The CBC part 1 – Erythrocytes

Hematology, or the complete blood count, is one of the most commonly performed blood tests in dogs and cats. It gives us valuable information about the number and appearance of blood cells, as well as the hemoglobin concentration.

Being able to interpret hematology results well can therefore give us important insight into what is happening in our patients.

In this Vet Info, we will focus on erythrocytes, or red blood cells. The other blood cells will be covered in a future Vet Info.

Changes in erythrocytes are common. They may involve an increased number of red blood cells, known as erythrocytosis; a decreased number, known as anemia; or changes in cell size, such as microcytosis or macrocytosis.

Changes in cell shape, also called poikilocytosis, can also be seen on a blood smear, but will not be discussed in this Vet Info.

Erythrocytosis

This can be a normal finding in some breeds, such as Greyhounds, some Jack Russell Terriers, and German Shepherds.

It can also be caused by dehydration. In that case, the increased red blood cell concentration is usually accompanied by an increased total protein concentration.

Causes of true erythrocytosis include:

• Primary erythrocytosis, also known as polycythemia vera. This can be hereditary or acquired, for example due to bone marrow neoplasia.
• Secondary erythrocytosis, which can occur as a result of:
  • Chronic hypoxia
  • Increased erythropoietin, or EPO, production, for example due to renal neoplasia or other tumors.

Anemia

A lower hematocrit can be a normal finding in young animals, especially small-breed dogs and cats under 1 year of age, and large-breed dogs under 2 years of age.

Anemia can be acute or non-acute, and regenerative or non-regenerative.

To determine whether anemia is regenerative, look at the absolute reticulocyte count and the blood smear. Regeneration is typically present when the absolute reticulocyte count is above 95,000/µL in dogs or above 60,000/µL in cats. On the blood smear, anisocytosis and polychromasia support regeneration.

Possible causes of anemia include:

Regenerative anemia

1. Hemolysis
   This is usually associated with a normal total protein concentration. Additional findings may include hyperbilirubinemia, bilirubinuria, hemoglobinemia, or hemoglobinuria.
2. Blood loss
   This is often associated with a decreased total protein concentration, unless the bleeding is peracute or chronically intermittent.

Non-regenerative anemia

Possible causes include:

1. The early stage of hemolysis or blood loss, before the bone marrow has had time to respond, usually during the first 3–5 days
2. Endocrinopathies, such as hypothyroidism or Addison’s disease
3. Bone marrow disease
4. Chronic kidney disease
5. Chronic inflammatory disease, also known as anemia of inflammatory disease, or AID
6. Iron deficiency

The Wintrobe indices, especially MCV, or mean corpuscular volume, and MCHC, or mean corpuscular hemoglobin concentration, can provide additional information.

• A microcytic anemia, with a decreased MCV, that is normochromic or hypochromic, with a normal or decreased MCHC, is typical of iron deficiency, anemia of inflammatory disease, and feline infectious peritonitis.
• A macrocytic, normochromic anemia, with an increased MCV and normal MCHC, can be seen in cats with FeLV affecting the bone marrow.

Microcytosis and macrocytosis

Microcytosis means that the red blood cells have a decreased MCV, while macrocytosis means that the red blood cells have an increased MCV.

Changes in MCV can sometimes be artefactual, for example due to an old sample, a diluted sample, or agglutination.

Possible causes of microcytosis include:

1. Familial microcytosis, without clinical signs, seen in several Oriental breeds, such as the Akita Inu and Abyssinian cat
2. Hyponatremia, for example in Addison’s disease
3. Iron deficiency or anemia of inflammatory disease
4. Liver disease, for example portosystemic shunts, where microcytosis is commonly seen

Possible causes of macrocytosis include:

1. Familial macrocytosis in Toy Poodles
2. Hypernatremia
3. Myelodysplasia, mainly associated with FeLV
4. Regenerative anemia, usually together with a decreased MCHC