What Is a Mean Corpuscular Volume (MCV) Blood Test?

A mean corpuscular volume (MCV) blood test measures the size of your red blood cells, which carry oxygen through your body.

The MCV count, also known as mean cell volume, is an important number listed on a complete blood count (CBC). Results may indicate that MCV levels are high, or that red blood cells are too large, or low, which means that red blood cells are smaller than average. High MCV levels may indicate a condition like liver disease or a vitamin deficiency, while low MCV levels are commonly associated with iron deficiency anemia.

This article explains how an MCV blood test is done, and what high and low levels in your results may mean. It discusses other tests that are used with the MCV to inform a diagnosis and treatment.

Purpose of MCV Blood Test

Mean corpuscular volume is one of the numbers provided as part of a CBC. So healthcare providers see the MCV any time a CBC is ordered.

An MCV blood test might be ordered as part of routine screening exams. It may also be done as part of diagnosis, treatment, and follow-up of many medical conditions.

However, there are times that a healthcare provider will want to look specifically at the MCV when evaluating symptoms or a medical condition. Some of these include:

• To evaluate possible symptoms of anemia such as fatigue, pale skin, and lightheadedness
• To distinguish between different types of anemia
• To evaluate other blood abnormalities such as an abnormal white blood cell or platelet count
• As an additional test in many medical conditions
• As an estimate of prognosis in people with some medical conditions
  

How MCV Is Measured

The MCV can be measured either directly by an analyzer or calculated using a formula. In calculating MCV, the hematocrit (how much of your blood is red blood cells, as a percentage) is multiplied by 10 and divided by the red blood cell count. This is measured in millions of cells per cubic millimeter (millions/mm3) of blood.

A low MCV (called microcytosis) means red blood cells are small. A high MCV (called macrocytosis) means RBCs are large.

What Does High or Low MCV Mean?

When MCV blood test results indicate a high MCV, it may point toward certain conditions in which the red blood cells are larger than average. A low MCV count may point to other types of medical issues.

Common causes of high MCV:

• Vitamin B12 deficiency
• Folate deficiency
• Liver disease
• Hyperglycemia, or high blood sugar
• Alcohol use disorder

Low MCV levels are commonly associated with thalassemia, an inherited blood condition, as well as iron deficiency anemia.

The value of MCV is usually quite stable over time. It changes slowly unless a person receives a blood transfusion.

Limitations

There are situations in which the MCV test isn't as helpful.

• Post-transfusion: The MCV offers little value if a person has had a blood transfusion. In this case, the MCV would show the average size of transfused red blood cells combined with a person's own red blood cells. Therefore, the MCV should be measured before starting a blood transfusion.
• Mixed anemias: If a person has more than one type of anemia, the MCV will be less helpful. For example, if a person has both severe iron deficiency anemia and severe folic acid deficiency anemia, their MCV could be normal. That's because the first type of anemia causes a low MCV, while the second causes a high MCV, leading to an overall normal reading.
• False positives: In certain settings, the MCV may be falsely high. This can occur when red blood cells clot. Sometimes this happens with cold agglutinin disease, paraproteinemias, multiple myeloma, and amyloidosis. It can also happen when a person's blood sugar is very high.

Similar Tests

Mean corpuscular hemoglobin (MCH) tracks closely with MCV. Since these readings offer similar information, healthcare providers usually rely on the MCV and pay less attention to the MCH on CBC readings.

MCH should not be confused with mean corpuscular hemoglobin concentration (MCHC), which is used in diagnosing anemia.

Complementary Tests

The MCV is not used alone. Rather, it is read along with other tests performed in a CBC. For example, using MCV alone may result in a misdiagnosis of thalassemia, a condition that causes red blood cells to be destroyed, instead of iron deficiency anemia. This can happen because both have a low MCV.

To avoid this diagnostic difficulty, healthcare providers will also look at other things for a more accurate picture. Other test results include:

• Red blood cell count: The red blood cell count (RBC) is the number of red blood cells in a blood sample.
• Hemoglobin and/or hematocrit: Hemoglobin is the substance that carries oxygen in the blood. Hematocrit is a measure of how much of the blood is made up of red blood cells.
• Mean corpuscular hemoglobin concentration (MCHC): MCHC is the average concentration of hemoglobin contained in a red blood cell.
• Red cell distribution width (RDW): RDW measures how much the red blood cells vary in size.

Your healthcare provider may order other tests for further clarification. These tests might include a reticulocyte count or iron levels.

What To Expect

Getting blood drawn is pretty straightforward. There is little risk associated with checking a CBC and MCV. However, there is a small risk of bleeding, bruising, or infection due to the blood draw.

Before the Test

You do not need to limit your activity level or what you eat before having a CBC. The test can be done in most healthcare provider's offices and hospitals.

It is usually covered by health insurance when there is a good reason for doing the test. Even so, it is best to check with your insurance company to confirm what they will cover.

Be sure to bring your insurance card, and if possible, previous CBC results with you to your appointment.

During the Test

An MCV is done on a sample of blood drawn from a vein. If you happen to be in the hospital and already have a port in a vein, it may be drawn from the port instead.

A lab technician or phlebotomist will clean the area first. They will then place a tourniquet on your arm. After they find your vein, they will insert the needle into the vein.

You will feel a sharp poke as the needle is inserted into the vein. In addition, you may feel pressure as the sample is taken.

When the test is done, the phlebotomist will remove the needle and hold pressure over the puncture wound until it stops bleeding. They will then apply a bandage or gauze wrap.

After the Test

As long as you are feeling well, you will leave as soon as the test is done.

Side effects are uncommon but may include:

• Bleeding: People who are on blood thinners or who have bleeding conditions may need to hold pressure on the site for some time to stop the bleeding.
• Hematoma: This is a bruise that may form at the site of a blood draw. It is most common in people who take blood-thinning medications or have a bleeding condition.
• Infection: Any time the skin is punctured, there is a small risk of infection.

Interpreting Results

The length of time until you get your results can vary. Sometimes it depends on whether the lab is on site or if your blood has to be sent to another lab. In a hospital or clinic with a lab, results might be available the same day.

When looking at your results, it's helpful for your healthcare provider to compare them to your prior CBCs. That way, they can see if your MCV has changed over time.

Most often, a healthcare provider wants to look at MCV if you have anemia so that they can determine which type you have. But it is also important even if you don't have anemia.

Several things can cause a high or low MCV. But your healthcare provider should always interpret the MCV alongside other measurements when looking for the cause of anemia.

Unusually high or low MCV can indicate several conditions. In addition to anemia, MCV outside of an expected range can indicate poisoning, vitamin deficiency, and liver disease.

Low MCV (Microcytosis)

A low MCV may be seen with:

• Iron deficiency
• Thalassemia
• Anemia of chronic disease
• Sideroblastic anemia
• Lead poisoning
• HgC and other hemoglobin hybrids
• Spherocytosis

The lowest MCV levels (for example, less than 70, called severe microcytosis) are usually a sign of iron deficiency anemia or thalassemia. However, conditions don't always fall neatly into these categories. Iron deficiency anemia may sometimes have a normal MCV.

High MCV (Macrocytosis)

MCV typically increases with age. In fact, around 30% of older adults will have an increased MCV without an obvious cause. Conditions associated with a high MCV include:

• Vitamin B12 deficiency
• Folate deficiency
• Liver disease
• Alcoholism
• Hypothyroidism
• Some hemolytic anemias
• Cold agglutinin disease
• Myelodysplastic syndromes/preleukemia
• Aplastic anemia
• Benign familial macrocytosis
• Some chemotherapy drugs
• Chronic hypoxia (low oxygen levels in the blood) such as with COPD
• Carbon monoxide poisoning

The highest MCV levels (for example, higher than 125, known as severe macrocytosis) are usually due to vitamin B12 or folate deficiencies or cold agglutinin disease.

In some cases, such as vitamin deficiency, a high MCV may be reversible, but it will depend on the underlying reason for the elevated count.

Anemias With a Normal MCV

Anemias that often have a normal MCV (normocytic anemias) include:

• Kidney disease (MCV may sometimes be low as well)
• Acute blood loss
• Anemia of chronic disease
• Endocrine diseases other than thyroid disease
• Some hemolytic anemias

Evaluation of Anemia Using MCV and Other Tests

When anemia is present, MCV can help determine the causes. Other tests can add to the picture, including reticulocyte count, MCHC, and RDW.

Reticulocyte Count

The reticulocyte count (a measure of new red blood cells) is an important first step in determining the cause of anemia. That's because it can separate anemias into two major categories:

• Low production of red blood cells
• High destruction of red blood cells

A normal or low reticulocyte count suggests that the body cannot keep up with making new red cells. For example, this may happen when someone is deficient in iron or folate.

However, on the other hand, a high count indicates the body is trying to raise the low red blood cell count. This is seen when red blood cells are lost from bleeding or cell damage.

Combination of MCV and MCHC

The combination of MCV and MCHC can help narrow down the possible diagnoses. Cells with a low MCHC are called hypochromic, which means light-colored.

The table below shows which conditions are associated with different combinations of MCV and MCHC measurements.

Combination of MCV and RDW

RDW describes the variation in the size of red blood cells. Anisocytosis is the medical term for red blood cells of various sizes.

For example, in sideroblastic anemia, most cells may be large, but some cells will be small. In this situation, MCV may be normal, but RDW will be high.

Other Tests

Further tests may be helpful in combination with MCV and other red blood cell indices as well. They include:

Blood differential: A blood differential may give further clues about anemia. This test shows variations in cell size, shape, or color. Other findings may include:

• Target cells and acanthocytes (abnormal red blood cells), as seen with thalassemia
• Hypersegmented neutrophils (a type of white blood cell), as seen with folic acid deficiency anemia
• Spherocytes, as seen with spherocytosis
• Sickle cells, as seen with sickle cell disease
• Howell-Jolly bodies, as seen in people without a spleen
• Nucleated red blood cells, as seen in infants or adults who are severely ill

Iron tests: Serum iron, iron-binding capacity, and serum ferritin can be helpful, especially with a low MCV. For example, the MCV will be low with sideroblastic anemia, but iron stores may be very high.

Vitamin B12 level: Vitamin B12 levels can be used to diagnose deficiency in macrocytic anemias.

Hemoglobin electrophoresis (HE): HE can test for a beta-thalassemia trait (an inherited blood disorder). It cannot, however, test for alpha thalassemia (an inherited anemia).

Bone marrow biopsy: A bone marrow biopsy may be helpful to look at the number and types of cells in the biopsy. It may also be used to evaluate iron.

Non-Anemia Uses of MCV

The MCV test can provide important information even when the red blood count is normal. A few examples include:

• Predicting mortality in esophageal cancer
• Estimating prognosis with chronic kidney disease (CKD)
• To predict how a person with rectal cancer might respond to chemotherapy and radiation
• Assessing cognitive function (thinking and memory) in older adults

For example, a 2017 study found that people with kidney disease who had a high MCV were more than twice as likely to die of all causes of mortality. In addition, they were over 3.5 times more likely to suffer from heart disease than those who had a normal MCV.

Follow-Up

Follow-up testing depends on the results of the MCV test and other measurements.

Summary

MCV is useful for evaluating anemia and some other medical conditions. But it is not usually looked at alone. Instead, your healthcare provider will consider MCV alongside other things in a CBC to get a complete picture.

In addition to anemia, MCV outside of an expected range can suggest poisoning, vitamin deficiency, and liver disease.

MCV can also help to determine a prognosis for certain cancers and chronic kidney disease. In addition, it may be used as a tool to evaluate cognitive function in older adults.

A Word From Verywell

The MCV test, especially when combined with other numbers on a CBC, can help to diagnose anemia, plan treatment, or evaluate a prognosis with other medical conditions. Ask your healthcare provider about any levels that are marked as abnormal.