Packed Cell Volume
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Also known as PCV — similar to Haematocrit
The packed cell volume (PCV) is the percentage of the blood volume which is occupied by red blood cells. The PCV is usually estimated by subjecting a small blood sample to centrifugation to compress the blood cells into a minimal volume before the PCV is measured against a standard scale. PCV differs from haematocrit values very slightly as haematocrit is measured automatically using techniques which exclude the microscopic spaces between the red blood cells - when comparing serial samples use the same technique (manual PCV or automated haematocrit) each time to make sure the samples can be compared.
Reductions in PCV are synonymous with anaemia and they occur most commonly with haemorrhage, haemolysis or failures in the erythroid lineage. In cases of acute haemorrhage, the PCV will not fall immediately because blood cells are lost in the same proportion as the aqueous solvent. The osmotic movement of fluid from the interstitial space results in haemodilution and subsequent reduction of the PCV.
Increases in PCV usually occur in dehydrated animals in which the blood cells come to occupy a greater proportion of the blood volume, a process called haemoconcentration or relative polycythaemia. Other reasons for increases in PCV include:
- Primary absolute polycythaemia, a form of myeloproliferative disease that results in the increased production of red blood cells.
- Secondary absolute polycythaemia which may be caused by the paraneoplastic production of erythropoietin (EPO) from renal tumours or it may occur as a response to the hypoxygenation of blood in animals with reverse-shunting patent ductus arteriosus (PDA) or Tetralogy of Fallot.
Since PCV is not a measure of the actual number of red blood cells, anaemic animals may appear to have a normal or even increased PCV if they also become dehydrated. An example of this phenomenon occurs with Addison's disease, where animals frequently have a non-regenerative anaemia which is masked by dehydration in an Addisonian crisis.
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