MCV, MCH and MCHC

Your full blood count has three lines that often raise questions: MCV, MCH and MCHC. They describe your red blood cells: how big they are and how much haemoglobin they carry. Especially when your haemoglobin is low, they help your doctor find the cause. In this article you'll read what each value measures, which patterns come up often and what to discuss with your GP.

What do MCV, MCH and MCHC measure?

Picture your red blood cells as tiny bags filled with haemoglobin, the protein that carries oxygen around your body.

MCV (mean corpuscular volume): how big the bag is on average.
MCH (mean corpuscular haemoglobin): how much haemoglobin one bag holds on average.
MCHC (mean corpuscular haemoglobin concentration): how full the bag is.

MCV and MCH usually move together: smaller cells hold less haemoglobin. MCHC tends to stay steadier, because your body guards how full a cell is more closely than how big it is.

Units sometimes differ between labs. Always compare your value with the reference range your lab prints next to it. RDW belongs to this group too: it shows how much your red blood cells vary in size.

Low MCV and MCH: small red blood cells

Small cells with little haemoglobin appear when your body can't make enough haemoglobin. The two most common causes:

Iron deficiency: without enough iron, your bone marrow makes smaller, less filled red blood cells. A shortage often comes from blood loss, such as heavy periods, or from needing more iron than your body takes in.
Thalassaemia trait: an inherited trait in which your body builds haemoglobin slightly differently. Carriers naturally have small red blood cells, sometimes with mild anaemia. It's more common in people with roots around the Mediterranean, in Africa or in Southeast Asia.

Less often, a long-lasting inflammation or illness plays a part. It holds back the use of iron, even when your stores are fine.

What will your doctor do? Usually check your ferritin first, which shows your iron stores. If it's low, your doctor will also look for where the iron is going. If it's normal, other causes such as thalassaemia trait come into view.

Worth knowing: a normal MCV doesn't rule out iron deficiency. In some people with anaemia from iron deficiency, the red blood cells are still a normal size.

High MCV and MCH: large red blood cells

Large red blood cells are seen with, among other things:

A shortage of vitamin B12 or folate: both are needed for red blood cells to mature properly.
Alcohol: drinking regularly can make your red blood cells larger, even without a vitamin shortage.
Some medicines: for example certain epilepsy medicines, some cancer treatments and methotrexate.
An underactive thyroid or liver disease: which is why your doctor sometimes also looks at your TSH and liver values.

A high MCV can also occur without anaemia. It's still worth finding the cause.

What will your doctor do? Often test your vitamin B12 and folate, and have your blood cells looked at under a microscope. Your doctor will also ask about your medicines and how much you drink. Honest answers make the search a lot easier.

Low or high MCHC

A low MCHC usually goes with a low MCV and MCH, in iron deficiency or thalassaemia trait.
A high MCHC is unusual. It fits red blood cells that are round like a ball instead of flat, as in the inherited condition spherocytosis.

MCHC is always read together with your haemoglobin and MCV, never on its own.

What to discuss with your GP

These patterns are leads for your doctor, not a diagnosis. Take your report along and go through it together:

Your haemoglobin: is there anaemia, or are only the indices off?
Follow-up values: have your ferritin, vitamin B12 and folate been measured?
Your story: tiredness, heavy periods, a vegetarian or vegan diet, stomach or bowel complaints, how much you drink and which medicines you take.
Your family: does anaemia or thalassaemia run in your family?
Earlier results: has your MCV been the same for years, or has it changed recently?

Hold off on iron tablets until the cause is clear. Extra iron only helps when there's a real shortage, and too much iron is harmful.

Conclusion

MCV, MCH and MCHC tell you how big your red blood cells are and how well filled they are. Small cells often point towards iron deficiency or thalassaemia trait, large cells towards vitamin B12, folate, alcohol or medicines. Which cause applies to you is something to work out with your GP. For more on the causes of anaemia, read Anemia, and for the other lines on your report, Blood test results explained.