Last Updated:
Sep 12, 2000

Diseases of the Blood
in Down Syndrome

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by Len Leshin, MD, FAAP

Copyright 2000, All rights reserved

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Red Blood Cells

In newborns, a common finding is an increased number of red blood cells. This does not appear to be harmful, and is often gone by 2 to 3 weeks of age. The cause for this is unknown, but doctors should check for heart defects in these babies.

In children and adults with DS, it is common to see red blood cells that are larger than normal; this is called macrocytosis. This has been speculated to be due to an altered folate metabolism seen in Down syndrome, along with structural changes in the red cell membrane. Unless accompanied by anemia, macrocytosis does not require treatment.

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White Blood Cells

Newborns with Down syndrome have an increased risk of having a condition called transient leukemia (also called transient abnormal myelopoiesis, transient blastemia or myeloproliferative syndrome). This condition resembles leukemia, but disappears on its own without treatment in just a few weeks or months. One researcher found that up to 10% of newborns with DS had evidence of transient leukemia on blood tests. On the blood test, the number of white blood cells is greatly above normal and there are immature white blood cells ("blasts") present in the blood as well. While this condition resolves without treatment, the concern is that there is an increased risk of these infants to develop leukemia later in childhood. In one study of 85 infants with transient leukemia, 30% went on to develop a type of leukemia called "nonlymphoid" (also called "myelocytic," or AML) within the next 3 years. The reason for the development of transient leukemia isn't understood, but is definitely linked to the extra 21st chromosome.

Leukemia is more common in children with DS, being seen anywhere from 10 to 30 times more often than in the general population of children. The vast majority of cases occur in the first 5 years of life. In the first 3 years of life, nonlymphoid leukemia is the most common form of leukemia in children with DS; after age 3, approximately 80% have acute lymphocytic leukemia and 20% have nonlymphocytic leukemia. The actual treatment of leukemia is beyond the scope of this article, but it is worth noting here that children with DS who develop AML seem to respond to chemotherapy better than do children without DS; with ALL, the response rate appears to be about the same.

For more information on leukemia, please visit the American Cancer Society's site.

There are also functional defects of white blood cells in Down syndrome. White blood cells in people with DS have a decreased response to infection, and a decreased killing ability of microorganisms. This may be one reason for the decreased immunity to infection seen in children with DS.


One abnormal finding in newborns with Down syndrome is a low platelet count, called thrombocytopenia. The platelets are the blood cells that assist in clotting blood. Rarely, the platelet count may be so low that transfusions of platelets may be needed to prevent bleeding problems. The reason for the thrombocytopenia is unknown; one study theorized that one reason could be that there is a temporary lack of regulation of the platelet precursor cells in neonates with DS. Newborns with thrombocytopenia should be watched carefully for any other signs of transient leukemia.

On the other hand, some infants with DS show an increase in platelet numbers, called thrombocytosis. One research team found 20% of all the infants they studied had thrombocytosis, but none had any complications from the condition and all had normal platelet counts one month later. The cause here is also unknown.

Older children and adults with DS rarely have any other problems with numbers of platelets, and problems with platelet function in DS is extremely rare.

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