MCV Blood Tests: Checking Red Blood Cells

A mean corpuscular volume (MCV) blood test measures the size of your red blood cells. It is included in a routine blood panel known as a complete blood count (CBC).

Red blood cells carry oxygen through your body. A low MCV means your red blood cells are smaller than average, which could indicate iron deficiency anemia. A high MCV means your red blood cells are too large, which could indicate liver disease or a vitamin deficiency.

This article explains the MCV blood test and what high and low levels may mean. It also discusses other tests that are used with the MCV to inform a diagnosis.

Purpose of MCV Blood Test

Mean corpuscular volume (MCV) is measured as part of a complete blood count (CBC). Also called a CBC with differential, it is ordered as part of routine bloodwork or as part of diagnosis, treatment, and follow-up of many medical conditions.

MCV blood tests are used to:

• Assess for anemia in people with symptoms like fatigue, pale skin, and lightheadedness
• Distinguish between different types of anemia
• Estimate of prognosis in people with some medical conditions
• Evaluate other blood abnormalities, such as an abnormal white blood cell or platelet count
• Follow-up on treatment of other medical conditions
  

Anemia and MCV

If you have anemia, the MCV test may be used to help determine the type:

• A low MCV is common in iron-deficiency anemia.
• A high MCV is seen in pernicious anemia and anemias related to vitamin B12 or folic acid deficiencies.

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

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

The MCV can be a helpful test even when the red blood cell count and other tests are normal. This is especially true with kidney disease.

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 three and a half times more likely to suffer from heart disease than those who had a normal MCV.

What Do the Results Mean?

MCV is calculated using a formula: hematocrit (the percent of your blood that is red blood cells) multiplied by 10, then divided by the red blood cell count.

Normal MCV results range from 80 to 100 femtoliter (fl).

Levels can vary based on sex and age. The value of MCV is usually quite stable but rises slowly as we age. A large analysis of 20 years' worth of MCV results found the following averages:

A dramatic shift in MCV levels outside of a blood transfusion may indicate an underlying health condition.

Low MCV

A low MCV means red blood cells are small, also known as microcytosis. Low MCV levels are commonly associated with thalassemia, an inherited blood condition, as well as iron deficiency anemia.

A low MCV (microcytosis) may also be seen with:

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

MCV levels less than 70, known as 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

A high MCV means red blood cells are large, also known as macrocytosis. MCV typically increases with age, and about 30% of older adults have a high MCV without an obvious cause.

Common conditions associated with a high MCV include:

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

Less common causes of a high MCV include:

• 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
• Colorectal, esophageal, and liver cancers

MCV levels higher than 125, known as severe macrocytosis, are usually due to vitamin B12 or folate deficiencies or cold agglutinin disease.

What to Expect

MVC blood test requires a venous blood draw. The test can be done at any time of day. You do not need to fast or limit your activity before the test.

Side effects are usually mild and may include:

• Bleeding
• Hematoma (bruising)
• Infection
  

Related Tests

The MCV test is not used alone but interpreted 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 also order additional tests for further clarification. These tests might include:

• Reticulocyte count: This measures new red blood cells and is used to determine the cause of anemia. A normal or low reticulocyte count suggests that the body cannot keep up with making new red cells, which may happen when someone is deficient in iron or folate.
• Blood differential: A blood differential may give further clues about anemia. This test shows variations in cell size, shape, or color.
• 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.

Limitations of an MCV Blood Test

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

• 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.
  

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.