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Updated: Jun 21, 2021

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I’m not going to lie, this is a long and articulated summary! But if you have half an hour to spend on it, it will help you understand how we read blood results.

Red Blood Cells

Packed Cell Volume (PCV) – This is the percentage of the blood that is made up of red blood cells. This figure can vary from as low as 30% up to 45%. The horse will show signs of shock if PCV goes below 10%.

Red Blood Cell (RBC) Count – This is the number of red blood cells in a given volume, typically 1 litre, of blood.

  • A low PCV and RBC count normally indicate anaemia (low RBC numbers); blood loss, bleeding into the gastrointestinal tract, immune disease, infections (equine infectious anaemia, piroplasmosis), cancer, can cause it. Signs of anaemia are weakness, dullness, reduced appetite, pale gums.

  • An increase in PCV and RBC count may be due to several factors. It usually means either that the horse is dehydrated or that the horse was excited prior to blood sample collection. In these cases the spleen contracts, releasing more red blood cells into the circulation. Rarely, other conditions can cause an increase in the PCV and RBC count.

Mean Corpuscular Volume (MCV) – This is the average volume of each red blood cell sampled. Differences in the average red blood cell volume explain why the PCV and RBC count do not always match. This can also be used to help identify causes of anaemia.

Mean Corpuscular Haemoglobin (MCH) – This is the average amount of haemoglobin in each red blood cell.

Haemoglobin (HGB): the substance, contained within in red blood cells, that carries oxygen around the body. This is important in racehorses. Higher values mean more efficient oxygen transport, and generally higher level of fitness.

White Blood Cells

There are five different types of white blood cells in the horse. The absolute number and % between different cells gives us information about horses' health.

White Blood Cell Count (WCC) – This is the total number of white blood cells in the blood. Increases in the WCC (leucocytosis) result from bacterial or viral infection, stress, or immune mediated disease.

A decrease in the total number of white blood cells (leukopenia) may be due to overwhelming bacterial or viral infection (in other words, WBC migrate from blood stream to an area that needs them), bone marrow disease or endotoxemia.

  • Neutrophils – They move rapidly to sites of infection or inflammation within the body. An increase in number of neutrophils is most often due to bacterial or viral infection, injury, stress, rarely bone marrow issues. A low number of neutrophils is most often a result of an increase in demand for them: neutrophils in the blood stream may have been used up in dealing with an infection somewhere in the body. There is a natural delay whilst the body adapts to this by synthesising and releasing more neutrophils.

  • Monocytes – These cells are important in the breakdown of damaged tissues and the destruction of microbes. An increase in monocyte numbers (monocytosis) may indicate bacterial infection, chronic inflammation or stress. Monocytosis may also be seen during the recovery phase, following viral infection. Low numbers of circulating monocytes is not clinically significant.

  • Eosinophils – These cells are most commonly associated with parasitic disease and with allergic conditions.

  • Basophils – Basophils are rarely found in blood samples.

  • Lymphocytes – These are the white blood cells with the greatest responsibility for managing the immune system. Lymphocytosis (increased numbers of lymphocytes) can be caused by excitement, exercise, cancer and ongoing viral infection. Lymphocytosis is a common incidental finding in young horses. Reduced lymphocyte numbers result from stress, acute viral infection or severe bacterial infection.

  • Platelets - They have a number of important functions such as blood clotting and the release of various beneficial chemicals but don’t have a huge importance in blood analysis in the horse.

Biochemical Tests

Substances that are measured in the liquid part of the blood. Many substances can come from more than one source, which makes it occasionally tricky to read the results.

Alkaline Phosphatase (ALP) – Raised levels may indicate chronic liver disease. Reference ranges vary with age.

Aspartate aminotransferase (AST) – Elevated AST levels are seen in cases of muscle damage, or more rarely, liver damage. Levels peak 24–48 hours following injury and will return to normal within 2 weeks. Combined with CK, AST provides a useful measurement of muscle damage in cases of ‘tying up’.

Bilirubin – may be increased in the horse in cases of anorexia or some liver conditions.

Bile acids – Their measurement can be useful in assessing liver function.

Creatine Kinase (CK) – Increased levels almost always indicate acute muscle damage. Levels peak 6–12 hours following injury and can return to normal levels in 3–4 days. CK is assessed alongside AST in cases of muscle damage.

Creatinine – Excreted from the blood by the kidneys. Increased levels of creatinine may be seen in horses with primary kidney disease or with other conditions affecting the kidneys such as dehydration or blockage.

Gamma Glutamyl-transferase (GGT) – Increased blood levels indicate liver disease. it can take several weeks for these levels to return to normal. Increases may also be seen in association with over training.

Glutamate Dehydrogenase (GLDH) – Raised GLDH levels are specific for acute liver disease. Following resolution of liver injury, levels of GLDH return to normal within 24 hours.

Insulin – This hormone may be tested for during investigation of equine metabolic syndrome.

Lactate – An increasing blood lactate concentration may indicate a worsening prognosis following colic surgery.

Lactate Dehydrogenase (LDH) – This enzyme is present in a number of different tissues. Elevated LDH may indicate liver, muscle or intestinal disease.

Protein – Total Protein (TP) – The total mass of protein in the blood. These is made of:

  • Albumin – Raised levels of albumin are almost always due to dehydration. When levels of albumin are low this suggests either a failure of protein production due to liver disease or loss through kidney or intestine.

  • Globulin – Globulin proteins carry out a number of tasks. Increased globulin levels are seen during infection.

Inflammatory Proteins – Measured to give information on inflammatory conditions. They are extensively used in the thoroughbred to detect inflammation or infection rapidly.

  • Serum Amyloid A (SAA) – Rises quickly in response to inflammation or infection and quickly returns to normal.

  • Fibrinogen – Will rise in response to tissue damage. Fibrinogen rises slowly, reaching its peak after about 5 days and takes around 2–3 weeks to return to normal levels.

Urea – Urea is produced in the liver and excreted by the kidneys. Elevated levels may signify kidney disease but may also occur with dehydration or fasting.

Electrolytes - Most of these are bound to proteins so levels should be assessed with reference to albumins.

Why different coloured blood tubes?

Each colour represents a specific substance within the blood tube: each substance interacts differently with the blood and allows us to run different tests.

When should the sample be collected?

  • A routine blood sample should be collected in the morning before exercise, to avoid any stress which might jeopardise results.

  • If your horse is ill, the sample will be collected during an examination, regardless of time of day.

How long does it take to get a result?

It depends. General turnaround time is less than 24 hours if done in house. We aim to provide results on the same day.

If a sample has to be sent away to a referral laboratory, it might take a bit longer.

What is a general health test?

This normally includes a red blood cell count, white blood cell count, estimate of haemoglobin, packed cell volume, globulins and albumins, muscle enzymes (CK and AST) and inflammatory proteins (Fibrinogen and/or SAA).

In which way does stress affect results? Pre-sampling excitement determines an alteration to white blood count and packed cell volume.

How much blood is in my horse?

The average 500kg horse has around 50 to 55 litres of blood pumping through its veins. That’s roughly 11gallons. If your horse is bleeding profusely from a wound, you’ve got about 3 gallons (13 litres) to go before you start panicking!

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