MCH and MCV: Differences in Red Blood Cells of People with Down Syndrome

February 2025 | Brian Chicoine, MD - Medical Director, Adult Down Syndrome Center

Key Points

The red blood cells of people with Down syndrome are often larger than normal.
Additional evaluation (testing) is indicated if the finding of large red blood cells (measured by MCV) is accompanied by anemia (low hemoglobin and/or hematocrit) or there are findings to suggest other health problems.
If large red blood cells are not accompanied by anemia, this is commonly considered a "typical" finding in an adult with Down syndrome and may not need further testing.
Red blood cells may be smaller than normal, too. This is not a "typical" finding in adults with Down syndrome and requires additional testing.

Complete blood count

A complete blood count (CBC) is a common blood test used to assess the health of an individual. There are several measurements reported including:

Amount of hemoglobin and red blood cells

Hemoglobin (Hgb) is a protein in the red blood cells that carries oxygen. Hematocrit (HCT) is a measure of the percentage of the blood volume that is the red blood cells. When one or both is low, the body doesn't have enough red blood cells, and the diagnosis is anemia. Anemia can be associated with fatigue, looking pale, shortness of breath, and many other symptoms.

Characteristics of red blood cells

Mean corpuscular volume (MCV) is the average size of the red blood cells. When the MCV is elevated, the red blood cells are larger than normal. When the MCV is low, the red blood cells are smaller than normal. A finding of abnormally large red blood cells is called macrocytosis. A finding of smaller red blood cells is called microcytosis.

Mean corpuscular hemoglobin (MCH) is the average amount of hemoglobin in the red blood cells. This is often low when the MCV is low.

A person can have macrocytosis (large red blood cells) but no anemia (normal hemoglobin/hematocrit). A person can also have macrocytosis with anemia (large red blood cells and low hemoglobin/hematocrit). Determining if the person has anemia is important when determining next steps.

Macrocytosis and Down syndrome

Macrocytosis (without anemia) is common in people with Down syndrome. More information is provided in this Normal Abnormal Lab Values article. As the article indicates, the general guideline is that macrocytosis without anemia does not need further evaluation unless the person has findings of some additional condition (such as hypothyroidism, liver disease - see the next section below).

Macrocytic anemia and Down syndrome

For individuals with anemia (low hemoglobin and/or hematocrit) along with macrocytosis (large red blood cells), further evaluation is indicated. Vitamin B12 and/or folate deficiencies are commonly the first considerations. They can both cause macrocytic anemia. There are other causes for macrocytic anemia as well.

In people with Down syndrome, some of the causes of macrocytic anemia are more common, including:

Vitamin B12 deficiency

Celiac disease can cause vitamin B12 deficiency, and celiac disease is also more common in people with Down syndrome.

Hypothyroidism
Myelodysplastic syndrome

Myelodysplastic syndrome is an abnormality in the blood marrow. It is more common in children with Down syndrome, but it has also been reported in adults with Down syndrome.

Additional causes of macrocytic anemia that are not more common in people with Down syndrome include:

Side effects of certain medications
Liver disease
Alcohol use disorder
Some genetic conditions

Microcytosis and microcytic anemia

Some of the reasons red blood cells may be smaller (microcytosis) and/or are smaller along with anemia (microcytic anemia) include:

Iron deficiency or iron deficiency anemia

Anemia of chronic illnesses

Some chronic illnesses have an indirect effect on the body's ability to make red blood cells and can cause anemia with small red blood cells.

Thalassemias

These are genetic conditions which affect the body's ability to make normal hemoglobin. Thalassemias are not more common in people with Down syndrome. Thalassemias are more common in certain ethnic groups. More information is available from the Cleveland Clinic.

Why the size of the red blood cell may be deceiving in people with Down syndrome

As noted above, with some types of anemia (e.g., iron deficiency anemia), the red blood cells tend to be smaller. However, sometimes, in a person with Down syndrome, because their red blood cells are always larger, when the cells decrease in size due to iron deficiency, they do not get small enough to be considered abnormally small (because they were so much larger to start with). Since iron deficiency anemia is more common in people with Down syndrome, it may be important to do additional testing for iron deficiency in a person with Down syndrome with anemia even if the red blood cells are not small.

Screening

The American Academy of Pediatrics recommends annual screening for anemia in children with Down syndrome (age birth to 18). There is not presently an official recommendation to screen adults with regular CBCs but if there is any concern of symptoms of anemia, a CBC blood test should be ordered.

Prevention

Healthy eating is an important means of preventing anemia. Adequate amounts of iron, vitamin B12, folic acid, protein, and other nutritional factors are all important. Additional information on prevention is available in the Resources section below.