What does it mean if my dog has a high PVC result? 

What is PCV? 

What many people don’t know is that blood is made up of several components. We can see this best when we take a sample of blood, place a small amount of it in a thin tube (sometimes called a haematocrit tube), and spin it in a centrifuge.  The blood separates out into different components. The heavier red blood cells settle to the bottom of the tube, the white blood cells form a layer above this (often called the buffy coat), and plasma is at the top.  

• Red blood cells (RBCs) – Red blood cells (RBCs), also known as erythrocytes, are responsible for carrying oxygen around the body. Oxygen in the lungs binds to haemoglobin, an iron-rich protein found within RBCs. RBCs then carry this to body tissues around the body. They also pick up carbon dioxide, a waste product of cell metabolism, carrying it back to the lungs to be breathed out. Too few RBCs could mean the dog is anaemic, and too many points to polycythaemia – more on this later! 
• White blood cells (WBCs) – White blood cells (WBCs) are one of the body’s defence mechanisms. They are responsible for consuming and destroying invading pathogens, as well as initiating inflammatory responses that help to stimulate healing. A thicker buffy coat may indicate an increase in WBCs due to infection or inflammation. 
• Plasma – plasma is primarily water, but it also contains proteins, glucose, clotting factors, and mineral ions. Plasma is usually clear, so a change in colour or appearance could indicate an issue, e.g., a yellow tinge seen in patients with increased bilirubin due to liver issues or RBC breakdown, or a cloudy, white appearance in hyperlipidaemia (high fat levels in the blood).  

When the blood has been spun down in this way, we can measure packed cell volume (PCV). PCV tells us the percentage of red blood cells to total blood volume. Normal dogs should have a PCV of around 35% – 45%, although results can vary depending on age and breed. Some breeds like Greyhounds can have a naturally very high PCV of up to 65%. 

If the PCV is too low, this indicates not enough red blood cells and the dog is anaemic. This can be from blood loss (haemorrhage), red blood cell destruction, or lack of red blood cell production.  

If the PCV is too high, this indicates polycythaemia, or blood that is too thick (hyperviscosity). Note erythrocytosis is a term that may also be used to describe this. 

What is polycythaemia? 

As we explored above, polycythaemia is the technical term for blood that is too thick due to an increased proportion of red blood cells.  

However, here are two different types of polycythaemia – 

Relative polycythaemia 

This is the most common cause for a high PCV reading. There is no actual increase in red blood cell numbers, but instead a decrease in plasma, the watery fluid part of the blood. 

This occurs when patients become dehydrated, which may be due to conditions like severe vomiting, severe burns, or heatstroke. The total circulating red blood cells stays the same, but they become more concentrated due to the reduction in plasma, making the blood appear thicker. However, any increases are usually only mild, and most of the patient’s clinical signs are from the underlying disease process rather than an increase in blood thickness. This type of polycythaemia can be treated with fluid therapy. 

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Absolute polycythaemia 

This occurs when your dog is producing too many red blood cells from their bone marrow. This can be further subdivided into – 

Primary reasons such as in the case of a rare bone marrow disorder called polycythaemia vera. 

Secondary reasons which are due to diseases that stimulate red blood cell production through an increase in the production of a hormone called erythropoietin. This is an appropriate response from the body in dogs that have chronic heart and lung disease or those that live in high altitudes. In both cases, low blood oxygen levels drive an increase in red blood cell production to try and compensate for this lack of oxygen. 

However, we can also see diseases where an increase in erythropoietin hormone is considered inappropriate. This can happen in certain types of kidney disease and tumours that cause an elevation in levels of this hormone. This in turn stimulates an overproduction of red blood cells, raising the PCV unnecessarily.  

Polycythaemia is usually diagnosed when a PCV measurement reaches over 60 – 65%. Your vet will consider some degree of breed variation when studying your pet’s blood results, especially if your pet is a Greyhound.  

What are the symptoms of polycythaemia? 

When a dog’s PCV is too high, the blood becomes thicker, and it struggles to circulate as normal. This means organs and tissues around the body can be deprived of oxygen. The blood is also more at risk of forming clots. 

Symptoms may include – 

• Exercise intolerance 
• Lethargy 
• Weakness 
• Bright red or blue/purple gums 
• Neurological issues such as seizures, blindness or ataxia (loss of coordination) 
• Nosebleeds 
• Bloody stools 
• Increased thirst and urination 

However, some patients with polycythaemia can be asymptomatic for many years before presenting. 

If polycythaemia is suspected, what happens next? 

More tests are often needed! Relative polycythaemia will respond to fluid therapy, and investigations are usually focused on the patient’s other presenting signs or disease.  

But diagnostic tests for absolute polycythaemia may include – 

• Further blood tests including a complete blood cell count, absolute reticulocyte count (immature red blood cells) and biochemistry screen (checking proteins, glucose, liver parameters and kidney values) 
• Urinalysis (urine examination) 
• Blood gas analysis, which can help assess a patient’s oxygenation and ventilation 
• Pulse oximetry, which is another measure of how well oxygenated the blood is 
• X-rays of the chest and abdomen to screen for lung and heart conditions, and/or to assess the kidneys 
• Ultrasound examination to provide more detailed information about the liver and kidneys 
• Serum erythropoietin values – a special test may be run to look at the levels of this hormone in the blood 

Bone marrow aspirate samples are not that helpful as they don’t really differentiate between primary and secondary polycythaemia. Polycythaemia vera has no characteristic features when studied under the microscope. 

Treatment 

Treatment for an elevated PCV will depend on the underlying cause. 

Relative polycythaemia  

Most cases are classified as relative polycythaemia, caused by dehydration due to other disease processes. In this instance, rehydration is recommended, usually with hospitalisation for intravenous fluids. Other treatments will be targeted at your pet’s symptoms and primary illness. 

Absolute polycythaemia  

Again, exact treatment will depend on the underlying cause. If there is a tumour creating an elevation in erythropoietin hormone, then this will need to be treated with surgery, radiation therapy and/or chemotherapy. Phlebotomy may also be needed; more on this below. 

If the rarer Polycythaemia Vera is diagnosed, then treatment will be as follows – 

Removing some of your dog’s blood – phlebotomy reduces the number of circulating red blood cells, thinning the blood. 10 -20 ml of blood per kg of bodyweight is collected from your dog during a session. This is performed in much the same way that we would give blood, using an intravenous catheter and collection set. The body will replace the fluid and white blood cells that are also extracted, but sometimes intravenous fluids are given to help. Phlebotomies may need to be carried out daily until the target PCV has been reached (usually less than 60%). 

Medication – Medication can be given to slow the production of red blood cells by the bone marrow. This medication is a chemotherapy agent called hydroxyurea, but it can take a while to see the effects of this drug, due to how long red blood cells live in the body (120 days). This is why phlebotomy is often required first. 

The medication is started at a high dose initially. Your pet’s complete blood cell count (CBC) is checked weekly during the first month of treatment, then monthly for 3 months, and then once every 3 months. As the red blood cell numbers decrease, the dose and frequency of hydroxyurea can also be decreased. 

Some dogs can be weaned off the medication after a year or two, but others will need to stay on it long-term. 

What’s my pet’s prognosis? 

Your pet’s prognosis will largely depend on the underlying disease causing their elevated PCV. Some issues are readily treatable, and PCV comes back to normal quickly, for example dehydration from a gastrointestinal disorder. However, the long-term outlook for pets with cancer or severe cardiorespiratory disease may be more guarded. Your vet will be able to advise you further. 

In cases of polycythaemia vera the outlook is fairly good, and many dogs can be successfully managed for several years. However, owners should note that hydroxyurea can cause side effects. This includes vomiting, reduced appetite, and diarrhoea. It can also cause hair loss, brittle nails, and mouth sores. Occasionally it may suppress white blood cell production too, affecting the pet’s immune function. This is why the effects of the drug need to be closely monitored. 

Summary 

A high PCV is not in itself a diagnosis! Instead, it just means that your pet’s blood is ‘too thick’, due to a high proportion of red blood cells compared to other components. In most pets, dehydration is to blame, but there are a few cases where an underlying tumour or condition called polycythaemia vera may be causing your dog to overproduce red blood cells. 

If you have any questions or concerns about your dog following this article, then don’t hesitate to contact your vet for advice. They will be able to advise further on the specifics of your dog’s case.