Overview
- High speed centrifugation separates layers of cells:
- Erythrocytes.
- Buffy coat (thrombocytes, leukocytes and nucleated erythrocytes).
- Plasma.
- Measurement of these components indicates the patient's erythrocytic and hydration status.
- PCV is calculated from a spun sample, hematocrit is usually calculated by automated counters from cumulative cell volumes of counted RBCs (Calculated hemocrit (%) = MCV (fl) x RBC (x10*12/l)/10).
- Relationship of Hbg and PCV: Hbg x 3 = PCV.
- Relationship of RBC and PCV: PCV / 6 = RBC. Is not valid when disease is present.
Uses
Alone
- Measurement of red blood cell mass - low error of 1-2%.
- Measurement of hemoconcentration - quick and accurate, useful for monitoring cases.
Estimated red blood cell counts and hemoglobin concentrations do not hold true in iron deficiency
anemia or in regenerative anemias .
- Icterus index: plasma appears yellow.
- Lipemia: plasma appears cloudy in non-fasted animals and secondary to diabetes, pancreatitis, hypothyroidism, hyperadrenocorticism and liver disease.
- Hemolysis: plasma appears red.
- Leukocyte counts may be approximated by measuring buffy coat in the Wintrobe method.
In combination
- As part of routine hematological screen.
- With RBC indices: in diagnosis and classification of anemia.
- In diagnosis of polycythemia .
Other points
Anemia and dehydration have opposite effects on PCV, therefore if both conditions are present, PCV may not be a true reflection of the animal's status.
Non-pathological changes
- Fear/excitement. Nervous animals or breeds, eg German Shepherd Dog, Poodle, Chihuahua, may show higher counts due to splenic contraction.
- Some breeds, eg Greyhound, Whippet, Lurcher, Borzoi, have higher normal counts (upper end of normal range or just above).
- Strenuous activity.
- Late pregnancy (low PCV).
- Age (lower PCV under 12 months of age).
Result data
Normal (reference) values
- 0.45 l/l (0.37-0.55 l/l = 37-55%).
- Varies with age:
- 3 weeks old - 0.32 l/l (32%).
- 2-4 months old - 32-45 l/l (32-45%).
- 4-6 months - 0.35-0.52 l/l (35-52%).
- 6-8 months - 0.41-0.55 l/l (41-55%).
- PCV, RBC and Hbg high at birth, falling as puppies nurse, with continuing reductions occuring during 1st month of life. Thereafter increase to adult levels by approximately 1 year of age.
- In late pregnancy PCV reduced.
- Certain breeds may have higher RBC, Hbg and PCV values (Greyhounds (0.5-0.6 l/l), Whippet, Lurchers, Borzoi; sometimes also seen in Poodles, German Shepherds, Boxers and Chihuahuas).
Abnormal values
PCV >0.55l/l
PCV <0.37l/l
- Anemia .
- Absolute polycythemia:
- Inappropriate (or appropriate) - may be physiologic because of increased erythropoietin or erythropoietin-like factors or associated with malignant proliferation (polycythemia vera).
- Relative polycythemia:
- Result of dehydration.
- Splenic contraction (excitement, exercise).
- Shock (splenic contraction).
- Exogenous anabolic steroids.
Decreased PCV
- Anemia - investigate underlying cause.
- General classifications may be:
- Decreased bone marrow production.
- Increased erythrocyte destruction/decreased survival.
- Sequestration - hypersplenism/splenomegaly.
- Erythrocyte loss (hemorrhage).
- Intravascular/extravascular hemolysis, rapid resonance.
- Toxicities resulting in anemia:
- Copper.
- Lead (usually only mild or borderline anemia).
- Zinc.
- Sulfa drugs.
- Onion toxicity.
- Blood-type incompatabilities:
- Transfusion reaction.
- Neonatal isoerythrolysis (rare in puppies).
Errors and Artifacts
- Evaporation from blood sample.
- Prolonged storage in EDTA/excess EDTA.
- Samples from sedated or anesthetized animals.
- Hemolysis.
- Clotting.
- Stress → splenic contraction, so hemoconcentration is common in canine blood samples.
Nervous animals or breeds, eg GSD often show higher than expected count due to splenic contraction. - Insufficent centrifugal force or time of spinning may result in elevated PCV.
- Rough handling with induced hemolysis may decrease PCV.
- Sample desiccation may result in increased PCV (spinning tubes without caps, improper handling or storage).
Sources
Publications
Other sources of information
- Cowell R J, Tyler R D & Meinkoth J H (1999) Diagnostic Cytology and Hematology of the Dog and Cat. 2nd edn. St Louis: Mosby.
- Duncan J R, Prasse K W & Mahaffey E A (1994) Veterinary Laboratory Medicine Clinical Pathology. 3rd edn. Ames, Iowa: Iowa University Press.
- Jain N C (1993) Essentials of Veterinary Hematology. Philadelphia: Lea & Febiger.
