Analyze This! (Blood Tests Part 2)
- Oct 1, 2003
Laboratory medicine, like a blood test, is used by your veterinarian--along with physical examination findings--to aid, confirm, or disprove a suspected diagnosis.
Photo: Anne M. Eberhardt/The Horse
Blood tests are a part of veterinary care known as laboratory medicine (for more information, see What Blood Can Tell You on TheHorse.com). Laboratory medicine is used by your veterinarian--along with physical examination findings--to aid, confirm, or disprove a suspected diagnosis. While the first part of this series discussed hematology (the study of blood), this part will discuss the second-most common laboratory analysis performed--the serum chemistry panel. What does that mean, and how can that information help your horse get well?
A Stable Horse
Remember that blood is approximately 60% water and approximately 40% red and white blood cells, with the rest of "blood" containing hundreds of elements, electrolytes, enzymes, and proteins. When measuring blood's different components, one should remember that each laboratory has its own set of normal ranges, which depend on their individual equipment, geographic population of horses, and many other factors. With serum tests, there are more gray areas of interpretation and a wider degree of variability. Your veterinarian will know which values he considers normal and abnormal for a specific laboratory.
We are now interested in some of the components in the blood's "serum." The serum is the clear-to-yellowish fluid remaining after the blood sample is allowed to clot and has been centrifuged (spun so that the red cells are removed). The serum contains electrolytes, a variety of proteins, enzymes, and many waste products of metabolism.
The mechanism by which the body maintains itself in a stable, consistent nature is called homeostasis. When all is normal, most of the components of blood (serum) are regulated to a very narrow concentration that varies only slightly from horse to horse. Many of the blood components change in a very predictable way relative to various disease processes. Therefore, blood tests can be an extremely valuable aid in disease diagnosis and determination of health status.
When evaluating serum chemistry profiles, the metric system is standard, using deciliter, milliliter, and microliter for fluids (1/10, 1/1,000, and 1/1,000,000 of a liter, respectively). The solids are measured in weight using gram, milligram, and microgram with the last values being 1/1,000 and 1/1,000,000 of a gram, respectively.
Some of the blood components are enzymes that are measured by their level of activity per volume of blood--these are typically reported as "IU" (International Units) of activity.
There are enzymes in the blood that are manufactured by and related to cellular metabolism of certain organ systems. These enzymes are routinely measured in blood for information about health status of the animal (more on this later).
Blood Chemistry Profiles
The first components measured in a blood chemistry profile are typically the electrolytes sodium, potassium, and chloride. The major function of electrolytes in the body is electrical in nature. For example, when a nerve ending is stimulated, the signal is transmitted to the brain, spinal cord, or muscle by the movement of sodium across the nerve cell membrane. Another example is the collection of cells controlling the trigger for contraction of the top part of the heart (the atria); the signal also depends of the movement of sodium across the cell membrane. Potassium also is closely involved with nerve conduction and muscle contraction.
Excessively high or low sodium concentrations can affect neurologic function and nerve conduction. Changes in sodium concentration can result from dehydration and its opposite (dilution or "water intoxication" typically following slowly developing dehydration followed by rapid consumption of pure water).
Electrolytes can also be "lost" through severe or chronic diarrhea or kidney dysfunction.
Another relatively common cause of electrolyte changes (particularly potassium) is a ruptured bladder in foals. The potassium in urine is absorbed into the bloodstream via the lining of the abdominal cavity--a clinical sign of high potassium is an abnormally low heart rate (bradycardia). Electrolyte alterations also can be caused by severe sweating.
The condition choke (blockage of the esophagus) can lead to loss of chloride due to this electrolyte's relatively high concentration in equine saliva, which is lost on the ground since the horse cannot swallow. The main objective of monitoring electrolyte concentrations is to aid in the treatment of various conditions.
Typically, initial electrolyte therapy is accomplished by the use of various intravenous fluids containing sodium, chloride, and/or potassium, and sometimes oral fluid administration if the horse's electrolyte imbalance is not severe.
The next components often measured in a chemistry profile include calcium, phosphorus, and magnesium. These electrolytes are very important in bone health and muscle contraction. For example, in addition to the actual contraction of the heart muscle, the trigger for contraction of the lower part of the heart (the ventricles) is related to movement of calcium across cell membranes in a collection of cells within the heart; various irregular heart rhythms can be related to abnormal calcium concentrations.
Changes in these electrolytes can be related to diet, various illnesses, sweating/ dehydration, kidney dysfunction, and other problems. For example, the inflammatory intestinal disease caused by the blister beetle (cantharidin toxicity) is often associated with a decrease in serum calcium.
Phosphorus and/or magnesium increases or decreases are typically associated with extreme dietary imbalances.
Serum protein is an important component of a serum chemistry profile that is typically differentiated into the two "main" proteins (there are many others in relatively small concentrations), the albumin and globulins. Albumin is a protein manufactured within the liver that is mostly responsible for water balance within the blood system. Typical causes of decreased albumin include kidney disease, a decrease in albumin production by the liver, or a malabsorption condition of the gastrointestinal system (sometimes caused by chronic parasitism).
Globulins, or immunoglobulins, are the proteins of the immune system. A common cause of increased globulins is excess production stimulated by chronic inflammation or infection--a long-standing chronic abscess is often a cause of increased globulins.
Two commonly measured enzymes are SGOT--standing for serum glutamic oxaloacetic transaminase (also called AST)--and CPK (standing for creatine phosphokinase), which are the muscle enzymes.
Elevations of CPK and SGOT are indictors of muscle inflammation--tying-up or rhabdomyolysis. The term "rhabdo" means muscle and "myolysis" means rupture of muscle cells.
The CPK and SGOT are very sensitive indicators of skeletal muscle damage, and they rise in concentration proportionally with the amount of damage. A bit of timing is required in order to obtain the most sensitive results; CPK rises (due to its leakage from muscle cells into the blood system) approximately six to eight hours after the onset of muscle inflammation, and SGOT rises after approximately 12-14 hours. The absolute peak of CPK concentration and the time it takes to return to normal are important indicators of the severity of muscle damage and the response to therapy.
The next compound is creatinine, which is a waste by-product of muscle metabolism that is eliminated very efficiently via the horse's kidneys. The serum concentration of creatinine is directly related to kidney function; therefore, an increase in serum creatinine is an indicator of renal insufficiency.
Creatinine monitoring is useful for a variety of illnesses and conditions that can affect kidney function. For example, severe cases of tying-up can release the protein myoglobin into the blood system, which can be toxic to the kidneys. Monitoring creatinine evaluates how well the kidneys are coping and if additional treatment is necessary.
When using the antibiotic gentamicin (or other drugs toxic to the kidneys, called nephrotoxic) long-term or on foals, creatinine is monitored during therapy to evaluate kidney function.
The next two take a look at liver function. The enzymes SDH (sorbitol dehydrogenase) and GGT (gamma-glutamyltransferase) are associated with liver function. Diseases that cause liver damage or inflammation cause an increased serum concentration of SDH and/or GGT.
For example, plants that are liver toxic such as Amsinckia (fiddleneck), Crotalaria (rattlebox), and Senecio (tansy ragwort) cause degeneration of the liver (pyrrolizine toxicosis) and a subsequent increase in serum SDH and/or GGT. Inflammatory conditions such as bacterial hepatitis also cause an increase in serum SDH and/or GGT concentrations.
Another liver-related compound is bilirubin, which is a compound secreted by the liver into the intestine to aid in digestion. Increases in serum bilirubin concentration can be caused by bile duct obstruction via infection and occasionally even bile stones (gall stones in humans, but horses don't have gall bladders). Due to the lack of a gall bladder, rises in serum bilirubin concentration can also be caused by relatively short periods of being off feed. Increased serum bilirubin is responsible for the yellowing of skin (jaundice or icterus) often observed with liver disease or hepatitis.
Of course, there are more blood tests than can be discussed in this one article, but these are the ones horse owners will most often hear about when they have a sick or injured horse. Remember that some of these tests will need to be done repeatedly over a period of time to see if the levels of various substances in your horse's blood are rising or falling as an indication of whether he's getting better, or worse. If you have questions about specific tests done on your horse, ask your veterinarian.
About the Author
Michael A. Ball, DVM, completed an internship in medicine and surgery and an internship in anesthesia at the University of Georgia in 1994, a residency in internal medicine, and graduate work in pharmacology at Cornell University in 1997, and was on staff at Cornell before starting Early Winter Equine Medicine & Surgery located in Ithaca, N.Y. He is also an FEI veterinarian and works internationally with the United States Equestrian Team.
Ball authored Understanding The Equine Eye, Understanding Basic Horse Care, and Understanding Equine First Aid, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.