What does it mean if my cat has a low PCV test? 

Your cat is ill, the vet’s run bloods and has come back to say the PCV is low. What on earth does that mean? Cats are a mystery. And when they’re ill, trying to figure out what’s wrong with them can sometimes be more puzzling than an episode of Poirot. Their nature means that they tend to hide symptoms of disease very well and can often be quite poorly by the time they’re presented to the vet.

Throughout university, veterinary students are reminded over and over by their lecturers “cats are not small dogs”. The obvious aside, this really does ring true in veterinary medicine, where the symptoms of disease in a dog can look vastly different to the symptoms of the same disease in a cat and that cats simply don’t follow the rules when it comes to what the textbooks say. PCV is one of the many parameters that are evaluated on a blood test – but what is it and what does it mean if it’s low? 

Blood tests – what do all the letters mean? 

Blood tests can be carried out to look at nearly every component of an animal’s body and its function, but a standard diagnostic profile includes a haematology, or complete blood count, and a biochemistry profile. The biochemistry panel looks at various proteins, electrolytes and enzymes required by or created by the body and can give indications as to the health of various organs such as the liver or kidneys. The complete blood count measures all the physical components of the blood such as red blood cells, white blood cells and platelets. If you ever look at a sheet of blood results, you’ll see a raft of acronyms – RBC, WBC, MCV, MCH, MCHC, PLT, PCV – which all measure slightly different aspects of the blood. To go into detail about all of them is beyond the scope of this article but we will concentrate on one of the easiest to measure and the most useful – the PCV.  

The PCV 

The acronym PCV stands for Packed Cell Volume. In simple terms, this is the percentage of the whole blood that is made up of actual red blood cells, with the remainder composed of white blood cells, platelets and plasma. Blood machines will measure this as part of their standard complete blood count, but it can also be measured manually. The blood sample is spun in a centrifuge machine which uses centrifugal force to separate out the four blood components with the heaviest at the bottom (the red blood cells), then the white blood cells and platelets together (known as the buffy coat), and finally the plasma layer at the top. A special reader is then used to measure the percentage of the total fluid that is made up of the layer of red blood cells. For cats, this percentage should be around 30-45%.  

A high PCV 

The most common reason for a high PCV measurement is clinical dehydration or hypovolaemia. This usually occurs when an animal is ill with vomiting or diarrhoea and they are not taking in enough fluids to replenish those which they are losing. There will be other indications on the blood sample to back this up and clinical signs such as tacky (sticky) mucous membranes and a skin tent (the skin stays lifted if gently pinched). If the losses are severe and the actual blood volume is decreased by the loss of fluid, they may also demonstrate a high heart rate and abnormal pulse quality. Correction of the underlying problem along with supportive care, such as administration of intravenous fluids, is key to recovery.  

One of the other, less common reasons for a high PCV is a condition known as polycythaemia which means ‘too many red blood cells’. This can be primary or secondary. The primary condition is known as Polycythaemia vera, which is a rare bone marrow disorder that causes an uncontrolled production of red blood cells by the bone marrow. Secondary polycythaemia comes about due to an increased amount of erythropoietin in the body, that in turn, stimulates the bone marrow to produce red blood cells. The increased erythropoietin can happen due to problems such as respiratory conditions, heart disease or hyperthyroidism.  

So, a high PCV can be caused by a common easy thing to correct, or a rare tricky thing to correct. A low PCV is another thing entirely.  

A low PCV 

Generally, a low PCV means the animal is anaemic. A low PCV would be one that falls below the usual reference ranges – a few percentage points are usually not much to worry about, but some cases can get very low indeed. Anaemia is not a diagnosis in itself, but a symptom of an underlying condition. Anaemia can also be acute or chronic, regenerative or non-regenerative. A regenerative anaemia is when the body responds to the drop in red blood cells by releasing immature red blood cells (reticulocytes) from the bone marrow. This can take 2-3 days but if this doesn’t happen by then, it is classed as a non-regenerative anaemia.  

A relative anaemia can be caused by over-hydration during intravenous fluid administration where the fluid given ‘dilutes’ the blood making it seem that the percentage of red blood cells is lower. However, if this isn’t the case it is vital that the underlying cause is found and rectified. This is likely to require further blood tests, urinalysis and imaging of the chest and abdomen.  

Causes of anaemia 

A true anaemia will either be caused by a decreased production of red blood cells or an increased loss of red blood cells.  

Decreased production: 

As seen above, the bone marrow is responsible for the production of red blood cells, therefore any disease process that damages the bone marrow or affects its function will lead to a decreased output. Often, and sadly, this is due to a type of cancer such as leukaemia and in cats, FeLV or FIV are also possible. In these cases, all blood cell lines will be affected, not just the red blood cells, and the anaemia will be non-regenerative. Autoimmune conditions can also be to blame as in these diseases, the body will attack itself, often focussing on the immune system, the blood or bone marrow. Certain toxins can impact on the bone marrow too, most commonly chemotherapy drugs or even certain human products such as oestrogen supplements or creams.  

Because red blood cell production is controlled by erthyropoietin, a decrease in this hormone will secondarily decrease the production. The most common reason for this would be in chronic, or even severe acute kidney disease, as the kidneys are responsible for 90% of the body’s production of erythropoietin, but it could also be seen with any chronic disease process.  

Increased loss: 

An increased loss of red blood cells will be either due to haemorrhage or haemolysis, and we’ll look at these one at a time.  

Haemorrhage  

Haemorrhage means bleeding, either externally, or internally into a body cavity or even into the gastro-intestinal tract. External haemorrhage is typically from wounds or conditions such as epistaxis (nose bleeds). Internal haemorrhage can be harder to find and imaging is usually required, especially if the blood is collecting in the abdomen or chest cavity. Bleeding into the gastro-intestinal tract is usually caused by a bleeding gastric or intestinal ulcer or severe inflammatory disease, though acute conditions such as AHDS (acute haemorrhagic diarrhoea syndrome) can also be to blame. 

Bleeding may also occur if there is a problem with the clotting of the blood which can be seen in certain inherited disorders such as feline haemophilia or if the animal has ingested a toxin like rat poison.  

Haemolysis  

Haemolysis is the destruction of red blood cells, by either the body itself, in an auto-immune condition, or by the action of a toxic substance. 

Immune mediated haemolytic anaemia is a relatively common condition seen as either a primary state, or secondary to another underlying condition or drug reaction.  

Certain toxins are also a very probable cause of haemolysis, specifically onions, garlic or zinc. In cats specifically, paracetamol is highly toxic and causes severe destruction of the red blood cells and liver failure. 

Other less common causes of haemolysis include damage to the red blood cells occurring as they pass through excessively narrow circulatory channels such as thickened heart valves or vessels, or by exposure to exotic parasites in animals which have travelled from abroad.  

Symptoms of anaemia 

The symptoms of anaemia will depend on the severity. Mild and transient anaemia can go unnoticed, or the animal may just be a bit lethargic. Severe anaemia can be life threatening, with symptoms such as pale mucous membranes or collapse. Animals will tend to cope with chronic anaemia much better than acute anaemia. In fact, cats are notoriously good at not showing any obvious symptoms of chronic anaemia even with an extremely low PCV 

Treatment of anaemia 

Because anaemia is a symptom of a disease or illness, the key to treating it is to treat that underlying cause. However, in some cases, the diagnosis of that underlying cause may take time and if the anaemia is severe, we may need to buy the animal time. Providing intravenous fluids to support the circulatory system can certainly help but if the PCV is dangerously low, a blood transfusion may be necessary. A bit like in people, cats have different blood types, and it is vital that they only receive the same type of blood as they have themselves to reduce the risk of transfusion reactions.  

Conclusion 

A low PCV is a common finding in an unwell cat, but the cause can range from minor to severe illness. By running a series of tests in a logical manner, the reason is often readily identified. Treatment is aimed at the underlying cause, though supportive measures may need to be put in place in the meantime. Just remember that cats are masters of hiding illness so it’s better to check a slightly off-colour cat sooner rather than later.