Hematocrit
The percentage of blood volume occupied by red blood cells, measured by centrifuging a blood sample or calculated from a complete blood count. Hematocrit (also called packed cell volume or PCV) is a primary indicator of anemia or dehydration in dogs.
Hematocrit (HCT), also called packed cell volume (PCV), is the fraction of whole blood volume composed of red blood cells (erythrocytes). It is expressed as a percentage. When a blood sample is placed in a capillary tube and centrifuged, the red cells pack to the bottom, the white cells and platelets form a thin buffy coat in the middle, and plasma sits on top. The proportion of the column occupied by red cells is the PCV.
Normal Ranges
Normal canine hematocrit ranges from approximately 37% to 55%. Breed-specific variation is significant:
- Sighthounds (Greyhounds, Whippets, Italian Greyhounds) normally run higher hematocrits (50-65%) due to higher red cell mass — an adaptation for aerobic performance. A “normal” hematocrit of 45% in a Greyhound may actually represent anemia.
- Puppies have lower hematocrits than adults due to growth dilution and fetal hemoglobin turnover.
- Toy breeds may run slightly lower normal ranges.
Low Hematocrit (Anemia)
A hematocrit below reference range indicates anemia, which is classified by mechanism:
- Blood loss anemia: Trauma, gastrointestinal bleeding, coagulopathy, splenic mass rupture (hemangiosarcoma)
- Hemolytic anemia: Accelerated red cell destruction — immune-mediated hemolytic anemia (IMHA), oxidative damage (zinc toxicity, onion ingestion), parasites (Babesia)
- Non-regenerative anemia: Bone marrow fails to produce adequate red cells — chronic kidney disease (reduced erythropoietin), bone marrow disease, chronic inflammation, myelophthisis
The complete blood count (CBC) provides additional parameters (reticulocyte count, MCV, MCHC) that help classify the type of anemia and guide treatment.
High Hematocrit (Polycythemia)
Elevated hematocrit can result from:
- Relative polycythemia: Dehydration concentrates the blood. This is the most common cause and resolves with rehydration.
- Absolute polycythemia: True overproduction of red cells, either primary (polycythemia vera, a rare bone marrow disorder) or secondary (response to chronic hypoxia from heart or lung disease, or inappropriate erythropoietin production by renal tumors).
How Hematocrit Is Measured
- Manual PCV: Capillary tube centrifugation — fast, inexpensive, available in any practice
- Automated HCT: Calculated by hematology analyzers as part of a CBC
Both methods are standard and generally concordant. Manual PCV is slightly higher than automated HCT due to trapped plasma between packed red cells.
Relevance to Longevity
Serial hematocrit tracking in annual wellness bloodwork can detect gradual anemia from chronic kidney disease, slow gastrointestinal blood loss, or early bone marrow disorders before clinical signs appear. A declining trend, even within the normal range, warrants investigation. In senior dogs, a 5-10% drop over 12-24 months may signal subclinical disease amenable to early intervention.