High MCV: causes of large red blood cells and next steps

MCV is one of the red cell measurements on a full blood count. Lab Tests Online UK explains that it gives the average size of your red blood cells, and that doctors use it to help understand the possible cause of anaemia. A high MCV means red cells are larger than normal, which the same source calls macrocytic anaemia when anaemia is present. A low MCV means the cells are smaller than normal.

A high MCV does not always mean you have anaemia; it can be found with a normal haemoglobin. It is a clue rather than a diagnosis, and your doctor will look at the rest of the full blood count, your medicines, your alcohol intake and your symptoms together. Lab Tests Online UK says many results slightly outside the normal range need no treatment and may simply be repeated.

Lab Tests Online UK lists the common causes of a high MCV as vitamin B12 deficiency, folate deficiency, liver disease, an underactive thyroid, alcohol excess and some bone marrow disorders. These fall into a few groups.

Vitamin deficiencies are the best known. The NHS says vitamin B12 or folate deficiency anaemia occurs when a lack of either vitamin causes large, underdeveloped red blood cells, a type called megaloblastic anaemia. In the UK the most common cause of B12 deficiency is pernicious anaemia. Diet can contribute, particularly a vegan diet without supplements or fortified foods; the NHS notes that B12 stores can last around 2 to 5 years, so problems can take a long time to appear. Stomach surgery and conditions such as Crohn's disease can reduce absorption.

Folate cannot be stored for long: the NHS says the body's store is usually enough for about 4 months. It lists an unbalanced diet, regular alcohol misuse, a restrictive diet, malabsorption such as coeliac disease, and higher demands in pregnancy, cancer, blood disorders and inflammation as causes.

Not all causes are deficiencies. Alcohol is listed by Lab Tests Online UK among the causes of a high MCV, and liver disease and an underactive thyroid can also be responsible.

• Vitamin B12 deficiency, including pernicious anaemia.

• Folate deficiency.

• Alcohol excess and alcohol-related liver disease.

• Underactive thyroid.

• Medicines that affect B12 or folate, such as PPIs, metformin, methotrexate, sulfasalazine and some anticonvulsants.

• Bone marrow disorders, which are less common.

Many people have no symptoms and the result is a surprise. When B12 or folate is the cause, the NHS lists extreme tiredness, lack of energy, pins and needles, a sore and red tongue, mouth ulcers, muscle weakness, problems with vision, psychological problems ranging from mild depression or anxiety to confusion, and problems with memory, understanding and judgement. It adds that some of these can occur in people who are deficient but do not have anaemia.

The NHS advises seeing a GP if you think you may have a B12 or folate deficiency. If you develop pins and needles, problems with vision or confusion, seek medical advice promptly rather than waiting for a routine appointment; contact NHS 111 if you are unsure.

Lab Tests Online UK says further tests may include vitamin B12 and folate levels, iron studies, a reticulocyte count and a blood film, in which cells are examined under a microscope, with bone marrow biopsy rarely needed. In practice a GP will often also check liver function and thyroid function, ask about alcohol and review medicines, because those are common explanations.

The NHS says B12 and folate deficiency can often be diagnosed from symptoms and a blood test. If a deficiency is confirmed, further tests may look for the underlying reason, such as pernicious anaemia or coeliac disease.

Treatment follows the cause. For B12 or folate deficiency, the NHS says most cases are easily treated with injections or tablets. B12 is usually given by injection at first, followed, depending on the cause, by tablets between meals or regular injections, and treatment may last until levels improve or for life. Folic acid tablets are used for folate, usually for 4 months. Improving the diet can help prevent it coming back.

If alcohol is the explanation, reducing intake is the main step, and a GP can offer support. An underactive thyroid is treated with thyroid hormone replacement, and a medicine that is lowering vitamin levels may be reviewed by the prescriber. Do not stop prescribed medicines without advice.

The NHS notes that some complications of long-standing deficiency, particularly nervous system problems, can sometimes be permanent, which is why prompt treatment matters.

A raised MCV is usually found on a GP blood test, and the next tests are usually arranged in primary care. Referral to haematology or gastroenterology is reserved for results that remain unexplained or where a specific condition is suspected. Private blood tests are also available, but results should be discussed with a doctor, who can interpret them alongside your history.