A blood count provides important information on a horse's health and is an essential part of veterinary diagnostics. However, interpreting a blood count is not so easy. Most horse owners don't know what to make of the lab values presented to them. Learn here what haemograms and complete blood counts can reveal, when to order which, and what further blood tests are available.
Why order a blood count for your horse?
About 45 litres of blood circulate through the body of a 500 kg horse. Blood supplies the organs with oxygen and nutrients, regulates heat balance, and removes metabolic products. Organic diseases and metabolic disorders can therefore be detected by examining the horse's blood.
A blood count can help the veterinary surgeon determine a diagnosis, as well assist in early detection of illness in horses that appear healthy. Blood counts can also point to nutritional deficiencies.
What is equine blood made of?
Blood (whole blood) has liquid and solid elements.
The liquid elements are called blood plasma. Blood plasma consists of 90% water and contains electrolytes and proteins. It also contains fibrinogen, a substance that is responsible for blood coagulation. Blood serum is the fluid supernatant of a coagulated blood sample, i.e., blood plasma minus the coagulation factors.
The blood cells – including red and white blood cells and blood platelets – are the solid components of blood. The volume percentage of blood cells (haematocrit) in a horse's blood is approx. 32–47%, lower than the amount of fluid plasma.
- Red blood cells (erythrocytes) are formed in the bone marrow. One litre of equine blood contains 4 billion of these. They transport fresh oxygen and the metabolism product carbon dioxide through the blood vessels with the help of the red pigment haemoglobin, a protein complex whose core consists of an iron ion. Erythrocytes live for about 140 days and must be constantly replaced by the body.
- White blood cells (leukocytes) defend the body against invading pathogens. They are formed in different types, such as lymphocytes granulocytes, monocytes, etc, allowing them to react to a variety of pathogens. A healthy horse will have a smaller number of leukocytes in relation to red blood cells. The number of white blood cells rises quickly, however, in the face of infection.
- Blood platelets (thrombocytes) are formed from bone marrow cells and play an important role in blood coagulation. They are responsible for blood clotting, which stops the flow of blood after injuries.
How is a blood sample obtained for testing?
The veterinary surgeon almost always takes blood from one of the jugular veins, which run along both sides of the horse's neck. The blood is filled directly via the injection needle into colour-coded tubes containing anticoagulants or, for serum collection, substances that promote coagulation. The type of tube used is determined by the type of test being carried out.
- The tubes are labelled with the horse's data and the test date and prepared by the vet: the sample tubes are spun in a centrifuge to obtain pure plasma or serum – whole blood is only rarely used, as it clots quickly.
- The sample material is heat sensitive. It must be kept chilled until transport. Factors that can interfere with the evaluation of the blood sample in the lab include haemolysis (rupturing of the red blood cells) and lipaemia (clouding of the serum by lipids). If the sample is handled correctly, it will remain stable for up to a week.
- In addition, a blood film is created for microscopic examination in the lab, because the blood cells begin to change within 4–6 hours after the sample is taken. This is done by applying a drop of blood onto a slide with a pipette, spreading it over the slide and drying it.
What do the values in laboratory results mean?
For each type of blood test, both the values determined from the horse's blood sample and the reference values are indicated in the report. Reference values indicate the normal range of the individual values and, like the individual values of the horse, are given in special measurement units.
Elevated or too-low values, designated with +/-, are not necessarily signs of the horse being seriously ill. The vet will always assess the individual values in relation to each other and include the horse's history, findings from the clinical examination, and information from the owner.
What is a haemogram?
A haemogram evaluates the number of different blood cells.
It shows the number of
- erythrocytes (red blood cells)
- leukocytes (white blood cells)
- and thrombocytes (platelets).
Closer examination of erythrocytes for a haemogram will include the following values:
- haematocrit (HCT) – the number of erythrocytes in the blood
- haemoglobin (Hb) – the concentration of red blood pigment in the blood
- MCV (mean corpuscular volume) – the volume of individual erythrocytes. Very large or very small red blood cells are characteristic of certain diseases.
- MCH (mean corpuscular haemoglobin) – the haemoglobin content of individual erythrocytes.
- MCHC (mean corpuscular haemoglobin concentration) – the measure of Hb concentration in relation to total number of erythrocytes.
- It will often include the number of reticulocytes (immature red blood cells) to assess blood formation in the bone marrow.
The determined quantities are then considered in relation to each other and can be used to form meaningful comparisons and mean values.
What do haemograms reveal?
A haemogram can provide a clinical picture for early detection of disease. For example, elevated leukocyte levels may signal an inflammation or infection, whilst low erythrocyte values point to the horse being anaemic.
Caution: A haemogram cannot clarify the causes for values outside the normal range! Deviations found through haemograms require further investigation!
What is a complete/differential blood count?
A complete blood count is most often ordered when a haemogram shows abnormalities or the horse shows signs of illness. A complete blood count will more closely examine the different leukocyte groups in the blood:
- Neutrophils are part of the immune system, detecting and combating microorganisms.
- Eosinophils and basophils fight off parasites and play an important role in the immune system.
- Monocytes develop into macrophages that engulf and digest invading pathogens.
- Lymphocytes produce antibodies against pathogens, regulate immune response, or kill off infected cells.
Search profiles which, in addition to the differential blood count (haematology), also include clinical-chemical parameters (kidney and liver values, metabolic and muscle values as well as the status of the supply of trace elements) are also referred to as blood counts for simplicity's sake.
What can be examined in a blood count?
Even a complete blood count is often not sufficient for making a precise diagnosis. The vet can request further laboratory tests in order to obtain information on the following based on screenings or search profiles:
- the function of specific organs
- metabolic disorders
- nutrient supply with trace elements and vitamins
- some allergic processes
The evaluation of clinical-chemical parameters includes enzymes and substrates formed in the organs as well as minerals and electrolytes. Veterinary laboratories also offer, alongside in-depth examinations of organ values (organ-specific screening), special blood sample evaluation for performance horses, senior horses (geriatric profile), and foals.
What information will my horse's blood results contain?
For a health check, a complete blood count will be accompanied by the following organ-specific values:
- Bilirubin (indirect) is formed when haemoglobin is broken down in the liver cells by binding to albumin. Abnormal values point to a problem before breakdown in the liver. Direct bilirubin is transported to the gut with bile and partly eliminated via the kidneys. Elevated values suggest liver dysfunction or bile disturbances. A blood count will show values for bilirubin (total) and bilirubin (direct).
- AP (alkaline phosphatase) is an enzyme that speeds up metabolism. AP is found in nearly all body tissue. Abnormal values suggest liver disease, bone disease, or anaemia.
- γ-GT (gamma-GT, GGT, gamma-glutamyltransferase) plays an important role in detoxification processes. This liver enzyme is an indicator for protein metabolism disorders.
- Total protein measures the plasma proteins that are responsible for binding to water, nutrient transport, coagulation, and immune resistance in the organism. Abnormal values are seen from diseases involving the liver, kidneys, or bowel.
- AST (aspartate transaminase) is an enzyme that is involved in the breakdown of specific amino acids. Abnormal AST values suggest damage to the liver or the skeletal muscles.
- GLDH (glutamate dehydrogenase) is an enzyme specific to the liver and an important marker for liver damages.
- Albumin is a protein that is formed in the liver. Abnormal values can indicate liver or kidney disease.
- Urea is formed through the breakdown of ammonia during protein metabolism and is excreted in the urine. An accumulation of urea in the blood is a sign that the kidney cannot eliminate these toxins.
It should be noted here that elevated urea levels can also occur through high protein intake.
- Creatinine (CREA) is a breakdown product of creatine, an acid produced in the body. It is filtered through the kidneys and excreted in the urine. Creatinine and urea levels are indicators of kidney function.
- Sodium (Na) is present in all cells and fluids of the body and is closely linked to chloride. Together, these two elements make common salt. Both are essential for life and have a direct connection to the body's fluid balance. Sodium is essential for the proper maintenance of many different bodily functions, including blood pressure and acid-base balance. It also plays an important role in stimulus transmission in the nervous system and activates various enzymes.
- Chloride (Cl) plays an important role in the formation of gastric acid. It is therefore essential for digestion, but also plays an important role in fighting off pathogens.
- Potassium (K) is arguably one of the most important electrolytes in the body with a positive charge. It plays an essential role in water balance and electrolyte balance. Even minor disturbances in potassium balance can lead to cardiac arrhythmia. When potassium levels are too high, this is known as acidosis and may be a sign of a kidney dysfunction or hormonal imbalance. Alkalosis from too-low potassium levels may be caused by respiratory disorders, hormonal imbalances, kidney disease, or severe diarrhoea.
Note: Blood test results can be skewed by exposure to heat and long transport times. Diuretic herbs administered for flushing the kidneys can temporarily lower sodium, potassium, and chloride levels.
- Phosphate (PO4, inorganic phosphate) in combination with calcium (Ca) is important for energy and bone metabolism. Phosphorus and calcium levels in the blood are regulated by hormones, with excess amounts excreted via the kidneys. Too-low levels may be an indication of a bone metabolism disorder, kidney disease, a hormonal disorder or a gastrointestinal disorder.
Some medications, for example preparations containing cortisone, can cause low calcium levels.
- Glucose is used to measure blood sugar levels.
Diet, but also stress, can cause fluctuations in glucose levels and lead to falsely elevated findings.
- Cholesterol and triglycerides are blood lipids. Contrary to its reputation, cholesterol plays an important role in calcium metabolism, in hormonal balance and in the production of Vitamin D. Abnormal values may indicate problems with liver metabolism but may be related to thyroid function.
- Triglycerides are absorbed directly as fats and provide the body with energy reserves. Abnormal levels are seen in overweight horses, but also in starving ones, and may indicate liver or thyroid dysfunction. The triglyceride level is dependent on nutrition and may therefore change quickly.
- Creatine kinase (CK) is an enzyme expressed in skeletal and cardiac muscles; shows increased activity when these muscles are damaged. CK increases after a horse's exercise and transport and reaches its highest level 4–6 hours later. Increases can even occur from the taking of a blood sample if the horse is agitated.
- LDH (lactate dehydrogenase) is found in many organs, primarily in the liver as well as in cardiac and skeletal muscle. This enzyme has little significance as an individual parameter, but is evaluated in conjunction with CK.
- Calcium (Ca) is responsible for muscle fibre contraction and nerve conduction. Calcium levels in the blood are regulated by hormones and do not signify calcium supply in feed.
- Magnesium (Mg) plays an essential role in muscle metabolism, as it is responsible for the release of energy in the muscle cells. Because the organism stores magnesium in the bones, teeth, and muscles, deficiencies are not initially detectable in blood work.
What other blood tests can be carried out?
Older horses can be given a geriatric profile which specifically examines senior horses' declining organ function as well as zinc and selenium deficiencies.
Tip: It is recommended that older horses have annual blood work done for early detection of any organic diseases.
Screenings for high performance horses will also measure lactate concentration in the blood which can rise through training. Other tests are used for diagnosing metabolic disorders (EMS, Cushing's), thyroid disorders, or allergies, for determining hormone status and immune status and for monitoring treatment.
Can blood work detect mineral deficiencies?
The equine organism relies on important minerals that must be supplied through its feed. To prevent deficiencies, minerals are deposited and stored in the bones, muscles, or skin. When needed, the body can turn to these reserves for the minerals it needs. A mineral deficiency will turn up in blood testing after these reserves are nearly depleted. The horse's general condition – the condition of its skin, coat, and hooves – is the first indicator of any deficiencies. One exception to this is the supply of selenium (Se). Many horses have low selenium levels without showing any symptoms.
Tip: Separate blood profiles can be requested to check selenium levels.
Will a blood count prove that my horse is healthy?
If all the values in your horse's blood count lie within the normal ranges, you can relax for now. But a single blood value that is too high or too low in comparison to the given reference value does not necessarily indicate severe problems.
A blood count captures a moment in time. Some of the values examined are influenced by feed, by work, or even by the horse's stress levels at the time when the blood sample was taken. Other accompanying circumstances may also influence some of the blood values – for instance medications.
Individual blood work results must always be looked at as part of a larger whole. To be able to make a diagnosis, the vet will look at all of the various values together with their own examination findings, medical imaging, etc., along with the horse's medical history and, especially, your own knowledge about your horse. The evaluation of a blood count should therefore always be left to a trusted veterinary surgeon!
- Svenja Stelse-Heine: Pferde richtig füttern: Blutwertanalysen - Futtermittel – Vitalstoffe, BoD 2021