Your vet may recommend a blood test for your dog for several reasons. It could be part of a wellness screen, a pre-anaesthetic health screen, or a diagnostic aid to investigate ill-health. One of the measurements within these tests is packed cell volume (PCV). If your dog has a low PCV, the cause needs to be identified. 

What is the PCV? 

Blood is a mix of cells suspended within a 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, called plasma, is mostly water with a small number of solids. 

Assessing the volume of cells as a percentage of the total blood volume highlights hydration and detects anaemia. It can be determined by a machine as part of a complete blood count, where it is referred to as the haematocrit. Alternatively, it can be manually calculated after the blood is spun in a centrifuge. 

When a thin tube of blood is centrifuged, the cells separate out of the fluid and pack 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 plasma at the top. The height of the red blood cell layer is recorded as a percentage of the whole volume, and this measurement is the PCV. A healthy dog would be expected to have a PCV between 35 – 55%. 

Significance of Low PCV 

A PCV below 35% would be classed as low and indicates an anaemia. This can be mild, moderate, or severe depending on how low the percentage is. A drop in PCV can occur suddenly (acute) or gradually (chronic). 

If the body recognises this drop by releasing immature red blood cells (called reticulocytes) from the bone marrow into the blood, it is a regenerative anaemia. If this does not happen, it is a non-regenerative anaemia. As it takes 2-3 days for the bone marrow to respond to a drop in PCV, a sudden or early anaemia can appear to be non-regenerative on initial blood tests but may become regenerative on repeat testing a few days later. 

A temporary drop in PCV can occur with overhydration which creates a relative anaemia. This is due to the dilution effect of too much fluid in the blood. Overhydration is not usually something that happens on its own but can occur with intravenous fluid administration. 

Anaemia is a symptom of a disorder, and not a disease in itself. In order to work out the cause of the low PCV, further testing will be recommended. These include an organ-function and protein panel (called a biochemistry), a complete blood count (which measures red cells, white cells, and platelets), and a urine sample. 

Causes of Anaemia 

Red blood cells normally live for 3 months before being removed from circulation and replaced with fresh cells. Anaemia can arise when either red blood cells fail to be adequately produced to replenish those expired cells, or if circulating cells are lost or destroyed before their natural expiry. Your vet will need to determine which of these scenarios is occurring, and why, in order to fix the anaemia. 

Reduced Production 

As the bone marrow produces new red blood cells, any health condition that affects the bone marrow can trigger an anaemia. Primary bone marrow disease includes blood cancers such as leukaemia or autoimmune disease (a condition where the body attacks its own cells as if they are foreign). But bone marrow activity can also be suppressed by exposure to drugs or toxins, such as chemotherapy, lead and oestrogen-based products. This can include accidental transfer from an owner if using topical oestrogen creams, for example. 

Chronic health conditions including kidney and liver decline can also suppress red cell production. The kidneys are responsible for producing the hormone erythropoietin which stimulates red blood cell production. When the kidneys fail to do this due to disease, anaemia can occur. 

Nutritional deficiencies of iron, copper and several B vitamins can also lower red blood cell production. This can occur if an inappropriate diet is fed but can also occur with diseases that prevent the absorption of these nutrients from the digestive system, such as hypothyroidism or bowel inflammation. 

A bone marrow disorder will be suspected when the body fails to respond to the anaemia. This will be indicated by a persistent non-regenerative anaemia on serial blood tests. Thyroid function testing and a review of your dog’s nutrition will likely be necessary, and your vet may discuss a bone marrow biopsy as well. 

Bleeding 

An external wound or bruising can be an obvious source of blood loss. But internal bleeding can be harder to identify. Internal bleeding may occur into a body cavity, which could cause swelling of the belly, or breathing issues, depending on the volume of bleeding and the location of the bleed. This could occur with trauma or a ruptured blood-filled tumour. Your vet will recommend imaging such as ultrasound or x-rays if this is suspected. 

Alternatively, there can be bleeding into the kidneys or bladder which may be visible in the dog’s urine as a red or pink colouration. Low levels of blood in the urine may only be visible on clinic testing, however, so visibly yellow or clear urine does not rule out blood loss through this system. 

Bleeding can also occur into the digestive tract due to ulcers or inflammation, such as with inflammatory bowel disease. If this occurs in the stomach or small intestine, the blood will be digested with food and will come out as black tar-coloured poos. If it occurs in the large intestine or rectum, it will come out looking like red blood. Ulcers can occur with some drugs, but also as a result of advanced kidney disease, due to the declining ability to filter waste byproducts out of the blood. 

Blood loss can also occur if there is a problem with forming blood clots. Some dog breeds can have a hereditary predisposition for clotting disorders, but the most common cause for clotting issues is rat bait ingestion. If there is a known, or suspected access to rat bait, your vet will recommend running a clotting blood test to screen for this. As the liver produces some of the components of clot formation, some forms of liver disease can affect blood clotting ability. 

Even without visible signs of blood loss, your vet may suspect bleeding if your dog’s blood protein level is low alongside the low PCV. Bleeding could be occurring without a visible poo colour change, so your vet may recommend having some poo tested for hidden (occult) blood. They may also discuss having an ultrasound to look for evidence of disease within the digestive tract, as well as disease of the kidneys and bladder. 

Iron Deficiency Anaemia 

Although relatively common in people, especially women, iron deficiency is not a common condition in animals. When it is identified, it is usually the result of chronic blood loss, rather than the primary problem. A heavy load of hookworms, one of several intestinal parasites that your dog can be exposed to, is also capable of causing iron deficiency anaemia. This is more likely to occur in a young pup when their immune system is still immature, or if they are fed a nutritionally incomplete diet. 

Cell Destruction 

Destruction of red cells before their natural expiry is called haemolysis. Haemolysis can be triggered by several methods. Onion and garlic are two food toxins that can cause haemolytic anaemia. Exposure to zinc, which is present in some sunscreens, can also cause a toxin-induced haemolysis. 

The tick-borne parasite, Babesia, can cause haemolysis. In the UK, Babesia infection is usually limited to dogs that have travelled within Continental Europe, however in recent years it has been identified in dogs without overseas travel. Diagnosing this infection involves using a microscope to look for the parasite within red blood cells. 

Red blood cells can also be damaged due to mechanical forces created by blood flow problems. Heart disease can cause narrowing of blood vessels or thickening of valves which can cause cells to fragment as they squeeze through a restricted space. A similar effect can occur in the rare event of the spleen becoming twisted. 

Some cancers can result in red blood cell destruction. This could either be due to mechanical forces created by the physical presence of the cancer, or because the cancer triggers an immune system reaction which destroys the blood cells as if they are diseased. This is known as an auto-immune reaction and is referred to as immune-mediated haemolytic anaemia (IMHA). 

When IMHA is triggered by another health condition, it is secondary IMHA. Aside from cancer, it can also be triggered by bacterial, viral, and parasitic infections. Drug reactions are also capable of triggering IMHA. Where no specific cause can be identified, it is a primary IMHA. Some dog breeds are predisposed to this, including cocker and springer spaniels, and Maltese terriers. 

Symptoms of Low PCV 

Because red blood cells are necessary to transport oxygen around the body, some symptoms of anaemia relate to altered oxygen circulation. Affected dogs may appear lethargic and have reduced exercise tolerance. They may get breathless easier and might want to sleep more. Their gums and the conjunctiva of the eyes may become pale in colour, either as a pale pink, or white if severe. 

Whether these symptoms are witnessed depends on the severity of the anaemia. A dog with mild anaemia may show no symptoms at all. With sudden or severe anaemias, affected dogs could collapse. 

Other symptoms will relate to the disease that has caused the anaemia. This could include visible signs of bleeding, weight loss, vomiting, reduced appetite, and increased drinking and peeing. Some affected dogs will start eating or licking non-food items which is a condition called pica. 

Treatment and Outlook for Affected Dogs 

If the PCV is low enough to threaten a dog’s life, a blood transfusion is a key part of treatment. It can take time and multiple tests to reach a diagnosis of the cause of the anaemia. A blood transfusion can provide a boost which buys precious time for these investigations to be carried out. A PCV of less than 15% is classed as critical, but a rapid drop even before this level, or if surgery is necessary (such as with a traumatic bleed), could warrant a transfusion even above 15%. 

Otherwise, treatment is targeted towards the underlying condition. Rat bait ingestion requires vitamin K1. Auto-immune disease requires immune-suppressive doses of steroids. Parasite burdens need worming medication. Bacterial infections need antibiotics. Surgery or chemotherapy may also be warranted. 

The outlook for affected dogs is determined by the cause and severity of their anaemia. A mild anaemia with a treatable cause has a reasonable prognosis of either recovery or compensation. If the PCV is critically low, or an untreatable cause is identified, the outlook can, sadly, be poor. Autoimmune disease is a complex condition to manage with recurrence possible. 50-75% of IMHA patients will respond to treatment, while many will not recover. 

Conclusion 

If your vet identifies that your dog has a low PCV, this generally means that they are anaemic. Anaemia can occur due to a failure to produce an adequate number of new red blood cells, or because cells in the blood are lost or destroyed. Once the cause is identified, your vet will discuss the expectations of how to treat your dog, and whether recovery is likely.