MCH is how much haemoglobin a red cell holds. MCHC is how concentrated it is, so it also takes the size of the cell into account. MCHC usually drops only with a marked lack of iron.

Does a high MCH mean my blood carries more oxygen? No. It usually means your red cells are bigger, not that they work better. What matters is finding out why.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it twice a year, with the blood count. If it is out of range, your doctor decides whether and when to check it again.

Sources

• British Society of Gastroenterology, guidelines for iron deficiency anaemia in adults. Gut, 2021. PMID 34497146
• Kaferle and Strzoda, evaluation of macrocytosis (red cells larger than normal). American Family Physician, 2009. PMID 19202968
• Dell’Edera and colleagues, how common beta thalassaemia trait is among Italian couples planning a pregnancy. Journal of Maternal-Fetal and Neonatal Medicine, 2013. PMID 23039078
• Zandecki and colleagues, spurious results on haematology analysers: red cells, haemoglobin and red cell indices. International Journal of Laboratory Hematology, 2007. PMID 17224005