Of the diagnostic methods available to veterinarians, the clinical
chemistry test has developed into a valuable aid for localizing pathologic
conditions. This test is actually a collection of specially selected individual
tests. With just a small amount of whole blood or serum, many body systems can
be analyzed. Some of the more common screenings give information about the
function of the kidneys, liver, and pancreas and about muscle and bone disease.
There are many blood chemistry tests available to doctors. This paper covers
the some of the more common tests.
Blood urea nitrogen (BUN) is an end-product of protein metabolism. Like
most of the other molecules in the body, amino acids are constantly renewed. In
the course of this turnover, they may undergo deamination, the removal of the
amino group. Deamination, which takes place principally in the liver, results
in the formation of ammonia. In the liver, the ammonia is quickly converted to
urea, which is relatively nontoxic, and is then released into the bloodstream.
In the blood, it is readily removed through the kidneys and excreted in the
urine. Any disease or condition that reduces glomerular filtration or increases
protein catabolism results in elevated BUN levels.
Creatinine is another indicator of kidney function. Creatinine is a
waste product derived from creatine. It is freely filtered by the glomerulus
and blood levels are useful for estimating glomerular filtration rate. Muscle
tissue contains phosphocreatinine which is converted to creatinine by a
nonenzymatic process. This spontaneous degradation occurs at a rather
consistent rate (Merck, 1991).
Causes of increases of both BUN and creatinine can be divided into three
major categories: prerenal, renal, and postrenal. Prerenal causes include
heart disease, hypoadrenocorticism and shock. Postrenal causes include urethral
obstruction or lacerations of the ureter, bladder, or urethra. True renal
disease from glomerular, tubular, or interstitial dysfunction raises BUN and
creatinine levels when over 70% of the nephrons become nonfunctional (Sodikoff,
Glucose is a primary energy source for living organisms. The glucose
level in blood is normally controlled to within narrow limits.Inadequate or
excessive amounts of glucose or the inability to metabolize glucose can affect
nearly every system in the body. Low blood glucose levels (hypoglycemia) may be
caused by pancreatic tumors (over-production of insulin), starvation,
hypoadrenocorticism, hypopituitarism, and severe exertion. Elevated blood
glucose levels (hyperglycemia) can occur in diabetes mellitus, hyperthyroidism,
hyperadrenocorticism, hyperpituitarism, anoxia (because of the instability of
liver glycogen in oxygen deficiency), certain physiologic conditions (exposure
to cold, digestion) and pancreatic necrosis (because the pancreas produces
insulin which controls blood glucose levels).
Diabetes mellitus is caused by a deficiency in the secretion or action of
insulin. During periods of low blood glucose, glucagon stimulates the breakdown
of liver glycogen and inhibits glucose breakdown by glycolysis in the liver and
stimulates glucose synthesis by gluconeogenesis. This increases blood glucose.
When glucose enters the bloodstream from the intestine after a carbohydrate-rich
meal, the resulting increase in blood glucose causes increased insulin secretion
and decreased glucagon secretion. Insulin stimulates glucose uptake by muscle
tissue where glucose is converted to glucose-6-phosphate. Insulin also
activates glycogen synthase so that much of the glucose-6-phosphate is converted
to glycogen. It also stimulates the storage of excess fuels as fat (Lehninger,
With insufficient insulin, glucose is not used by the tissues and
accumulates in the blood. The accumulated glucose then spills into the urine.
Additional amounts of water are retained in urine because of the accumulation of
glucose and polyuria (excessive urination) results. In order to prevent
dehydration, more water than normal is consumed (polydipsia). In the absence of
insulin, fatty acids released form adipose tissue are converted to ketone bodies
(acetoacetic acid, B-hydroxybutyric acid, and acetone). Although ketone bodies
can be used a energy sources, insulin deficiency impairs the ability of tissues
to use ketone bodies, which accumulate in the blood. Because they are acids,
ketones may exhaust the ability of the body to maintain normal pH. Ketones are
excreted by the kidneys, drawing water with them into the urine. Ketones are
also negatively charged and draw positively charged ions (sodium, potassium,
calcium) with them into urine. Some other results of diabetes mellitus are
cataracts (because of abnormal glucose metabolism in the lens which results in
the accumulation of water), abnormal neutrophil function (resulting in greater
susceptibility to infection), and an enlarged liver (due to fat accumulation)
Bilirubin is a bile pigment derived from the breakdown of heme by the
reticuloendothelial system. The reticuloendothelial system filters out and
destroys spent red blood cells yielding a free iron molecule and ultimately,
bilirubin. Bilirubin binds to serum albumin, which restricts it from urinary
excretion, and is transported to the liver. In the liver, bilirubin is changed
into bilirubin diglucuronide, which is sufficiently water soluble to be secreted
with other components of bile into the small intestine. Impaired liver function
or blocked bile secretion causes bilirubin to leak into the blood, resulting in
a yellowing of the skin and eyeballs (jaundice). Determination of bilirubin
concentration in the blood is useful in diagnosing liver disease (Lehninger,
1993). Increased bilirubin can also be caused by hemolysis, bile duct
obstruction, fever, and starvation (Bistner, 1995).
Two important serum lipids are cholesterol and triglycerides.
Cholesterol is a precursor to bile salts and steroid hormones. The principle
bile salts, taurocholic acid and glycocholic acid, are important in the
digestion of food and the solubilization of ingested fats. The desmolase
reaction converts cholesterol, in mitochondria, to pregnenolone which is
transported to the endoplasmic reticulum and converted to progesterone. This is
the precursor to all other steroid hormones (Garrett, 1995).
Triglycerides are the main form in which lipids are stored and are the
predominant type of dietary lipid. They are stored in specialized cells called
adipocytes (fat cells) under the skin, in the abdominal cavity, and in the
mammary glands. As stored fuels, triglycerides have an advantage over
polysaccharides because they are unhydrated and lack the extra water weight of
polysaccharides. Also, because the carbon atoms are more reduced than those of
sugars, oxidation of triglycerides yields more than twice as much energy, gram
for gram, as that of carbohydrates (Lehninger, 1993).
Hyperlipidemia refers to an abnormally high concentration of
triglyceride and/or cholesterol in the blood. Primary hyperlipidemia is an
inherited disorder of lipid metabolism. Secondary hyperlipidemias are usually
associated with pancreatitis, diabetes mellitus, hypothyroidism, protein losing
glomerulonephropathies, glucocorticosteroid administration, and a variety of
liver abnormalities. Hypolipidemia is almost always a result of malnutrition
Alkaline phosphatase is present in high concentration in bone and liver.
Bone remodeling (disease or repair) results in moderate elevations of serum
alkaline phosphatase levels, and cholestasis (stagnation of bile flow) and bile
duct obstruction result in dramatically increased serum alkaline phosphatase
levels. The obstruction is usually intrahepatic, associated with swelling of
hepatocytes and bile stasis. Elevated serum alkaline phosphatase and bilirubin
levels suggest bile duct obstruction. Elevated serum alkaline phosphatase and
normal bilirubin levels suggest hepatic congestion or swelling. Elevations also
occur in rapidly growing young animals and in conditions causing bone formation
Aspartate aminotransferase (AST) is an enzyme normally found in the
mitochondria of liver, heart, and skeletal muscle cells. In the event of heart
or liver damage, AST leaks into the blood stream and concentrations become
elevated (Bistner, 1995).AST, along with alkaline phosphatase, are used to
differentiate between liver and muscle damage in birds.
Alanine aminotransferase (ALT) is considered a liver-specific enzyme,
although small amounts are present in the heart. ALT is generally located in
the cytosol. Liver disease results in the releasing of the enzyme into the
serum. Measurements of this enzyme are used in the diagnosis of certain types
of liver diseases such as viral hepatitis and hepatic necrosis, and heart
diseases. The ALT level remains elevated for more than a week after hepatic
injury (Sodikoff, 1995).
Fibrinogen, albumin, and globulins constitute the major proteins of the
blood plasma. Fibrinogen, which makes up about 0.3 percent of the total protein
volume, is a soluble protein involved in the clotting process. The formation of
blood clots is the result of a series of zymogen activations. Factors released
by injured tissues or abnormal surfaces caused by injury initiate the clotting
process. To create the clot, thrombin removes negatively charged peptides from
fibrinogen, converting it to fibrin. The fibrin monomer has a different surface
charge distribution than fibrinogen. These monomers readily aggregates into
ordered fibrous arrays. Platelets and plasma globulins release a fibrin-
stabilizing factor which creates cross-links in the fibrin net to stabilize the
clot. The clot binds the wound until new tissue can be built (Garrett, 1995).
The alpha-, beta-, and gamma-globulins compose the globulins. Alpha-
globulins transport lipids, hormones, and vitamins. Also included is a
glycoprotein, ceruloplasmin, which carries copper and haptoglobulins, which bind
hemoglobin. Iron transport is related to beta-globulins. The glycoprotein that
binds the iron is transferrin (Lehninger, 1993). Gamma-globulins
(immunoglobulins) are associated with antibody formation. There are five
different classes of immunoglobulins. IgG is the major circulating antibody.
It gives immune protection within the body and is small enough to cross the
placenta, giving newborns temporary protection against infection. IgM also
gives protection within the body but is too large to cross the placenta. IgA is
normally found in mucous membranes, saliva, and milk. It provides external
protection. IgD is thought to function during the development and maturation of
the immune response. IgE makes of the smallest fraction of the immunoglobulins.
It is responsible for allergic and hypersensitivity reactions.
Altered levels of alpha- and beta- globulins are rare, but immunoglobulin
levels change in various conditions. Serum immunoglobulin levels can increase
with viral or bacterial infection, parasitism, lymphosarcoma, and liver disease.
Levels are decreased in immunodeficiency.
Albumin is a serum protein that affects osmotic pressure, binds many
drugs, and transports fatty acids. Albumin is produced in the liver and is the
most prevalent serum protein, making up 40 to 60 percent of the total protein.
Serum albumin levels are decreased (hypoalbuminemia) by starvation, parasitism,
chronic liver disease, and acute glomerulonephritis (Sodikoff, 1995). Albumin
is a weak acid and hypoalbuminemia will tend to cause nonrespiratory alkalosis
(de Morais, 1995). Serum albumin levels are often elevated in shock or severe
Creatine Kinase (CK) is an enzyme that is most abundant in skeletal
muscle, heart muscle, and nervous tissue. CK splits creatine phosphate in the
presence of adenosine diphosphate (ADP) to yield creatine and adenosine
triphosphate (ATP). During periods of active muscular contraction and
glycolysis, this reaction proceeds predominantly in the direction of ATP
synthesis. During recovery from exertion, CK is used to resynthesize creatine
phosphate from creatine at the expense of ATP.After a heart attack, CK is the
first enzyme to appear in the blood (Lehninger, 1993). CK values become
elevated from muscle damage (from trauma), infarction, muscular dystrophies, or
inflammation. Elevated CK values can also be seen following intramuscular
injections of irritating substances. Muscle diseases may be associated with
direct damage to muscle fibers or neurogenic diseases that result in secondary
damage to muscle fibers. Greatly increased CK values are usually associated
with heart muscle disease because of the large number of mitochondria in heart
muscle cells (Bistner, 1995).
When active muscle tissue cannot be supplied with sufficient oxygen, it
becomes anaerobic and produces pyruvate from glucose by glycolysis. Lactate
dehydrogenase (LDH) catalyzes the regeneration of NAD+ from NADH so glycolysis
can continue. The lactate produced is released into the blood. Heart tissue is
aerobic and uses lactate as a fuel, converting it to pyruvate via LDH and using
the pyruvate to fuel the citric acid cycle to obtain energy (Lehninger, 1993).
Because of the ubiquitous origins of LDH, the total serum level is not reliable
for diagnosis; but in normal serum, there are five isoenzymes of LDH which give
more specific information. These isoenzymes can help differentiate between
increases in LDH due to liver, muscle, kidney, or heart damage or hemolysis
Calcium is involved in many processes of the body, including
neuromuscular excitability, muscle contraction, enzyme activity, hormone
release, and blood coagulation. Calcium is also an important ion in that it
affects the permeability of the nerve cell membrane to sodium. Without
sufficient calcium, muscle spasms can occur due to erratic, spontaneous nervous
The majority of the calcium in the body is found in bone as phosphate
and carbonate. In blood, calcium is available in two forms. The nondiffusible
form is bound to protein (mainly albumin) and makes up about 45 percent of the
measurable calcium. This bound form is inactive. The ionized forms of calcium
are biologically active.If the circulating level falls, the bones are used as
a source of calcium.
Primary control of blood calcium is dependent on parathyroid hormone,
calcitonin, and the presence of vitamin D. Parathyroid hormone maintains blood
calcium level by increasing its absorption in the intestines from food and
reducing its excretion by the kidneys. Parathyroid hormone also stimulates the
release of calcium into the blood stream from the bones. Hyperparathyroidism,
caused by tumors of the parathyroid, causes the bones to lose too much calcium
and become soft and fragile. Calcitonin produces a hypocalcemic effect by
inhibiting the effect of parathyroid hormone and preventing calcium from leaving
bones. Vitamin D stimulates calcium and phosphate absorption in the small
intestine and increases calcium and phosphate utilization from bone.
Hypercalcemia may be caused by abnormal calcium/phosphorus ratio,
hyperparathyroidism, hypervitaminosis D, and hyperproteinemia. Hypocalcemia may
be caused by hypoproteinemia, renal failure, or pancreatitis (Bistner, 1995).
Because approximately 98 percent of the total body potassium is found at
the intracellular level, potassium is the major intracellular cation. This
cation is filtered by the glomeruli in the kidneys and nearly completely
reabsorbed by the proximal tubules. It is then excreted by the distal tubules.
There is no renal threshold for potassium and it continues to be excreted in the
urine even in low potassium states. Therefore, the body has no mechanism to
prevent excessive loss of potassium (Schmidt-Nielsen, 1995).
Potassium plays a critical role in maintaining the normal cellular and
muscular function. Any imbalance of the body’s potassium level, increased or
decreased, may result in neuromuscular dysfunction, especially in the heart
muscle. Serious, and sometimes fatal, arrythmias may develop. A low serum
potassium level, hypokalemia, occurs with major fluid loss in gastrointestinal
disorders (i.e., vomiting, diarrhea), renal disease, diuretic therapy, diabetes
mellitus, or mineralocorticoid dysfunction (i.e., Cushing’s disease). An
increased serum potassium level, hyperkalemia, occurs most often in urinary
obstruction, anuria, or acute renal disease (Bistner, 1995).
Sodium and its related anions (i.e., chloride and bicarbonate) are
primarily responsible for the osmotic attraction and retention of water in the
extracellular fluid compartments. The endothelial membrane is freely permeable
to these small electrolytes. Sodium is the most abundant extracellular cation,
however, very little is present intracellularly. The main functions of sodium
in the body include maintenance of membrane potentials and initiation of action
potentials in excitable membranes. The sodium concentration also largely
determines the extracellular osmolarity and volume. The differential
concentration of sodium is the principal force for the movement of water across
cellular membranes. In addition, sodium is involved in the absorption of
glucose and some amino acids from the gastrointestinal tract (Lehninger, 1993).
Sodium is ingested with food and water, and is lost from the body in urine,
feces, and sweat. Most sodium secreted into the GI tract is reabsorbed. The
excretion of sodium is regulated by the renin-angiotensin-aldosterone system
Decreased serum sodium levels, hyponatremia, can be seen in adrenal
insufficiency, inadequate sodium intake, renal insufficiency, vomiting or
diarrhea, and uncontrolled diabetes mellitus. Hypernatremia may occur in
dehydration, water deficit, hyperadrenocorticism, and central nervous system
trauma or disease (Bistner, 1995).
Chloride is the major extracellular anion. Chloride and bicarbonate
ions are important in the maintenance of acid-base balance. When chloride in
the form of hydrochloric acid or ammonium chloride is lost, alkalosis follows;
when chloride is retained or ingested, acidosis follows. Elevated serum
chloride levels, hyperchloremia, can be seen in renal disease, dehydration,
overtreatment with saline solution, and carbon dioxide deficit (as occurs from
hyperventilation). Decreased serum chloride levels, hypochloremia, can be seen
in diarrhea and vomiting, renal disease, overtreatment with certain diuretics,
diabetic acidosis, hypoventilation (as occurs in pneumonia or emphysema), and
adrenal insufficiency (de Morais, 1995).
As seen above, one to two milliliters of blood can give a clinician a
great insight to the way an animals’ systems are functioning. With many more
tests available and being developed every day, diagnosis becomes less invasive
to the patient. The more information that is made available to the doctor
allows a faster diagnosis and recovery for the patient.
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Therapy XII. Ed. John Bonagura. Philadelphia: W. B. Saunders, 1995.
Bistner, Stephen l. Kirk and Bistners Handbook of Veterinary
Procedures and Emergency Treatment. Philadelphia: W. B.
de Morais, HSA and William W. Muir. Strong Ions and Acid-Base
Disorders. Current Veterinary Therapy XII. Ed. John
Bonagura. Philadelphia: W. B. Saunders, 1995.
Fraser, Clarence M., ed. The Merck Veterinary Manual, Seventh
Edition. Rahway, N. J.: Merck ; Co., 1991.
Garrett, Reginald H. and Charles Grisham. Biochemistry. Fort
Worth: Saunders College Publishing, 1995.
Lehninger, Albert, David Nelson and Michael Cox. Principles of
Biochemistry. New York: Worth Publishers, 1993.
Schmidt-Nielsen, Knut. Animal Physiology: Adaptation and
environment. New York: Cambridge University Press, 1995.
Sodikoff, Charles. Labratory Profiles of Small Animal Diseases.
Santa Barbara: American Veterinary Publications, 1995.