High MCV Blood Test: What Does It Mean and What Are the Causes?

A high MCV (mean corpuscular volume) means your red blood cells are, on average, larger than normal. This is called macrocytosis. It is a finding on a complete blood count rather than a diagnosis, and common causes include alcohol, low vitamin B12 or folate, some medicines, thyroid problems and liver disease.

This guide is for adults who have seen a raised MCV on a blood count report. It explains what the number measures, the main groups of causes and what to ask. It cannot tell you why your own MCV is high.

A high MCV often causes no symptoms by itself. Symptoms, when present, usually come from the underlying cause or from anemia (a low red cell or hemoglobin level). Healthdirect lists tiredness, a fast heartbeat, dizziness, shortness of breath, pale skin and chest pain among anemia symptoms.

Seek urgent help through local emergency services for chest pain, severe breathlessness or fainting. Contact a clinician promptly for new numbness, pins and needles, problems with balance or walking, vision changes, or new confusion or memory problems. The NHS notes that some problems caused by vitamin B12 deficiency can become permanent if diagnosis and treatment are delayed.

Also get checked sooner if you notice yellowing of the skin or eyes, or any new symptom that worries you while you wait for follow-up tests.

Cleveland Clinic gives a normal adult MCV range of 80 to 100 femtoliters (fL), and an MCV above 100 fL is the usual definition of macrocytosis. Every laboratory sets its own reference range, so compare your number with the range printed on your report.

A result only slightly above the range, on a single test, may need nothing more than a repeat and a look at earlier counts. Cleveland Clinic notes that an abnormal MCV on its own does not provide enough information to diagnose a condition.

MCH (mean corpuscular hemoglobin) is the average amount of hemoglobin in each red cell. Larger cells usually carry more hemoglobin, so MCH often rises along with MCV. Cleveland Clinic gives an adult MCH range of 27 to 33 picograms per cell, and lists B12 or folate deficiency, long-term heavy alcohol use, thyroid problems and some treatments such as chemotherapy among causes of a high MCH.

MedlinePlus explains that the red cell indices, including MCV, MCH, MCHC and RDW, are interpreted together and with the other blood count results, rather than one at a time.

Cleveland Clinic and a review in American Family Physician describe several groups of causes. More than one can apply to the same person.

• Alcohol: regular heavy drinking is one of the most common causes. The review notes that stopping alcohol typically brings the MCV back down.
• Low vitamin B12 or folate: these vitamins are needed to make normal red cells. The NHS notes that deficiency produces abnormally large red cells.
• Medicines: some chemotherapy drugs, hydroxyurea and some HIV medicines can raise MCV. The NHS adds that certain anticonvulsants and proton pump inhibitors can reduce absorption of B12 or folate.
• Underactive thyroid (hypothyroidism), which can cause macrocytosis with or without anemia.
• Liver disease, with or without alcohol use.
• Extra young red cells (reticulocytes), which are larger than mature cells and increase after bleeding or when red cells are being destroyed faster than usual.
• Bone marrow conditions such as myelodysplastic syndrome, which are less common.

A high MCV can appear even when your hemoglobin is normal. Cleveland Clinic states that about 60% of people with macrocytosis also have anemia, so many do not. It also notes that macrocytosis without anemia is more common in older adults, and that sometimes no clear cause is found.

A normal hemoglobin does not rule out a vitamin deficiency. If your clinician is checking B12 in particular, the SameCase guide on B12 deficiency without anemia covers that question in more detail. Macrocytosis on its own is often not serious, but the cause still deserves an explanation, especially if the MCV is rising over time.

Clinicians usually start with a careful history, including alcohol, diet, medicines and supplements. Cleveland Clinic and the American Family Physician review list follow-up tests such as a blood film (looking at cells under a microscope), a reticulocyte count, B12 and folate levels, liver tests and a thyroid test. A bone marrow test is considered only in some situations, for example when a marrow problem is suspected and other causes have been excluded.

Give honest information about alcohol and all medicines and supplements, including over-the-counter ones. Do not stop a prescribed medicine or start supplements to change the number before testing. Ask the team first.

Cleveland Clinic notes that treating the underlying cause often reverses macrocytosis, so a later blood count can help show whether the plan is working.

Take your report, earlier blood counts if you have them, and a list of medicines and supplements.

• How high is my MCV compared with the laboratory's range, and is it new or has it been rising?
• What do my hemoglobin, MCH, white cells and platelets show?
• Which causes are you considering, and which tests will check them?
• Could any of my medicines, my alcohol intake or my diet be relevant?
• When will the blood count be repeated, and what result would change the plan?
• Which symptoms should make me contact you sooner?

On SameCase, patients and verified doctors share de-identified cases. Seeing how others with a high MCV described their tests, timelines and conversations can help you list what to bring and what to ask.

Another person's cause will not necessarily be yours, and a similar case cannot diagnose you. We suggest using cases only to prepare questions for your own clinician.