• 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.
• Packed cell volume (PCV) is calculated from a spun sample, hematocrit is usually calculated by automated counters from cumulative cell volumes of counted red blood cells (RBCs).

Uses

Alone

• Measurement of red blood cell mass - low error of 1-2%.
• Measurement of hemoconcentration - quick and accurate, useful for monitoring cases.

• Icterus index: plasma appears yellow.
• Lipemia: plasma appears cloudy in non-fasted animals and secondary to diabetes Diabetes mellitus, pancreatitis Pancreatitis, hypothyroidism Hypothyroidism, hyperadrenocorticism Hyperadrenocorticism and liver disease Liver: acute disease.
• Hemolysis: plasma appears red.

• Leukocyte counts may be approximated by measuring buffy coat in the Wintrobe method.

In combination

• RBC indices: PCV, total RBC count and hemoglobin concentration are used to calculate MCV, MCH and MCHC; these are required to classify types of anemias.
• Polycythemia - secondary, eg right to left cardiac shunts, alveolar disease, hyperadrenocorticism. Primary polycythemia is extremely rare.

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.
• The feline red blood cell is highly susceptible to oxidative injury and may have a shortened life span in the presence of many drugs, eg aspirin Acetyl salicylic acid.

Non-pathological changes

• Fear/excitement. Nervous animals may show higher counts due to splenic contraction.
• Strenuous activity.
• Exogenous anabolic steroids.
• Late pregnancy (low PCV) Pregnancy / gestation.
• Age (neonate has hematocrit of 0.35 l/l, reduces after birth to minimum of 0.25 l/l at 3-4 weeks and then increases up to adult values of 0.35 l/l by 16 weeks).