MCV (mean corpuscular volume): high and low results explained

Red blood cells are not all the same size, and the average size changes with the way the bone marrow is working and with the supplies of building materials such as iron, vitamin B12 and folate. MCV puts a number on the average cell volume. Lab Tests Online UK describes it as part of a group of red cell indices, along with MCH, MCHC and RDW, that relate to the volume, size and haemoglobin content of the red cells and are measured or calculated as part of a full blood count.

The result is reported in femtolitres. You do not need to know that unit to interpret your report; what matters is whether your figure sits inside the interval printed by the laboratory, below it, or above it.

MCV is reported with every full blood count, which may be requested for tiredness, breathlessness, bruising, a routine check, monitoring of a long-term illness or before surgery. Lab Tests Online UK explains that it helps in the diagnosis or monitoring of conditions that affect red cell size, shape and haemoglobin content, such as anaemia, iron or B12 deficiency and thalassaemia.

Its main strength is sorting anaemia into types. When haemoglobin is low, the MCV points the clinician toward the likeliest cause and therefore toward the most sensible next test.

You do not need to fast or prepare. A blood sample is taken from a vein in the arm. Lab Tests Online UK notes that normal ranges differ by age and sex, that medicines and heavy drinking can alter red cell measurements, and that vitamin and mineral deficiencies affect them too. A recent blood transfusion can also temporarily change the results.

Because red cells live for about four months, MCV changes slowly. A deficiency that has just begun may not show in the figure for some time, and a corrected deficiency may take weeks to months to bring it back to normal.

Lab Tests Online UK says that when MCV is high the red cells are larger than normal, a state called macrocytic, and lists vitamin B12 deficiency, folate deficiency, liver disease, an underactive thyroid, alcohol excess and some bone marrow disorders among the common causes. The NHS explains that vitamin B12 or folate deficiency anaemia occurs when a lack of either vitamin causes the body to produce abnormally large red blood cells that cannot function properly.

The NHS says pernicious anaemia, an immune condition that stops B12 absorption, is the most common cause of B12 deficiency in the UK, and that certain medicines including anticonvulsants and proton pump inhibitors can affect how much of these vitamins the body absorbs. The follow-up is usually a B12 and folate level plus thyroid and liver tests, and a conversation about alcohol.

A mildly raised MCV with normal haemoglobin is common and often has no serious cause. It is nevertheless worth a check of B12, folate, thyroid and liver function, since early deficiency can cause nerve problems even before anaemia appears.

A low MCV means small red cells, called microcytic. Lab Tests Online UK lists iron deficiency, long-standing inflammation and thalassaemia as common causes. The NHS describes iron deficiency anaemia as the most common type of anaemia, often caused by blood loss or pregnancy, and lists heavy periods, stomach and bowel bleeding and a lack of iron in the diet as causes.

Thalassaemia carriers can also have small cells. The NHS says carriers may get mild anaemia because their red blood cells are smaller than usual and their haemoglobin may be slightly lower than normal, and that this is not the same as iron deficiency anaemia and does not need treatment. This matters because iron tablets are not helpful when the cause is thalassaemia trait, so iron studies are normally checked first.

Your doctor will look at the MCV together with haemoglobin, the other red cell indices and your symptoms. Lab Tests Online UK lists iron studies, vitamin B12 and folate, and a blood film as likely next tests, with a reticulocyte count and rarely a bone marrow biopsy in selected cases.

If the NHS pathway suggests iron deficiency and no obvious cause such as heavy periods is found, tests to look for bleeding in the digestive tract, such as a faecal immunochemical test, may be used. Do not start iron or B12 supplements before the cause is understood, because they can mask or delay the diagnosis.