Why Is My Dog Or Cat's Average Red Blood Cell MCV Value High Or Low ?













Ron Hines DVM PhD

To see what normal blood and urine values are, go here

For an explanation of causes of most abnormal blood and urine tests, go here

To see how tests are grouped, go here

Mean Corpuscular Volume = The Size Of Your Pet's Red Blood Cells = MCV

Your dog’s normal red blood cells are about 7 microns wide - about the same size as yours. Your cat’s RBCs are a bit smaller. When they are normal in size they are quite uniform (normocytic RBCs) and there are about 6-8 million of them in a single, small drop of blood. When your pet has certain health issues, the size of its red blood cells (their volume or MCV) can increase (=macrocytosis of its RBCs) or decrease (=microcytosis of its RBCs). Either condition changes its MCV value number.


Reasons Why Might Your Pet’s MCV Might Be High (Macrocytosis) :

An elevated MCV number means that your pet’s red blood cells are larger than normal.

If your pet has lost blood and is anemic, but it is still able to make replacement red blood cells (a regenerative anemia), those new cells are often larger than normal. That will elevate the pet's MCV reading.

Much rarer causes of higher MCV values occur. Abnormal red blood cell outer wall (its cell membrane) that leads to RBC swelling and increased MCV has been reported in a number of breeds that include Pomeranians, malamutes and miniature schnauzers.

Feline leukemia is occasionally the underlying causes of elevated MCV (primary myelodysplasia syndrome) values in cats.

In some studies, hyperthyroidism has been associated with elevated MCV in cats.

A folate or vitamin B12 deficiency can cause elevated MCV, since intestinal disorders can interfere with folate and vitamin B12 absorption. In that situation MCV can also be high. Certain medications (eg trimethoprim sulfa) or a cobalt deficiency can also raise MCV.

Some dog breeds seem to have larger than average red blood cells (=elevated MCV). That has been reported in poodles and greyhounds.

If your pet’s blood partially clotted during collection due to technical issues or if health issues caused the RBCs in the sample to clump or agglutinate before the test was run, its MCV value may be falsely high when read by some of the older automated systems. Those systems can mistake a clump of normal-size RBC for a single abnormally large one.

Red blood cells swell with storage, increasing their MCV value. Blood samples need to be run promptly and refrigerated or iced immediately.

Blood electrolyte disturbances (high blood sodium levels = hypernatremia) can falsely elevate MCV readings in some automated blood analyzers.

Reasons Why Your Pet’s MCV Might Be Low (Microcytosis) :

An lower than normal MCV number means that your pet’s red blood cells are smaller than normal.

Puppies and kittens less than 16 weeks of age can have low MCVs. Veterinarians believe that is because the iron content of mother’s milk is quite low and they have not had time to build up their iron stores.

Anything that causes iron levels to be low in your pet can cause a low MCV reading. It can be an iron deficiency in their diet (if they are fed bizarre stuff), starvation, intestinal inflammation that prevents iron absorption, chronic diarrhea or significant blood loss for any reason. Heavy intestinal parasite infection (hookworms) can also be the cause.

Certain medications (eg chloramphenicol), pyridoxine deficiency or a copper deficiency have all been associated with low MCV (If you feed a healthy dog or cat a diet that is high in meat and liver, it will get all the iron, pyridoxine, B12 and folate it needs to produce normal blood).

Liver disease and altered liver blood flow (portosystemic shunts) can reduce body iron stores and result in microcytic anemia (=low MCV). So can zinc or lead poisoning.

Some akitas, shibas, and perhaps other oriental breeds seem predisposed to having slightly smaller RBCs (ie a low MCV). They are not anemic and it does not appear to affect their health. In English Springer Spaniels, however, low MCV can be associated with ill health.

Too much anticoagulant (EDTA) in the blood collection tube can shrink the size of RBCs lowering the MCV.

Abnormally low blood sodium levels can trick some automated blood analyzers into reporting a low MCV.

Asiatic Dogs

Dog breeds that originate in Asia can have naturally low MCV, MCH and high end RBC (erythrocyte) numbers. Not all do. They can also be mistaken for husky and arctic breed crosses that do not share this natural peculiarity. Asian breeds can also have higher blood potassium levels. None of these features seem to affect their health. (ref1, ref2, ref3)

Complementary Tests :

All I suggested for determining the cause of low PCV, hemoglobin, iron, TIBC. blood chemistry panel and if any clinical chemistry values are abnormal, the complementary tests suggested for each of those values need to be performed.