Icterus (Jaundice) is a yellowish discoloration of the skin, the conjunctival membranes over the sclera (whites of the eyes), and other mucous membranes caused by hyperbilirubinemia (bilirubin ≥ 1.5 mg/dL).
Bilirubin is the major biliary pigment and is derived from the breakdown of haemoglobin. Consequently, jaundice may result from increased breakdown of red blood cells (prehepatic jaundice), dysfunction of hepatic biliary metabolism (hepatic jaundice) or reduced excretion of bile from the liver into the gastrointestinal tract (post-hepatic jaundice). Prehepatic jaundice is easily diagnosed by documenting marked anaemia coincident with hyperbilirubinemia. Obviously, prehepatic jaundice requires investigation as to the cause of the anaemia.
Bilirubin is formed in cells of the mononuclear phagocytic system and is unconjugated. It is released into the circulation and bound to albumin, limiting its glomerular filtration. The liver removes bilirubin from the plasma and is stored within hepatocytes until conjugated with glucuronic acid and is released back into circulation. The kidney has a high threshold for conjugated bilirubin in cats (9 times that of dogs) and hence bilirubinuria always occurs with hyperbilirubinemia in this species. Conjugated bilirubin is then excreted into the bile by an energy-dependent saturable process - a number of diseases can interfere with this process directly, including septicaemia and pancreatitis. Once in the intestinal tract, bilirubin is deconjugated and excreted into the faeces. Assays differentiating conjugated from unconjugated bilirubin are inaccurate and many hepatopathies result in elevations in both, therefore differentiation is of little clinical use.