MCV Blood Test: Abnormal MCV Explained

What is MCV on a blood test?

MCV (mean corpuscular volume) is the average size of your red blood cells. It is reported automatically as part of a complete blood count (CBC), in femtolitres (fL) – one femtolitre is a millionth of a billionth of a litre. A typical adult MCV is 80–100 fL (NBME reference values).

MCV matters because it is the single most useful number for working out why someone has anaemia. Doctors sort anaemia into three groups by MCV:

• Low MCV (under 80 fL) – microcytic: small red cells. Most often iron deficiency or thalassaemia trait.
• Normal MCV (80–100 fL) – normocytic: normal-sized cells. Often blood loss, chronic disease, kidney disease or a mix of causes.
• High MCV (over 100 fL) – macrocytic: large red cells. Common causes include alcohol, vitamin B12 or folate deficiency, liver disease, an underactive thyroid and some medicines.

An abnormal MCV can also appear without anaemia – a high MCV with a normal haemoglobin is quite common and still worth understanding. The interpreter above classifies your MCV and, if you add the RDW (red cell distribution width), shows which causes the combination favours. The second tab calculates MCV from your haematocrit and red-cell count if your report does not show it.

MCV normal range and categories

Children have lower MCV values than adults – a healthy toddler's MCV can be in the low 70s – so paediatric ranges are used for anyone under about 16. In newborns, MCV is high and falls over the first months of life.

How MCV is calculated

Most modern analysers measure red-cell size directly as cells pass through a sensor. MCV can also be calculated from two other CBC values:

MCV (fL) = haematocrit (%) × 10 ÷ red blood cell count (× 10¹²/L)

Worked examples

• Haematocrit 42%, RBC 4.7 × 10¹²/L: 42 × 10 ÷ 4.7 = 89.4 fL – normocytic.
• Haematocrit 33%, RBC 4.9 × 10¹²/L: 33 × 10 ÷ 4.9 = 67.3 fL – microcytic, with a relatively high red-cell count (a clue to thalassaemia trait, see below).
• Haematocrit 36%, RBC 3.1 × 10¹²/L: 36 × 10 ÷ 3.1 = 116 fL – markedly macrocytic.

The other red-cell indices

MCV is one of several red-cell indices printed on a CBC:

MCH usually moves with MCV: small cells contain less haemoglobin (hypochromic), large cells more. MCHC is less variable; a high MCHC is unusual and can suggest hereditary spherocytosis, cold agglutinins or a sample artefact.

Low MCV (microcytosis)

Small red cells mean the bone marrow could not make enough haemoglobin for each cell. The main causes:

Iron deficiency

The commonest cause worldwide. MCV falls gradually as iron runs out, usually after ferritin has already fallen. The RDW is typically high, because new small cells mix with older normal-sized ones. Platelets are often mildly raised. Confirm with ferritin – see low ferritin – and an iron panel.

Thalassaemia trait

Inherited conditions that reduce production of one of the globin chains of haemoglobin. Carriers ("trait") are usually well, with mild or no anaemia but a strikingly low MCV (often 60s to low 70s), a normal or high red-cell count and a normal RDW. Thalassaemia trait is common in people with Mediterranean, Middle Eastern, South Asian, Southeast Asian and African ancestry. It matters mostly for family planning and to avoid unnecessary iron treatment.

Anaemia of chronic disease (inflammation)

Usually normocytic, but about a quarter to a third of cases become mildly microcytic. Ferritin is normal or high.

Less common causes

Lead poisoning, sideroblastic anaemia and copper deficiency.

Telling iron deficiency from thalassaemia trait

The Mentzer index, described in 1973, is a quick screen: divide MCV (fL) by the RBC count (× 10¹²/L). For the second worked example above, 67.3 ÷ 4.9 = 13.7 – borderline, which illustrates that the index is a clue, not a diagnosis. The RDW pattern described by Bessman and colleagues (high RDW in iron deficiency, normal in thalassaemia trait) is another helpful clue. Both can coexist – a thalassaemia carrier can also become iron deficient.

High MCV (macrocytosis)

Macrocytosis is found in a few percent of adults having routine blood tests, often without anaemia. A 2009 review in *American Family Physician* (Kaferle and Strzoda) divided the causes into megaloblastic (from impaired DNA synthesis, mainly B12 or folate deficiency and some medicines) and non-megaloblastic (everything else).

A very high MCV – above about 115 fL – points more strongly towards B12 or folate deficiency or a medicine effect. Typical next tests include vitamin B12, folate, reticulocyte count, liver tests and TSH, and a blood film. Our vitamin B12 converter and folate converter help with unit differences between laboratories.

Why B12 deficiency matters

Vitamin B12 deficiency can damage nerves – causing numbness, tingling, balance problems and memory changes – and nerve damage can occur even before anaemia appears and is not always fully reversible. Causes include pernicious anaemia (an autoimmune condition), previous stomach or bowel surgery, strict vegan diets without B12 sources, and long-term use of some medicines such as metformin and acid-reducing drugs. If your MCV is high, it is sensible to ask whether your B12 has been checked.

Three example reports

These simplified examples show how MCV is read alongside the rest of the complete blood count. They are illustrations, not diagnoses.

Example A: small, variable-sized cells, a normal-to-low red cell count and a Mentzer index above 13 – the classic pattern of iron deficiency. A low ferritin would confirm it.

Example B: very small cells but a *high* red cell count, uniform cell size and a Mentzer index below 13 – more typical of thalassaemia trait, which is confirmed with haemoglobin electrophoresis. Iron treatment does not help thalassaemia trait, which is why telling these apart matters.

Example C: large cells and a high RDW. The next step is usually vitamin B12 and folate levels, a reticulocyte count, and a review of alcohol intake, medicines, thyroid and liver tests.

The Mentzer index is a quick screening clue, not a diagnosis – it can mislead when iron deficiency and thalassaemia trait occur together, or in children and pregnancy.

Normal MCV with anaemia (normocytic)

When haemoglobin is low but MCV is normal, common causes are:

• Acute blood loss – before the marrow has had time to respond.
• Anaemia of chronic disease – infections, autoimmune disease, cancer.
• Chronic kidney disease – reduced erythropoietin.
• Mixed deficiencies – iron (pulls MCV down) plus B12 or folate (pulls it up) can "average out" to a normal MCV. A high RDW is the clue.
• Haemolysis – early, or when reticulocytes are not high enough to raise MCV.
• Bone-marrow problems and endocrine disorders.

The reticulocyte count helps here: a high count means the marrow is responding (bleeding, haemolysis); a low count means production is the problem.

RDW: the partner of MCV

This grid is a simplified version of the classification proposed by Bessman and colleagues in 1983. It is a guide to likely causes, not a substitute for clinical assessment.

MCV over time

Because red cells live for about 120 days, the MCV reflects a mixture of cells made over the past three to four months. That has practical consequences:

• MCV is a slow marker. In developing iron deficiency, ferritin falls first, then transferrin saturation, and only later does the MCV drift down as small new cells gradually replace older ones.
• After a deficiency is corrected, the marrow releases a wave of new, normal-sized (or, for a while, slightly larger) cells. The RDW often rises temporarily during this recovery because two populations of cells are present, then settles as the older cells are cleared.
• A trend is more informative than one value. An MCV that has crept from 88 to 99 fL over several years is worth noticing even though both values are "normal", whereas a stable MCV of 78 fL that has been present since childhood in someone with known thalassaemia trait is expected.

If you have access to older CBC results, compare them – many patient portals show a graph.

MCV in particular groups

Pregnancy

MCV rises slightly in normal pregnancy. Because iron and folate requirements increase, both microcytosis (from iron deficiency) and macrocytosis (from folate deficiency) can develop, and a mixed picture can leave the MCV in the normal range. Antenatal blood counts are interpreted with this in mind, and many maternity services screen for thalassaemia and other haemoglobin disorders early in pregnancy.

Older adults

Macrocytosis becomes more common with age, and myelodysplastic syndromes – bone-marrow disorders that can cause a high MCV with low counts – are mainly diseases of older adults. B12 deficiency is also more common in older people because of reduced stomach acid and some medicines. A new macrocytosis in an older adult is therefore usually investigated.

People who drink alcohol

Regular heavy drinking is one of the most common causes of a raised MCV, and it can take two to four months of abstinence for the MCV to normalise, matching red-cell lifespan. A raised MCV together with a raised GGT is a common pattern.

Vegans and vegetarians

Plant-based diets contain little or no vitamin B12 unless foods are fortified or a B12 source is included. A rising MCV can be an early clue, though neurological symptoms can occasionally appear before the MCV changes.

Preparing for the test

No preparation is needed for an MCV – it comes from a standard CBC tube. Results can be distorted if the sample is very old, refrigerated incorrectly, or affected by cold agglutinins (antibodies that clump red cells at room temperature), which can produce a falsely high MCV and low red-cell count. Very high blood glucose can also make cells swell in the analyser's diluent and raise the measured MCV. Laboratories usually recognise these artefacts and comment on them.

When to see a doctor

• Your MCV is outside 80–100 fL with anaemia or symptoms such as tiredness, breathlessness, numbness or tingling.
• Your MCV is over 115 fL, or keeps rising.
• More than one blood count line is abnormal (for example low haemoglobin plus low white cells or platelets).
• You have a low MCV and ferritin has not been checked.