Bilirubin
Definition
Bilirubin is a breakdown product of heme, derived primarily from senescent erythrocytes. It is carried by albumin to the liver, where it is detoxified by the glucuronic acid pathway, conjugated, and excreted into the bile. Biliverdin produced in reptiles and birds also undergoes this detoxification in the liver and is excreted into the bile.
Physiology
Three forms of bilirubin have been identified: conjugated, unconjugated, and a fraction that is irreversibly bound to protein. Unconjugated bilirubin is the most clinically significant fraction because it is the form that is most likely to cause tissue damage.
Typical Normal Range
The typical normal range for this laboratory test varies greatly among species. The reader is referred to the following Elsevier publications for additional information:
Clinical Applications
Next Diagnostic Steps to Consider If Levels Are High
Complete blood count, chemistry panel, urinalysis, bile acids, radiographs ± abdominal ultrasound
Important Interspecies Differences
• Birds have an enzyme called heme oxygenase, which converts the protoporphyrin in heme to biliverdin. Both birds and reptiles have decreased hepatic production of biliverdin reductase, which converts biliverdin to bilirubin. Although decreased, biliverdin reductase is still present in some birds. Bacteria in the intestine may produce biliverdin reductase as well, and bilirubin can be absorbed from the gastrointestinal tract.
• Bilirubin is found only in small quantities in the avian and reptilian plasma, making it a relatively useless clinical parameter in these species.
• Rabbits also produce biliverdin as the primary heme metabolite, but in contrast to birds and reptiles, bilirubin is present in the blood of rabbits at measurable levels.
• Although the rabbit produces significantly more bile than a dog of equal size, rabbits have low activity of biliverdin reductase, and only about 30% is converted.