Equine Blood Work

Blood Analysis and its Relation to the Diet

Blood analysis is a useful tool to help assess equine health. Blood tests provide insight into a horse’s internal functioning, helping veterinarians diagnose a variety of conditions, monitor ongoing treatments and assess overall well-being. Blood is vital in the transportation of oxygen, nutrients, hormones and waste products throughout the body, and any abnormalities in the blood can signal issues with one or more of the organ systems. Routine blood testing, especially for performance horses or for those with health issues, can help ensure problems are detected early and managed appropriately.

While blood tests provide valuable information regarding circulating nutrients, they are one of several factors that can indicate the overall nutritional status. A diet analysis provides a more complete look at what is being ingested and nutrient availability, and it can be a more accurate way to assess the overall nutrition provided to a horse.

Show and Tell: Equine Blood Analysis

Blood tests are performed for many reasons. Common reasons for blood testing in equine medicine include: diagnosis of disease, pre- and post-surgical monitoring, evaluation of organ function, monitoring treatment as well as routine health check-ups. A blood analysis can give a snapshot of a horse’s health or can help diagnose specific conditions.

A blood analysis shows several measurements of blood components including:

1. Complete Blood Count (CBC)

A CBC is one of the most common and essential blood tests. It evaluates the number and types of blood cells, providing insight into the horse’s immune system, oxygen-carrying capacity and overall health.

Parts of the CBC:

Red Blood Cells (RBC) are responsible for transporting oxygen from the lungs to tissues and carbon dioxide back to the lungs for exhalation. A low RBC count (anemia) may indicate blood loss, nutritional deficiencies or chronic disease. Conversely, a high RBC count could be a sign of dehydration or other rarer causes.

Hemoglobin (Hb) is a protein in RBC that binds to oxygen. Low hemoglobin levels can indicate anemia or blood loss, while high levels may occur in cases of dehydration or polycythemia.

Hematocrit (PCV) measures the percentage of blood volume made up of RBCs. A low hematocrit suggests anemia, while a high hematocrit can indicate dehydration, as the blood becomes more concentrated.

White Blood Cells (WBC) are part of the immune system and are responsible for fighting infections. An elevated WBC count can indicate infection, inflammation or an immune response. A low WBC count can suggest an immune deficiency or bone marrow problem, or may be low early in an infectious process, where most circulating WBCs are at the site of infection.

Platelets are involved in blood clotting. A low platelet count (thrombocytopenia) may indicate a clotting disorder or a viral infection, while a high platelet count can indicate inflammation or bleeding disorders.

2. Serum Biochemistry Panel

The serum biochemistry panel provides information about the chemical composition of the blood measuring a variety of substances, including electrolytes, proteins and metabolites.

Parts of the Serum Biochemistry Panel:

Electrolytes

• Sodium (Na): Sodium helps regulate fluid balance and nerve function. Abnormal sodium levels can indicate dehydration, kidney problems or diarrhea.
• Potassium (K): Potassium is crucial for muscle function, particularly the heart. Abnormal potassium levels can cause muscle weakness, arrhythmias or colic.
• Chloride (Cl): Chloride works with sodium to maintain fluid balance. Abnormal chloride levels may point to acid-base imbalances.
• Calcium (Ca): Calcium is vital for bone health, muscle function and blood clotting. Abnormal calcium levels may be indicative of kidney disease, bone disorders or endocrine issues.
• Magnesium (Mg): Magnesium is essential for muscle and nerve function. Low magnesium levels can cause muscle tremors and other neurological issues.

Total Protein (TP) reflects the overall concentration of proteins in the blood, including albumin and globulins. Low protein levels can indicate malnutrition, liver disease, or kidney problems. Low albumin specifically may point to liver dysfunction or protein-losing enteropathy (a condition where proteins are lost through the gastrointestinal tract).

Liver enzymes including aspartate aminotransferase (AST), alanine aminotransferase (ALT), gamma-glutamyl transferase (GGT) and alkaline phosphatase (ALP). Elevated levels of these enzymes often indicate liver damage or disease. Since the liver detoxifies ammonia present in the body, elevated levels of ammonia can be associated with significant liver dysfunction. Elevated ammonia levels are typically associated with acute neurological symptoms and behavior changes. Ammonia is detoxified by creating urea (see BUN in kidney section), so low BUN levels may suggest liver dysfunction. Similarly, since glucose is made in the liver, low levels could reflect liver dysfunction.

Muscle Function

Enzymes contained in muscle include creatine kinase (CK), AST and lactate dehydrogenase, as well as potassium. Elevations of these values can be associated with muscle damage or various forms of myositis.

Lactate levels are measured experimentally in horses to help tune athletic ability. These levels are also measured in critically ill horses, as lactate can also be produced by lack of blood supply to parts of the intestine in certain forms of colic.

Cardiac muscle damage will also show elevations of CK and AST, although usually at lower levels, and these enzymes are not specific to cardiac muscle. The less commonly run cardiac troponin I (cTnI) is specific to cardiac muscle.

Kidney Function

• Blood Urea Nitrogen (BUN): BUN is a waste product from protein breakdown that is filtered by the kidneys. High BUN levels can indicate dehydration, kidney disease or a high-protein diet.
• Creatinine: Like BUN, creatinine is a waste product filtered by the kidneys. Elevated creatinine levels typically suggest kidney dysfunction.
• Urine Specific Gravity: This test measures the concentration of urine and is a good indicator of kidney function.

Symmetric Dimethylarginine (SDMA): This biomarker is an important biomarker of kidney disease in humans and small animals. In horses, the levels correlate well with creatinine levels but have not proven to be superior to creatinine, so are only sometimes offered on a kidney panel.

Glucose is the body’s primary energy source, and the levels can provide insight into a horse’s metabolic state. Elevated glucose levels may suggest stress or metabolic syndrome, with true diabetes in horses being extremely rare. Low glucose levels may indicate starvation or liver disease.

Bilirubin is a breakdown product of hemoglobin from red blood cells. It is processed by the liver and excreted in bile. High bilirubin levels may indicate liver dysfunction, hemolysis (destruction of red blood cells) or bile duct obstruction.

3. Endocrine Testing

Endocrine tests assess hormone levels that regulate critical bodily functions, such as metabolism and growth.

Thyroid Function

Thyroxine (T4) and triiodothyronine (T3) are measured in their total and free levels. Thyroid hormones regulate metabolism. Low levels are common due to pulsatile secretion and numerous factors affecting their levels. True hypothyroidism in horses is very rare. True hypothyroidism can be confirmed by measuring levels after stimulation with thyrotropin releasing hormone (TRH)

Insulin and Glucose

Insulin resistance and equine metabolic syndrome (EMS) are common conditions in horses that affect blood glucose and insulin levels. Horses with EMS often have high insulin levels, which can lead to laminitis and other health issues. While resting insulin levels can be helpful, normal levels do not necessarily mean that the horse will have a normal response to glucose or to corticosteroids. An oral sugar test, where insulin levels are measured after a dose of glucose, is more accurate.

Leptin is a hormone secreted by adipocytes (fat cells), and can be helpful in tracking weight loss trends.

Adrenocroticotropic hormone (ACTH) is a hormone produced by the pituitary gland. Elevated levels are associated with equine Cushing’s disease, more correctly called pars pituitary intermedia dysfunction (PPID). In borderline cases, the administration of TRH will amplify the ACTH response, with blood samples taken afterwards better differentiating between normal and abnormal cases.

4. Blood Gas Analysis

A blood gas test measures the levels of oxygen (O2), carbon dioxide (CO2) and pH in the blood, either venous or arterial. It provides insight into respiratory function and the acid-base balance of the horse’s body. Blood gas analysis is useful for diagnosing conditions such as respiratory diseases and certain metabolic disorders. This test is most commonly run on arterial blood during general anesthesia to help monitor and manage these critical levels.

5. Coagulation Profile

A coagulation profile assesses a horse’s ability to form blood clots, which is critical in preventing excessive bleeding. Key tests include prothrombin time (PT), activated partial thromboplastin Time (aPTT) and fibrinogen, a protein involved in clot formation.

6. Inflammatory Biomarkers

While fibrinogen can be abnormally low with certain clotting conditions (like diffuse intervascular coagulation aka DIC), these levels often rise with ongoing inflammatory conditions, although the elevation is often delayed.

Serum amyloid A (SAA) is an acutephase inflammatory protein with a good correlation with infectious causes. While elevations can sometimes occur for other reasons, an elevation in SAA is useful for both determining the presence of an infectious disease as well as for tracking response to treatment.

7. Disease Titers

Exposure to equine protozoal myeloencephalitis (EPM), Lyme disease, Corynebacterium pseudotuberculosis (pigeon fever) and other diseases can be evaluated indirectly by measuring the body’s immune response to the disease. Because previous exposure to a disease does not mean active infection, these values must be evaluated in the context of clinical signs and other testing.

Blood Test Results

There are a few things to consider when interpreting blood tests: context, the individual horse and trends over time. Interpreting blood test results in horses requires an understanding of normal reference ranges, as these can vary depending on the laboratory as well as the individual horse. Different laboratories may have variations in reference ranges, and it is important to interpret results within the context of the lab’s specific guidelines. It is also important to consider the horse’s clinical signs, history and any other diagnostic results. A single abnormal result might not always be a definitive sign of a disease. Veterinarians often rely on these reference ranges along with knowledge of the specific patient to identify patterns of abnormal results and diagnose potential health problems.