| Indications for performing this test: This is a
hepatic function test and should be run in any case of suspected heptatic disease or
hemolysis. Function tests depend on the ability of the liver to uptake, conjugate,
and excrete compounds. |
 |
http://web.vet.cornell.edu/public/
popmed/clinpath/cpmodules
Notice the tube with
icteric serum.
What could that be due to?
|
|
Contraindications for running this
test: In a case that presents for icterus, there is
usually no need to run a
bilirubin test. Icterus is already ample clinical evidence that bilirubin is elevated
beyond the normal range. |
|
|
| Pathophysiology: Bilirubin is a bile pigment that originates from the
hemoglobin of the senescent red blood cells, as well as non-heme porphyrins. It is
the job of either the macrophages or the hepatocytes to convert the heme to bilirubin
through an enzymatic pathway. It is
bilirubin that results in the yellow discoloration of the tissues know as icterus or
jaundice. |
 |
|
Bilirubin may be found in the blood either conjugated with glucuronic
acid or unconjugated and bound to albumin. Conjugation makes bilirubin soluble and
allows for excretion into the bile canaliculi. Once excreted into the intestine, it
is converted to urobilogen and excreted with the feces. A small amount of the
conjugated bilirubin is hydrolyzed back into the conjugated form and reabsorbed into the
blood. |
The Test: Assays for bilirubin
may be done for conjugated (known as direct bilirubin) or total bilirubin, which considers
both conjugated and unconjugated bilirubin. The assay uses a dye as the reagent,
known as the diazo dye. When added to the sample, the diazo dye interacts directly
with the conjugated bilirubin to create a color change. Then alcohol is
added, producing another color change indicating the presence of both the conjugated and
unconjugated bilirubin. This result is used for the measurement of total bilirubin. The amount of
unconjugated bilirubin is calculated indirectly by subtracting the direct bilirubin
value from
the total bilirubin. Occasionally, measurements for direct bilirubin will also
include some amount of unconjugated bilirubin making this assay unreliable and
variable. Therefore, elevated direct bilirubin should be considered insignificant if
total bilirubin is not also elevated beyond the normal range.
Interpretation of Hyperbilirubinemia:
- Pre-hepatic icterus: Accelerated destruction of red blood cells
will lead to an initial rise in unconjugated bilirubin due to the inability of the liver
to keep up with uptake. This is closely followed by an increase in the
conjugated bilirubin as the liver becomes more hypoxic and unable to secrete bile.
The rise in conjugated bilirubin may quickly exceed that of unconjugated bilirubin.
Therefore, a depressed PCV or CBC is a much more reliable indicator of
hemolysis.
|
RBCs experiencing
intravascular hemolysis.
|
- Intra-hepatic icterus:
Primary hepatobiliary disease
interfering with the bilirubin cycle.
|
Bilirubin
(bile) casts in hepatic canaliculi in a
horse with chronic liver
disease.
|
- Extra-hepatic icterus: Extrahepatic obstruction of bile flow.
|
Cholesterol
stones can be found blocking a bile duct. |
Important Species Differences:
- Hyperbilirubinemia in the horse is due primarily to
unconjugated bilirubin and often occurs secondary to periods of fasting or anorexia.
- In sheep and cattle, most major increases are due to
hemolytic disease.
- In dogs, a low renal threshold for conjugated bilirubin will
often lead to bilirubinuria before the onset of hyperbilirubinemia.
For more information . . .
- Veterinary Laboratory Medicine, Duncan, Prasse, and Mahaffey
page 137-140.
- Veterinary Laboratory Medicine, Interpretation & Diagnosis.
Meyer & Harvey, pages 172-177.
