Why Should You Measure Mean Corpuscular Volume?

What to expect when undergoing this test

Mean corpuscular volume (MCV), also known as mean cell volume, is an important number listed on a complete blood count (CBC) that can help diagnose different types of anemia as well as other health conditions. The MCV is a value that describes the average size of red blood cells (erythrocytes) in a blood sample.

A low MCV indicates small red blood cells and is called microcytosis, whereas a high MCV is called macrocytosis. While the MCV can provide important information, it is not used alone—it is interpreted along with blood counts and other red blood cell indices such as mean corpuscular hemoglobin concentration (MCHC) and red cell distribution width (RDW) to narrow diagnoses.

The MCV can be a helpful test even when the red blood cell count and other tests are normal, especially in the setting of kidney disease.

Purpose of MCV Test

Mean cell volume (MCV) is one of the numbers provided as part of a complete blood count (CBC), so doctors have access to the MCV any time a CBC is ordered. This can include routine screening exams and as part of the diagnosis, treatment, and follow-up of a vast array of medical conditions.

There are times, however, that a doctor 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

Measurement

The MCV can be measured either directly by an analyzer or calculated using a formula. In calculating MCV, the hematocrit is multiplied by ten and divided by the red blood cell count measured in millions of cells per cubic millimeter of blood.

  • MCV = hematocrit (percent) x 10 / red blood cell count (millions/mm3 blood)

The Meaning of MCV

Mean corpuscular volume (MCV) is a number that describes the average size of red blood cells circulating in the bloodstream.

Thus, a high MCV would mean that the red blood cells are larger than average and a low MCV would mean they are smaller than average. Determining the average size of red blood cells is very helpful in determining the type of anemia present and more:

  • high MCV is seen with macrocytic anemias such as vitamin B12 deficiency anemia
  • low MCV is seen with microcytic anemias such as iron deficiency anemia

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

Limitations

There are several things to keep in mind when considering the limitations of the MCV test.

  • Post-Transfusion: The most obvious limitation of the MCV is that it offers little value if a person has had a blood transfusion. In this case, the MCV would reflect the average size of transfused red blood cells combined with a person's own red blood cells.
  • This is one reason that doctors try to draw a blood sample from people with severe anemia 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, a person could have severe iron deficiency anemia (which ordinarily causes a low MCV) as well as a severe folic acid deficiency anemia (which causes a high MCV), and their MCV could be normal.
  • False Positives: In certain settings, the MCV may be falsely elevated. This can occur when red blood cells clot such as with cold agglutinin disease and conditions (paraproteinemias), multiple myeloma and amyloidosis, or when a person's blood sugar is very high (red blood cells swell).

Similar Tests

Mean corpuscular hemoglobin (MCH) closely parallels MCV. Since these readings offer similar information, doctors usually rely on the MCV and dismiss the MCH on CBC readings. (MCH should not be confused with MCHC which is different and used in diagnosing anemia).

Complementary Tests

The MCV is not used alone; rather, it is interpreted along with other tests performed in a CBC. For example, using MCV alone may result in thalassemia being misdiagnosed as iron deficiency anemia since both have a low MCV.

  • 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 molecule that carries oxygen in the blood, and hematocrit represents the total number of red blood cells in a particular volume of blood.
  • Mean corpuscular hemoglobin concentration (MCHC): MCHC is the average concentration of hemoglobin contained in a red blood cell.
  • Red cell distribution width (RDW): RDW is a measure of how much the red blood cells vary in size.

Other tests may be ordered for further clarification, such as a reticulocyte count or iron levels.

Risks and Contraindications

There is little risk associated with checking a CBC and MCV other than a very small risk of bleeding, bruising, or infection due to the blood draw.

Before the Test

There are no dietary or exercise restrictions needed before having a CBC. The test can be done in most physician offices as well as hospitals. It is usually covered by health insurance if there is a valid reason for doing the test, though it is best to check with your healthcare provider prior to receiving any test to confirm what the coverage will be. You will be asked to bring your insurance card, and if possible, previous CBC results.

During the Test

An MCV is done on a sample of blood drawn from a vein (or in people with a chemotherapy port, it may be drawn from the port). A lab technician will cleanse the area for the blood draw and place a tourniquet. She will then insert the needle into a vein.

You will feel a sharp poke as the needle is inserted into the vein and may feel pressure as the sample is taken. When the test is done, the needle will be removed and the lab technician will hold pressure over the puncture wound until it stops bleeding. A bandage or gauze wrap is then applied.

After the Test

You will be able to leave as soon as the test is done if there is no evidence of bleeding.

Possible side effects are very 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: Most common in people predisposed to bleeding by medications or bleeding conditions, a bruise (hematoma) may form at the site of the blood draw.
  • 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 depending on whether the lab is on-site or if your blood is sent out to another lab. In a hospital or clinic with a lab, results are available shortly. When looking at your results, it's helpful for your doctor to have prior CBCs so she can see if your MCV has changed over time.

Most often, MCV will be evaluated in the process of determining the type of anemia that is present, but is also important even if there is no evidence of anemia. There are several possible causes of a high or low MCV, but the MCV should always be interpreted along with other blood cell indices when looking for the cause of anemia.

Reference Range

A normal MCV is 80 to 96 femtoliters per cell. (A femtoliter is a cubic micrometer.)

Possible Causes of Low MCV (Microcytosis)

A low MCV may be seen with:

  • Iron deficiency (there are many different causes of iron deficiency anemia)
  • Thalassemia (there are several types and it is thought to occur in around 30 percent of African Americans)
  • Anemia of chronic disease
  • Sideroblastic anemia
  • Lead poisoning
  • HgC and other hemoglobin hybrids
  • Spherocytosis

The lowest MCV levels (for example, less than 70 or severe microcytosis) are usually a sign of iron deficiency anemia or thalassemia. There is overlap between these categories, and iron deficiency anemia may sometimes have a normal MCV.

Possible Causes of a High MCV (Macrocytosis)

MCV ordinarily increases with age, and around 30 percent of older adults will have an elevated 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 with CO2 retention
  • Carbon monoxide poisoning

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

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, but these can be broken down further using MCHC and RDW.

First Step: Reticulocyte Count

The reticulocyte count is an important first step in determining the cause of anemia as it can separate anemias into two major categories: decreased production of red blood cells or increased destruction of red blood cells.

A normal or low reticulocyte count suggests that the body is unable to keep up with the production of red cells, for example, due to a deficiency of iron or folate. However, on the other hand, a high reticulocyte count indicates the body is trying to raise the low red blood cell count and is seen when red blood cells are lost via blood loss or cell degradation.

Combination of MCV and MCHC

The combination of MCV and MCHC can help narrow down possible diagnoses. (Cells with a low MCHC are hypochromic or light colored.)

MCV and MCHC in Anemia
MCV MCHC Examples
Low (Microcytic) Low (Hypochromic) Iron deficiency anemia
Low (Microcytic) Normal (Normochromic) Thalassemia
Normal (Normocytic) Normal (Normochromic) Anemia of chronic disease
High (Macrocytic) Normal (Normochromic) Vitamin B12 deficiency

Combination of MCV and RDW

RDW describes the variability in size of red blood cells (anisocytosis). For example, in sideroblastic anemia, most cells may be macrocytic, but some cells will be small. MCV may be normal, but RDW will be high.

Examples of Anemias Based on MCV and RDW
Anemia Type Normal RDW High RDW
Microcytic Thalassemia -Iron deficiency anemia
-Some hemolytic anemias
Normocytic -Acute blood loss anemia
-Anemia of chronic disease
-Spherocytosis
-Combined anemias
-Sickle cell anemia
-Sideroblastic anemia
-Chronic blood loss
Macrocytic -Aplastic anemia
-Preleukemia
-Liver disease
-B12/folate deficiency
-Cold agglutinin disease

Other Tests

There are further tests that may be helpful in combination with MCV and other red blood cell indices as well.

Blood Differential: A blood differential may give further clues as to anemia such as variations in cell size (anisocytosis), cell shape (poikilocytosis), or color (polychromasia). Other findings may include:

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

Iron Tests: Serum iron, iron binding capacity, and/or serum ferritin are helpful, especially with a low MCV. For example, with sideroblastic anemia, the MCV will be low 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 can test for a beta thalassemia trait (not for alpha thalassemia).

Bone Marrow Biopsy: A bone marrow biopsy may be helpful to look at the number and types of cells in the biopsy or to do an iron stain on the aspirate.

Non-Anemia Uses of MCV

In recent years, the MCV test has been found to 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 response to chemotherapy and radiation with rectal cancer
  • assessing cognitive function (a higher MCV in older adults is associated with poorer cognitive function)

A 2017 study found that people with kidney disease who had a high MCV were more than twice as likely to die (all causes of mortality) and over 3.5 times more likely to suffer from heart disease than those who had a normal MCV.

Follow-Up: Follow-up testing will depend on the results of the MCV test and other red blood cell indices and counts.

A Word From Verywell

The MCV test, especially when combined with other numbers on a CBC, can be helpful in both diagnosing anemia and planning treatment or predicting prognosis with other medical conditions. These small numbers on a CBC can easily be overlooked, and it's a good idea to be your own advocate and ask your doctor about any levels that are marked as abnormal.

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Article Sources

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