MCV: what it is, what it tells you and what high or low means

MCV, or mean cell volume, shows how big your red blood cells are on average. On its own it says little: it mainly helps explain another result, such as a low haemoglobin. If it is low your red cells are small, if it is high they are large, and the causes are different.

Why we test it

The bone marrow builds red cells with what it has. Short of iron, they come out smaller; short of vitamin B12 or folate, they come out larger.

So MCV is a trace of how your red cells are made. It does not show how well they work, but it points to where to look for the cause of a problem.

It is part of the full blood count. On an Italian report it appears as MCV, sometimes written out as volume corpuscolare medio or volume globulare medio.

What it tells you, and what it does not

If it is low, your red cells are small. Usually iron is short, or you carry thalassaemia trait, which is inherited and common in Italy.

If it is high, your red cells are large. Often vitamin B12 or folate is short, or alcohol or some medicines are behind it. More rarely a liver or thyroid problem.

A normal MCV does not rule out iron deficiency: ferritin falls well before red cells get smaller. And with a normal haemoglobin, an MCV just outside the limits is usually a clue, not a problem.

That is why it is read together with other tests:

• Haemoglobin: Whether there is anaemia
• Ferritin: How much iron you have in store
• Vitamin B12: If it is short, red cells get bigger
• Red blood cells: How many there are: many small ones point to an inherited trait

Normal MCV levels

The lab decides what is normal. It is usually 80 to 94 femtolitres (fL), for women and men alike. Your own report may print different limits.

In the same person MCV moves very little over time. So even a small change between two tests can mean something.

Above 100 fL it is usually called macrocytosis, meaning large red cells. Below your report’s limit it is called microcytosis, meaning small red cells.

In the Rosa report you also see your optimal range, read together with haemoglobin, ferritin and vitamin B12.

What your result means

Find the row for your result, in fL. Which values are dangerous? MCV on its own usually is not: it is a clue. Haemoglobin is what shows whether there is anaemia.

• Below your report’s limit, Usually below 80: Low. Your red cells are small. Usually iron is short, or it is an inherited trait. Check your haemoglobin and ferritin, and show your report to your doctor.
• Within your report’s limits, Usually 80 to 94: Normal. Your red cells are a normal size. But iron can still be short. In the Rosa report you also see whether you are in your optimal range.
• Above your report’s limit, Usually above 94: High. Your red cells are large: too little vitamin B12 or folate, alcohol or some medicines are often behind it. If it is just above and your haemoglobin is normal, it usually matters little. Check your vitamin B12 and haemoglobin. If either is out of range, or your MCV is well above the limit, show your report to your doctor.

If your MCV is high or low

• Low MCV: Small red cells: usually too little iron or an inherited trait. The causes are below.
• High MCV: Large red cells: often too little vitamin B12 or folate, or alcohol. The causes are further down.

Low MCV: the causes

It is usually one of the first two. Ferritin helps tell them apart.

• Too little iron: The most common cause, for example from heavy periods or small blood losses. Ferritin is usually low.
• Thalassaemia trait: An inherited trait common in Italy. The red cells are small but numerous, and ferritin is usually normal.
• Long-lasting inflammation: More rarely, a disease that has lasted a long time locks iron away in the stores, and red cells come out a little smaller.

High MCV: the causes

Large red cells have different causes from small ones. The most common:

• Too little vitamin B12 or folate: without them the bone marrow cannot divide cells properly, so they come out larger.
• Alcohol: drinking regularly makes red cells bigger, often together with a high GGT, a liver test.
• Some medicines: if you have taken any for a long time, bring the list to your doctor.
• Many young red cells, which are larger: this happens after bleeding, or when red cells break down too early.
• More rarely, liver disease, an underactive thyroid or a bone marrow problem.

Your doctor usually starts with vitamin B12, folate and liver tests. If they are normal, they look further for the cause.

What can skew the result

MCV changes slowly, because red cells are replaced slowly. But a few things are worth knowing.

• A sample that waits: If the blood sits at room temperature for over a day before analysis, the red cells swell a little and MCV reads higher.
• Two shortages at once: With too little iron and too little vitamin B12 together, small and large cells cancel out and MCV can look normal. RDW, on the same blood count, helps spot it.
• Recent bleeding: The bone marrow quickly makes young red cells, which are larger, and MCV can rise for a while.
• Treatment that just started: After treatment with iron or vitamin B12, MCV returns to normal slowly: it usually takes a few months.

When to worry

Talk to your doctor if:

• Your MCV is out of range and your haemoglobin is low too.
• It is low but your ferritin is normal: your doctor can ask for a test to see whether you carry thalassaemia trait.
• You are a man, or a woman after menopause, and iron is short.
• It is well above the limit and your vitamin B12 is normal: your doctor looks for the other causes.
• It changes a lot from one test to the next, for no clear reason.
• You have tingling in your hands and feet or feel unsteady: with a high MCV, it can come from too little vitamin B12.
• 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

Can a low MCV mean cancer? Usually not: the most common causes are too little iron and thalassaemia trait. In men and after menopause, if iron is short, doctors look first for blood loss from the stomach or gut. Sometimes that comes from a tumour, which is why a low MCV with too little iron should be checked by a doctor.

What MCV do people with thalassaemia trait have? MCV is usually low, often very low for how low the haemoglobin is. The red cell count is normal or high, and ferritin is usually normal. A number alone is not enough: haemoglobin electrophoresis, a test that separates the types of haemoglobin, confirms it, and your doctor can ask for it.

Does a high MCV mean I drink too much? Not necessarily. Alcohol is one of the most common causes, especially if your GGT is high too. But so are too little vitamin B12 or folate and some medicines: your doctor looks at everything together.

MCV out of range but normal haemoglobin: should I worry? Usually not: with a normal haemoglobin, an MCV just outside the limits is more a clue than a problem. It is worth checking your ferritin and vitamin B12, and showing your report to your doctor.

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. MCV changes slowly: if it is out of range, your doctor decides when to check again, often after about three months.

Sources

• British Society of Gastroenterology, guidelines for iron deficiency anaemia in adults. Gut, 2021. PMID 34497146
• Kaferle and Strzoda, evaluation of macrocytosis. American Family Physician, 2009. PMID 19202968
• Dell’Edera and colleagues, beta thalassaemia carriers among couples planning a pregnancy in Italy. Journal of Maternal-Fetal and Neonatal Medicine, 2013. PMID 23039078
• Coşkun and colleagues, how much blood count values vary within one person and between people. Clinical Chemistry and Laboratory Medicine, 2018. PMID 29605821
• Gulati and colleagues, how blood count results change when blood is kept at room temperature. Archives of Pathology and Laboratory Medicine, 2002. PMID 11860310