Macrocytosis

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Macroyctosis is an increased red blood cell population that has a mean cell volume (MCV) above normal. The RBCs are larger than normal.

When a dog is regenerating in response to anemia you may see the MCV elevated above normal on the lab test and wonder what it means. There can be a number of causes.

The red cells may clump (stick together), or poor storage of the sample can lead to RBCs swelling with fluid, and there can be an increased concentration of chemical particles in the fluid part of blood. This would be an artificial elevation of this value.

Folate or vitamin B12 deficiencies can lead to defects in the DNA production and maturation of the RBC. Both B12 and Folate are required for DNA synthesis. The division of cells with be delayed and the result will be Macrocytic Anemia. B12 and Folate deficiencies can be present due to gastrointestinal problems. A common disorder in dogs is small intestinal bacterial overgrowth (SIBO) that can lead to poor absorption of these nutrients. Some drugs may prevent the absorption or metabolism of B12 and Folate such as trimethoprim sulphur (an antibiotic) and hydroxyurea (used in treating a rare platelet disorder.)

Some breeds like Greyhounds, Miniature and Toy Poodles may have macrocytosis but not show anemia.

When a dog has regenerative anemia the reticulocytes (baby red blood cells) are usually larger and the MCV can be increased above normal high values (along with a decreased MCHC ). Laser analyzers are less likely to show this than a blood smear examined under a microscope.

There is also a inherited defect called Hereditary Stomatocytosis that has been seen in Malamutes, Miniature Schnauzers, Pomeranians and other breeds. Their cell walls can have defects that allow too much fluid into the cells. This will show up as macrocytic and hypochromic (low hemoglobin content) RBC’s. These animals may not show signs of anemia.