Drawing blood for laboratory testing has been a routine part of equine veterinary care for decades. Along with a physical examination and review of a horse’s history, bloodwork can offer valuable insights into the health and function of the body’s systems. What’s more, routine screenings can detect brewing trouble long before a horse might show outward signs of anything amiss.

The procedure is simple enough—the blood is drawn, placed into test tubes, and then a few days later your veterinarian calls with new information and possible recommendations for management changes or treatments. And many blood samples are taken when a horse is perfectly healthy. “It’s good to have baseline values that are taken when your horse is normal because then you can compare them to additional results taken later on in his life and see how things have changed,” says Liz Arbittier, VMD, CVA, of the University of Pennsylvania’s New Bolton Center. “For instance, certain organ values that are creeping up in older horses can be a harbinger of problems.”

But blood analysis also has many specialized applications. “We run screening tests for any number of reasons,” says Arbittier. “We might run tests on a horse with a fever of known or unknown origin, one with an obvious disease process such as nasal discharge or cough, one who is lethargic or has lost weight, or for a prepurchase exam or as part of annual exams on geriatric horses, for instance. Once we have baseline values in a sick horse, we can use some of them to help monitor how a certain treatment is working.”

Of course, your veterinarian will decide what sort of blood analysis needs to be done for your horse and will explain what the results mean for his health. And, as useful as blood tests can be, they do have their limitations. However, the following overview of the more common blood tests will help you understand what tests your veterinarian has called for and why.

Complete blood Count (CBC)

The CBC quantifies the types of cells in a blood sample—specifically the red cells, the white cells and the platelets. Run on a sample of whole blood that has been blended with an anticoagulant, the CBC is a general-purpose screening tool that can yield a great deal of information about a horse’s general health. “Results of the CBC can help determine a variety of physiological responses,” says Peggy Marsh, DVM, DACVIM, DACVECC, of Equine Medical Associates in Lexington, Kentucky.

The results of a CBC are broken out into several different components. For example, a test may describe the total number of red blood cells (RBC count) as well as hemoglobin, the total amount of oxygen-carrying protein in the blood. Another indicator is the hematocrit, also called the packed cell volume (PCV), which is the percentage of the whole blood that consists of red cells. More specific tests, such as mean corpuscular volume (MCV), might look at the average sizes and composition of the red blood cells.

Low red cell and hematocrit values might indicate anemia; high values might point toward dehydration. But this information is only a starting point. “The red blood cell information can sometimes help us focus on specific diseases, but is not very definitive,” says Katherine Wilson, DVM, DACVIM, of the Virginia–Maryland Regional College of Veterinary Medicine.

In fact, she adds, “RBC count is probably the least helpful information because horses usually don’t have big changes in red blood cell numbers. It is not uncommon for horses to have an RBC count a little lower than normal range, however. The term we use for low RBC is anemia, but unless the count gets very low, a horse doesn’t necessarily need to be treated for that condition. A lot of diseases or any chronic long-term disease can cause mild anemia. Usually if we see mild anemia on the bloodwork and the horse has other issues, the anemia is just an indication that we need to fix/treat another problem.”

Blood analysis also focuses on the white blood cell count (WBC count), a measure of the number of the infection-fighting white blood cells in the sample. This test may also be broken down into a white blood cell differential, which is a count of the numbers of each of the five specific types of white cells: neutrophils, lymphocytes, monocytes, eosinophils and basophils.

An elevated WBC count usually indicates infection or inflammation, and a closer look at the specific types of cells can yield clues to the type of process at work. “Usually if there is some kind of infectious disease, one or two types of white blood cells may be elevated,” says Wilson. “If it’s a bacterial infection like pneumonia, the neutrophil count may be elevated. If it’s an early stage of bacterial infection, we would see a lot of immature neutrophils in the blood, because the body is putting out a lot more of them to try to fight the infection.”

Also, says Arbittier, “The percentage breakdowns of different WBCs can help diagnose an allergic or parasitic process if one particular white cell, the eosinophil, is high.”

A low WBC count could be an indication of endotoxemia, a systemic inflammatory condition that develops when toxins released by some intestinal bacteria get into the bloodstream. “When horses become endotoxic, the white cells go out into the tissues rather than staying in the blood, so we’d see a low count in the blood sample,” says Wilson. “ I usually become more concerned if a horse has a low white cell count than if it’s high, because a low count usually means the horse is very sick at the time you are taking the blood sample.”

Finally, a CBC will show the platelet count, the total volume of the cells that are instrumental in forming clots and stimulating healing after injury. More specific tests—mean platelet volume (MPV) and platelet distribution width (PDW)—record the average sizes of the platelets. A low platelet count, which can be caused by certain drugs or toxins, may leave the horse at risk of uncontrolled bleeding. Platelet counts usually return to normal soon after the drug is discontinued. Abnormally high platelet counts are rare in horses.

Serum chemistry profile

Focusing on the clear, yellowish liquid left after all the solid blood cells are removed by clotting and then centrifuging, the serum chemistry profiling can identify levels of various proteins and enzymes, electrolytes and other biochemicals that indicate overall health as well as individual organ function.

“Results of the serum chemistry help me evaluate systems such as the liver, kidneys or muscles,” says Marsh. “It helps us know if the body is in good health or if there is something abnormal going on that needs further exploration.” Some of the major values a veterinarian may look for include:

• Proteins. The tests measure levels of the proteins albumin and globulin. “It is important to check protein levels because protein in the blood is responsible for keeping fluid inside the blood vessels and not leaking out into body tissues,” says Wilson. “Usually the horse loses protein from the blood through severe diarrhea. Horses can also lose it through the kidneys if the kidneys are not functioning properly.”

Low protein levels may also signal a liver problem. “The protein is produced in the liver, so if the protein is low this might mean the horse has a liver problem—if there are other things on the profile that are consistent with liver damage,” says Wilson.

The albumin/globulin ratio can also help to distinguish health issues that will affect both together, such as dehydration, from illnesses that will affect one more than the other.

Another important protein is fibrinogen, which aids the formation of blood clots. “Fibrinogen is an acute-phase protein generated by the equine liver that can indicate an inflammatory process if it is moderately elevated and an infectious process if it is significantly elevated,” says Arbittier. “This value can be utilized not only to diagnose an infection but also as a relatively inexpensive tool to gauge the horse’s response to treatment.”

Serum amyloid A (SAA) is another protein that is useful for gauging a horse’s level of infection and inflammation. “New advances in running this test have made it more readily available to practitioners and it is an extremely sensitive marker of inflammation and infection,” says Arbittier.

“Fibrinogen and more recently SAA are known as acute phase response, since these may rise more quickly than any changes in the CBC,” says Claudia True, DVM, of Woodside Equine Clinic in Ashland, Virginia. “Fibrinogen and SAA become elevated with inflammation, infection, neoplasia [abnormal growth] and trauma.”

Plasma proteins and plasma fibrinogen may also be recorded on the CBC test. “This is roughly the same thing, whether measured in the CBC or the serum chemistry profile,” says Wilson.

• Electrolytes. These dissolved ions, including sodium, potassium, chloride, bicarbonate, calcium and phosphorus, are essential to many biological functions, and abnormally high or low levels can have a significant impact on health. “Electrolytes in the blood can change quickly, due to dehydration or loss through diarrhea, or via urine, if the kidneys are not functioning normally,” says Wilson. Elevated potassium may indicate kidney disease or muscle damage. Bicarbonate is important because it helps to regulate the blood pH. Too much bicarbonate creates an alkaline environment (metabolic alkalosis) and too little makes the blood acidic (metabolic acidosis), both of which can have serious consequences.

• Glucose. This sugar is an energy source inside the horse’s cells. Serum glucose fluctuates widely in normal individuals, in response to factors such as stress, pain or recent feeding, so an elevated reading on a single test isn’t necessarily significant. However, chronically elevated levels of glucose may also indicate a metabolic disorder, such as insulin0 resistance (IR).

•Kidney waste products. Urea, which is checked for in a blood urea nitrogen (BUN) test, and creatinine are two waste products found in the blood and filtered out by the kidneys to be eliminated in the urine. If levels of these substances are elevated, it may mean that the kidneys are not functioning properly. It could also mean that the horse is dehydrated.

• Liver enzymes. The liver produces several enzymes, commonly abbreviated as AST, ALP and GGT. “If these values are elevated this usually means there has been some damage to the liver,” says Wilson. “Those cells essentially rupture and those enzymes are floating around in the bloodstream at higher concentration. One thing we emphasize with our students is that just the fact those enzymes are elevated or there is evidence of liver damage doesn’t necessarily mean the liver is not functioning normally. It may still be able to do all the things it needs to do in spite of the damage, and it can sometimes heal from that damage. It might be temporary.”

•Bilirubin. Another indication of liver health is a pigment called bilirubin, which is formed from the breakdown of red blood cells. Elevated levels can mean unusual loss of red cells or liver dysfunction. However, in horses, unlike other animals, elevated levels of bilirubin often isn’t serious. “This value can increase fairly rapidly when horses go off feed, and this is something that is unique to the horse,” says Wilson. “Often we get phone calls from veterinarians who don’t work on horses much or owners who see the bloodwork and note that the bilirubin is above normal range and are concerned about liver disease. If the horse is off feed for 24 to 48 hours, that value will increase, but this is just a temporary elevation.”

• Muscle enzymes. The most important muscle enzyme is creatine kinase, but there are others as well, including some of the same ones that are produced in the liver. “These enzymes are found inside the cells, and when the cell is sick or damaged, these leak into the bloodstream. Thus if these enzyme levels in the blood are increased, this could indicate cell damage,” Marsh says. “However, a lot of these enzymes are not found in just one place in the body, so we have to put this all together to figure out what’s happening.”

Tests for specific diseases

A number of tests are available to determine whether a horse has been exposed to particular organisms that cause diseases. “The disease bloodwork falls into two categories,” says Wilson. “One is testing for antibodies against that disease, produced by the animal after being exposed to that disease. The other category is tests that look for the organism itself. By far the majority of tests are simply testing to see if antibodies are there.”

Sometimes, a veterinarian might run more than one type of test to confirm a result. “Many disease processes have multiple types of tests available to help diagnose them, and new literature is constantly being published as to which methods are most reliable,” Arbittier says. The options include:

• Enzyme-linked immunosorbent assay (ELISA). The ELISA test consists of a plastic plate with columns of wells that come from the manufacturer coated with a reactive substance, usually an antigen0. When fluid samples from the horse are placed in the wells, any antibodies present will bind with the antigens. Then, the wells are emptied and refilled with other solutions that will bind to any antibodies present to produce a color change that will be visible to the naked eye. The result is an on/off positive or negative. However, false positives are possible, and a light color change might be an inconclusive result.

ELISA tests can also be used to measure a horse’s hormone levels, such as insulin or adrenocorticotropic hormone (ACTH), to look for metabolic diseases. “Leptin [a hormone produced by fat cells] is another thing the lab can look for. Some are offering this as an insulin resistance panel when checking for equine metabolic syndrome,” True says.

• Western blot or immunoblotting. Like ELISA, Western blotting works by providing antigens to bind to antibodies in a horse’s blood in a way that can be seen. The biggest difference is that the Western blots first separate the proteins in the horse’s blood into bands on a sheet of gel so that veterinarians can see exactly which ones are causing the reactions. This offers more specificity than the ELISA.

• Indirect immunofluorescent antibody technique (IFAT or IFA). IFAT is another, newer method that utilizes the antigen-antibody bond to produce a visible result. In this test, fresh or frozen tissue samples are bathed in solutions that contain anti-bodies combined with fluorescent dyes, which then bind with any antigen proteins on the surface of the cells. The fluorescent dyes can then be visualized by any of several imaging techniques. Both IFAT and ELISA are used to test horses for antibodies to equine0 protozoal myeloencephalitis (EPM).

The downside of all of these tests is that the presence of antibodies in a blood sample does not necessarily mean that the horse has that disease. For example, many horses are exposed to the organisms that cause EPM or Lyme disease, but only a fraction develop signs of illness. “It depends on the disease, but in general terms [a positive result] simply means that the animal became infected at some point in the past and developed antibodies to fight the disease, or it could indicate that the animal actively has the disease right now,” says Wilson. “With some diseases it can sometimes be difficult to determine which scenario this is.”

Of course, vaccination can also influence antibody level. “There are many tests that become difficult to interpret if the horse has been vaccinated,” Wilson adds. “It’s hard to determine whether the antibodies are from the vaccine or from previous exposure, or active disease—unless it’s a disease for which there’s no vaccine.” Some antibody tests need to have multiple samples run at different times in the disease process to interpret whether the animal has the disease currently or has been previously exposed.

Generally speaking, if an animal has been recently exposed or infected, the antibody levels should increase over a period of a couple of weeks. Blood from the horse at the time of examination can be compared to blood taken two weeks later and an increase in antibody levels to a specific disease would support infection with that disease. A constant level of antibodies over time might be more indicative of exposure to the disease previously, or previous vaccination. The major downside to having to take multiple samples is that by the time the results are obtained, the horse may be better or much worse; it makes it difficult to diagnose the disease at the time the horse is showing signs of illness. There is, however, an alternative approach:

• Polymerase chain reaction (PCR). Unlike these other tests, which test a horse’s blood, bodily fluids or tissues for the presence of antibodies, PCR is a method of determining whether a disease-causing organism itself is present in the sample by identifying its DNA. PCR has myriad applications in research, but in veterinary practice it is used mainly to diagnose bacterial and viral infections that would be difficult to identify by other means. “If the pathogen is detected, it would be much more indicative of an active infection, for most diseases, because the pathogen is still there,” says Wilson.

But the test does have its limitations. “The caveat: These really only test for the DNA of the organism, so you could have a dead bacterium or dead virus that isn’t causing disease but still shows up,” Wilson adds. “That’s not usually the case, with most of the diseases that horses get. It would be something to think about, however, if the horse is positive on a test for a disease that affects the lungs but has diarrhea, for instance. It doesn’t fit together.” You have to look at the whole picture.

Putting it all together

Lab reports based on blood tests can yield very specific and detailed information about what might be going wrong inside your horse. However, the results still need to be interpreted carefully and compared to the horse’s outward signs of illness.

“Some owners want us to come take a blood sample and tell them what their horse has, but it’s not that easy,” says Marsh. “One of the main points to remember is that doing bloodwork is just one piece of the puzzle when we are trying to figure out what is going on with that horse.”

The blood tests are part of the sequence a veterinarian goes through to diagnose a horse’s illness. “We do a physical exam, get a history, and then we can run a CBC and a serum biochemistry profile to try to determine which body system is sick,” says Wilson. “Then we might individually test for the most common diseases that would affect that body system in that particular horse, depending on his exposure to other animals, what part of the country he is in, etc. Unfortunately, you can’t just run a screening test and the answer shows up. You have to request a test for what you think it could be, and you don’t want to run too many tests or this becomes expensive for the horse owner.”

But even all of that might not be enough: “Often we are presented with animals that we test for all the things we know could cause the clinical signs they have, and everything comes back negative,” says Wilson. “We are still scratching our heads to try to figure out what is making the horse sick.”

Sometimes a veterinarian might also test other bodily fluids. “We might need a urine sample or perhaps a sample of lung fluid, or the fluid that lines the GI tract,” says Marsh. “We can test different things for each specific situation.”

A urinalysis might be especially important to look at in conjunction with bloodwork. “The minimum data-base for most animals—especially dogs and cats and humans—is generally a CBC, a serum biochemistry profile and a urinalysis,” says Wilson. “Unfortunately, we tend to forget the urinalysis with horses, but we really need to look at that in comparison with the blood to determine kidney function and hydration status. There are also other things we can see in the urine. If there are red blood cells or white blood cells in the urine, this might indicate inflammation or infection of the urinary tract. Ideally, it would be nice if we could get blood and urine from every horse we need to test.”

When your horse is ill, you want to do anything you can to restore his health as quickly as possible. Fortunately, modern laboratory tests are one of the fastest and most effective ways to zero in on the source of a problem. They also help keep tabs on recovery. 

This article first appeared in EQUUS issue #450.

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