46
Complementary Tests
Sodium, potassium, and chloride should always be assayed together to determine electrolyte
balance. The additional measurement of bicarbonate will allow accurate assessment of metabolic
acid-base physiology.
Reaction Sequence
Total Bilirubin (TBIL)
Hemoglobin from degenerated erythrocytes is converted to bilirubin in the monocyte-macrophage
system. Free unconjugated bilirubin is transported to the liver bound to albumin, where it is
conjugated with glucuronic acid and eliminated in the bile. In obstructive liver disease, the
concentration of conjugated bilirubin in the blood increases.
During intravascular or extravascular hemolysis, very large numbers of erythrocytes may be
destroyed quickly and the conjugation mechanism in the liver may become overloaded so that
high concentrations of unconjugated bilirubin are found in the blood. If the loss of hemoglobin and
erythrocytes is very large, anoxia may occur. Hepatocyte dysfunction follows leading to cellular
swelling, which occludes the bile canaliculi preventing the elimination of conjugated bilirubin. A
concomitant rise in circulating conjugated bilirubin then occurs.
Principal Reason for Performing the Test
To detect hepatobiliary disease and excessive erythrocyte destruction.
Note:
In healthy dogs and cats, the concentration of total bilirubin in the serum is very low. Visual
inspection of the sample will frequently indicate whether bilirubin determination is necessary (serum
and plasma only).
Most Common Abnormality Indicated by the Test
Increased bilirubin—cholestatic liver disease (conjugated bilirubin) and hepatic insufficiency
(unconjugated bilirubin), hemolytic disease (unconjugated and possible conjugated bilirubin), and
intrahepatic obstruction.
Sample Type and Precautions
Remove plasma or serum promptly from cells or clot. Samples should be analyzed immediately as
bilirubin degrades rapidly in light. If immediate analysis is impossible, the sample must be kept in
the dark and preferably at 4°C–8°C (36°F–40°F) in a refrigerator. Sample must be allowed to come
to room temperature before analysis. If plasma is collected, use only lithium heparinized samples.
It is critical that samples be properly centrifuged. Otherwise, leukocytes and platelets may remain
in suspension, even when red blood cells have been separated. Cellular material on the slide may
cause significant positive error. Also, hemoglobin increases total bilirubin results, so avoid even
moderately hemolyzed samples.
Complementary Tests
Total bilirubin should be determined with other tests of hepatic function or damage. Hematocrit
should also be performed to eliminate or confirm the presence of hemolytic disease. Determination
of urinary urobilinogen and bilirubin may also be useful.
Reaction Sequence
Appendices
