Top 10 Blood Tests

Learn what blood tests are most meaningful to your veterinarian—and what basic lab work will (or won’t) tell you about your horse’s health.
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Learn what blood tests are most meaningful to your veterinarian—and what basic lab work will (or won’t) tell you about your horse’s health.

Your horse is under the weather, and your vet doesn’t know what’s wrong. “We should probably run some basic lab work,” he explains. “Maybe that’ll help us figure out what to do.”

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Just about every horse owner has given the green light for “basic lab work” that includes a complete blood count and chemistry panel. I’ll unravel some of the mysteries about basic blood work. I’ll start by explaining the most common reasons I suggest blood work and what factors I consider when interpreting the results. Then I’ll tell you about the 10 tests I always look at first—including what they might mean and how they relate to other tests. You’ll learn why blood work is an important tool in your vet’s tool box, and how to make the most of the results.

BLOOD-TEST BASICS
Your horse’s blood is his basic internal transport system. It carries oxygen from his lungs to his other organs, delivers nutrients from his intestines, transmits proteins or other specialized cells to places where they’re needed, and carries waste materials away for elimination. When your horse is healthy, the substances in the blood typically stay within a certain range. When something’s wrong, one type of cell or substance may go out of whack. Knowing this provides information to help identify the problem.

Hundreds of tests can be performed on a sample of blood, ranging from a simple count of red blood cells to a test for hormone levels that might indicate a specific disease. I’ll limit my focus to the basics, made up of a complete blood count (or CBC) and chemistry panel.

A CBC provides information about the number and characteristics of red and white blood cells circulating in your horse’s system, as well as a breakdown of the different types of white blood cells. The chemistry panel consists of a number of tests that help evaluate the health and function of internal organs, as well as measurement of proteins that are involved with inflammation. Your vet will most likely suggest basic blood work for one of the following five reasons.

Your horse is losing weight. The most common reasons for weight loss include parasites, dental problems, and poor nutrition. If your vet determines that these basic issues aren’t the cause and he finds nothing unusual on a physical examination, he’ll suggest blood work that might identify more serious causes for your horse’s weight loss, such as liver or kidney disease.

Your horse has a fever. A fever is usually caused by one of three things: an inflammatory problem, a viral infection, or a bacterial infection. Blood work can help your vet determine the likelihood of bacterial involvement, and help him decide whether to treat with antibiotics. It can also help your vet evaluate the severity of your horse’s problem.

Your horse is off his feed with no other signs. If your horse stops eating yet shows no other symptoms, and your vet can’t find anything amiss on a physical exam, blood work might help provide an answer—whether it’s a low-grade infection, source of inflammation, or organ dysfunction.

Your horse just doesn’t seem right. Your horse might be eating and passing manure normally, but something just “seems off.” Blood work can give you peace of mind that nothing is wrong—or it might give you a clue about a problem brewing.

Routine screening. Routine blood work establishes a baseline for comparison if your horse later becomes ill. This is especially useful for older horses that develop chronic problems that sneak up over time. Routine screening is also valuable for performance horses that experience high levels of stress and often are administered medications such as non-steroidal anti-inflammatories.

Although basic blood work can give you and your vet a lot of information about your horse’s health status, a single lab value or set of tests doesn’t always tell the whole story. To make the most of blood results, make sure your vet has an accurate history about your horse’s condition and is able to do a thorough physical exam.

TOP TEN TESTS
When you see the report from a CBC and chemistry panel, you might be overwhelmed by a list of as many as 40 or 50 tests—accompanied by results, a normal range, an indicator of whether the test is normal or abnormal, and comments from the lab. It can be confusing, even for your vet!

Not every single number on that report is significant for your horse’s situation. The tests we pay the most attention to in a general-practice setting differ from the tests an internal-medicine specialist might pay close attention to when managing a post-operative colic in intensive care, for example.

Even more important? Every single abnormal value doesn’t mean your horse has a problem. A value just slightly outside the normal range may not mean anything at all, while for a different test even a small variation is meaningful. And sometimes a high value may indicate a problem, while a result below the normal range doesn’t mean much.

With this in mind, here’s a rundown of the tests I pay attention to when running blood work in a general-practice setting.

Test #1: Red Blood-Cell Count (RBC)
What it is: The RBC tells you the total number of red blood cells circulating in your horse’s bloodstream.

What it tells you: A low RBC might indicate anemia, while a high RBC is most commonly seen with dehydration. Anemia is most likely in horses secondary to some other chronic disease. Your vet might consider fluid therapy if he notices a very high RBC.

Related tests: The RBC is always accompanied by a couple of other tests including the hematocrit (also called “packed-cell volume” or PCV), which indicates the percentage of red blood cells compared with the total volume of blood, and the hemoglobin, which measures of the amount of the protein that carries oxygen within the blood.

Test #2: White Blood-Cell Count (WBC) and Differential

What it is: The WBC tells you the total number of white blood cells circulating in your horse’s bloodstream. The differential breaks down the different types including neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

What it tells you: A high WBC indicates inflammation or infection, and the differential can help determine more specifics. For example, a high neutrophil count is most likely with a bacterial infection, a high eosinophil count is most likely with parasites or allergies, and a high monocyte count points toward chronic inflammation. If the WBC is low, it could indicate endotoxemia—a severe, body-wide toxic state.

Related tests: If your horse’s WBC and differential are out of whack, your vet will pay close attention to fibrinogen and proteins. WBC results can also help your vet decipher underlying causes of organ dysfunction (such as kidney or liver failure), so he’ll look closely at lab values related to organ function.

Test #3: Fibrinogen
What it is: Fibrinogen is a protein that’s produced in the liver, and released early during inflammation.

What it tells you: A moderately elevated fibrinogen indicates chronic inflammation. If it’s very elevated, it may indicate infection somewhere in the body, and can assist your vet in deciding to administer antibiotics. An exceptionally high fibrinogen level accompanies many abscess-producing diseases.

Related tests: If fibrinogen is high, your vet will look closely at your horse’s WBC and differential to determine the likelihood of a bacterial infection requiring antibiotics. A test for another acute-phase inflammatory protein called Serum Amyloid A provides similar information. This test can be run “stall-side” for immediate results, and can be useful for making the decision whether to treat with antibiotics.

Test #4: Creatinine
What it is: Creatinine is a product of muscle metabolism excreted through the kidneys.

What it tells you: Elevated creatinine indicates that your horse’s kidneys aren’t functioning properly.

Related tests: Blood urea nitrogen (see below) is also likely elevated if your horse’s kidneys are failing. Electrolyte imbalances including elevated calcium and decreased phosphorus will also accompany kidney failure, along with lowered RBC.

Test #5: Blood Urea Nitrogen (BUN)
What it is: Urea is a waste product of protein breakdown in the liver and, similar to creatinine, it is eliminated by the kidneys.

What it tells you: Elevations in BUN are most commonly seen with kidney failure, although this test can also mean other things, such as dehydration. Low BUN levels may be related to liver disease or a protein-deficient diet.

Related tests: Creatinine and BUN are often considered together when evaluating kidney disease, along with calcium, phosphorus, and RBC.

Test #6: Gamma Glutamyl Transferase Enzyme (GGT)
What it is: GGT enzyme aids in metabolism of nutrients as well as regulation of inflammation in the body. It’s especially important in the liver, where it helps break down drugs and toxins.

What it tells you: Elevated GGT is most commonly associated with liver or bile-duct disease. The bile duct is a tube that transports bile (a substance produced by the liver) to the intestine where it aids in absorption and digestion, especially of fats. Blockage of the bile duct causes the largest increases in this enzyme, followed by chronic (long-term) liver-cell damage. GGT can also elevate with acute (sudden) liver injury. Rarely, an elevated GGT will be seen with excessive, hard training.

Related tests: A low RBC and decreased albumin (see Test #10) may be seen with chronic liver damage. Changes in the WBC differential can indicate an inflammatory condition of the liver or bile duct. Other diagnostic tests including liver-function tests, ultrasound, or biopsy make a specific diagnosis.

Test #7: Bilirubin
What it is: Bilirubin is released from hemoglobin (the oxygen-carrying component of red blood cells) and excreted in bile. Unconjugated bilirubin is the initial breakdown product that’s sent to the liver. Once in the liver, bilirubin becomes conjugated with another substance to make it dissolvable in water so it can be excreted. The lab reports total, conjugated, and unconjugated bilirubin levels.

What it tells you: Conjugated bilirubin is likely to increase with bile-duct blockage or disease (it’s trapped inside the liver). Unconjugated bilirubin increases with liver damage, or when your horse is simply off his feed, so this value must be interpreted with care.

Related tests: Elevations in bilirubin are usually considered along with other liver tests, including GGT, function tests, ultrasound, and biopsy.

Test #8: Creatine Kinase Enzyme (CK)
What it is: CK enzyme breaks down substances related to energy storage to release energy for muscle contraction.

What it tells you: Elevated CK values point toward muscle breakdown, and are most commonly seen after a tie-up (severe muscle cramping) episode. CK values can increase to very high levels within several hours after a significant event. Mildly elevated or “just above normal” values can be seen after something as simple as an intramuscular injection or particularly hard training session.

Related tests: Aspartate amino transferase (AST) enzyme also elevates with muscle damage. This enzyme takes a little longer to peak in your horse’s bloodstream and stays around a little longer than CK. Sequential testing for these two enzymes after an event can help guide the recovery process for your horse to go back to work.

Severe episodes of muscle breakdown also result in the release of a myoglobin into your horse’s bloodstream that can be damaging to the kidneys. Kidney-related blood tests such as creatinine and BUN can be important to pay attention to if your horse has a significantly elevated CK test.

Test #9: Glucose
What it is: Sugar circulating in your horse’s bloodstream.

What it tells you: Elevated blood sugar can be seen with a number of conditions, including Cushing’s disease and insulin resistance. If your horse is very sick, glucose might be elevated because of shock or a severe body-wide infection. Rarely, it’s elevated immediately following a meal or a “fight-or-flight” response.

Related tests: If your horse’s blood glucose is elevated, testing for Cushing’s disease or insulin resistance might be suggested.

Test #10: Protein
What it is: Total protein usually includes measurement of albumin and globulin. Albumin is a protein made by the liver that has many important functions, such as helping to maintain fluid balance in the blood and transport of important substances throughout the body. Globulins are proteins produced by the liver and by the immune system. They also have many functions and include antibodies that help fight infection.

What it tells you: Low protein levels can indicate liver disease (decreased production) or kidney disease (increased loss). Protein levels also drop with severe diarrhea or other gastrointestinal diseases. Protein levels increase with dehydration, and globulin levels may increase with an infection.

Related tests: Protein levels can be altered with many different conditions. Tests for kidney disease and liver disease as well as the values on the CBC will all be considered carefully if protein levels are abnormal.

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