There are several reasons that your vet may recommend running a blood test on your cat. Blood tests can help assess an individual’s health before a general anaesthetic. They can help to determine the cause of an illness or be used to monitor response to a treatment. One of the common blood factors that is measured is the PCV, which is shorthand for packed cell volume. If your vet reports that your cat’s level is high, it raises the question of why. Understanding what the PCV is measuring and how it is affected, can help to understand what might give a high result.

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What is the PCV?

Blood is made up of cells suspended within fluid. The cells are red blood cells (which carry oxygen), white blood cells (which patrol the body ready to respond to disease), and platelets (which help form clots). The fluid is called plasma, which is mostly water, with a small number of solids. Measuring the components of blood is usually a quick procedure to do, with most veterinary clinics being able to perform this test in-house.

A thin tube of blood is spun in a centrifuge to separate the cells out of the fluid and pack them together. This creates three layers within the tube: red blood cells at the bottom, a layer of white blood cells and platelets in the middle, and the plasma at the top. The height of the red blood cell layer is recorded as a percentage of the whole volume, and this is the PCV. A healthy cat would be expected to have a PCV between 30-45%.

A high value occurs for two reasons

It may be that the red blood cell mass is increased (known as absolute); or that the proportion of red blood cells relative to the plasma is increased (known as relative). Old or poorly stored blood samples can give falsely high results due to swelling of cells, so your vet will want a fresh blood sample to run this test.

High PCV: Relative

Loss of fluid within the blood is the most common cause of a high PCV and is known as haemoconcentration. Dehydration is a common cause of this. There may be other abnormal blood results to confirm dehydration as the cause, or your cat may physically appear dehydrated. In this situation, the high PCV will improve with intravenous fluid therapy.

Haemoconcentration can also occur if fluid is pulled out of the blood and redistributed in other parts of the body. This occurs with some types of inflammation, or if fluid is lost internally into body cavities. There will usually be other evidence of inflammation on blood tests such as an increased white blood count. With this cause, animals may appear unwell, may have a fever, or may have swelling or pain of the abdomen.

When an animal becomes aroused, usually through fear or excitement, the spleen (which stores blood) can release red blood cells into the blood vessels. This is a normal part of the ‘fight or flight’ response, to provide more oxygen-carrying capacity in preparation for a sudden increase in activity. This increases the red blood cell volume relative to the plasma volume and is another cause of a high PCV.

High PCV: Absolute

An absolute increase occurs due to the body producing more red blood cells. Red blood cell production is usually triggered by the hormone erythropoietin. This hormone is mostly produced in the kidneys, but also in the liver. An absolute increase in red blood cell mass (erythrocytosis) can be primary or secondary.

Primary causes are rare and occur with familial disorders or a type of blood cancer known as polycythaemia vera. With this condition, red blood cells are produced without the stimulation of the erythropoietin hormone due to disease in the bone marrow. Not only is it rare, but it is a diagnosis that can only be reached by ruling out other causes of disease.

Secondary causes result from an increase in erythropoietin hormone which causes an increase in red blood cell production. This hormone can be stimulated when oxygen levels in the body are low (hypoxia). This can occur with heart or lung disease. An inappropriate increase occurs in the absence of hypoxia and is associated with some types of cancer, notably in the kidney, which are capable of producing erythropoietin.

Conclusion

There are several processes that can lead to an elevated PCV. The cause may be evident from a physical examination, or it may require further tests to determine. Your vet can discuss with you the level of concern that they have for your cat, and what other investigations or treatments they recommend.

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