What does an MCV that is too high or too low mean?

Your blood contains billions of red cells, and MCV describes how large an average one of them is. The abbreviation stands for mean corpuscular volume, and the unit is the femtolitre (fL). Modern cell counters measure that volume directly: the analyser sizes each red blood cell individually and reports the mean of that distribution. The haematocrit is the derived value, MCV multiplied by the red cell count. Older, manual methods did back-calculate MCV from a spun haematocrit. As a result MCV almost always arrives as part of a full blood count, and no separate tube has to be drawn for it.

An MCV between 80 and 100 fL is considered normal in adults. The median rises slightly with age, so a value of 92 or 94 fL in an adult is ordinary and not in itself an abnormality.

The figures in this table are medians in people without anaemia, collected at a single South Korean hospital, and not cut-offs. Your own laboratory always states the reference value belonging to the method it used.

MCV is not sex-specific: the same range of 80 to 100 fL applies to men and to women. That sets MCV apart from haemoglobin and haematocrit, which do carry separate reference values for men and women. MCV is, however, sensitive to how the sample was stored. In an EDTA tube red cells swell during longer storage, so a sample analysed only after 24 hours gives an artificially raised MCV alongside a lowered MCHC. An isolated, mildly raised MCV without any other abnormality is therefore first a question about the sample and only then a clinical question.

MCV is the classic signpost in anaemia. A low haemoglobin on its own says only that oxygen transport is short; it says nothing about the cause. MCV divides that problem into three tracks and so gives the investigation a direction: microcytic below 80 fL, normocytic between 80 and 100 fL, and macrocytic above 100 fL.

With small cells, iron deficiency is by far the leading explanation, and ferritin is the next value that brings clarity. Alongside it sits thalassaemia trait, an inherited pattern in which the cells are small while the red cell count is normal to high and the ferritin is perfectly fine. The Mentzer index, MCV divided by the red cell count, is used as an indication of which way things point: above 13 fits iron deficiency better, below 13 fits thalassaemia trait. That is explicitly an indication and not proof, and the outcome belongs in a conversation with a doctor. Anaemia of chronic disease can also make the cells smaller.

With large cells there are two families. The megaloblastic group arises from a vitamin B12 or folate deficiency, where the laboratory sometimes also sees hypersegmented neutrophils. The non-megaloblastic group covers chronic alcohol use, liver disease, an underactive thyroid, a strong output of young cells, myelodysplasia, and medicines such as methotrexate, hydroxyurea or zidovudine. Alcohol raises MCV directly, even without any B12 deficiency, and the value falls back only slowly, roughly on the timescale over which red cells are replaced: up to about three months.

The most important pitfall of MCV is that a normal value does not rule out a deficiency. MCV is an average, not a description of the spread. If two deficiencies exist side by side, for instance iron together with vitamin B12 or folate, the small and the large cells cancel each other out within that average, and the MCV can be entirely normal while something is genuinely going on. In that scenario the RDW, which describes the spread in cell size, is usually raised. That is why MCV should always be read together with RDW and never on its own.

MCV is rarely requested on its own. It forms part of the full blood count and therefore comes along with almost any general blood test. Even so, there are situations in which this particular value supplies the answer.

The first is persistent tiredness, breathlessness on exertion, pallor, dizziness or headache without a clear explanation. If a low haemoglobin is found alongside those, MCV determines which direction the follow-up takes: ferritin for small cells, vitamin B12 and folate for large ones.

The second is following up a previously abnormal result. Because red cells live for roughly one hundred and twenty days, MCV changes slowly. A recheck after two to three months is the first that genuinely shows whether anything has shifted; repeating sooner mostly produces noise.

The third is a known reason to expect a deficiency, for example a strongly restricted diet, reduced absorption after stomach surgery, long-term use of certain medicines, regular alcohol use, or blood loss. In those cases a baseline value is useful, so that a later measurement has something to be compared against.

You do not need to fast for the test itself. It is sensible to have the blood analysed on the day it is drawn, because longer storage raises MCV artificially. Always discuss an abnormal result with your doctor, and particularly one that repeats.