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Today's Ve T erinary Prac T ice | s eptember/ o ctober 2016 | tvpjournal.com TH e ye LL o W ca T: dia G nos T ic & TH era P e UT ic s T ra T e G ies Peer r eviewed 38 There is no mystery when it comes to a "yellow" cat. Icterus and jaundice—both of which describe a yellowish pigmentation of the skin—indicate hyperbilirubinemia, a 5- to 10-fold elevation in serum bilirubin concentration. However, this is where the certainty ends and the diagnostic challenge begins. The icteric cat presentation is not a sensitive or specific marker of disease, despite the visually obvious and impressive clinical sign (Figure 1). 1 The objective of this article is to briefly review differentials for hyperbilirubinemia in the cat, and present a diagnostic and therapeutic strategy that will help practitioners approach this problem in an efficient and effective manner. HYPERBILIRUBINEMIA: ORGANIZATION BY LOCATION The differentials for hyperbilirubinemia should be organized by location: prehepatic, hepatic, and posthepatic. While, in cats, it is common to find concurrent disease processes, starting from this foundation is the first step toward an effective and efficient diagnostic workup of icteric cats. Prehepatic Disease Hemolysis releases hemoglobin, which is then metabolized through biliverdin to bilirubin in the liver. Hepatocytes in the healthy feline liver have a large capacity for uptake, conjugation, and excretion of bilirubin. Prehepatic icterus is most likely due to the combination of a large increase in bilirubin from hemolysis and a degree of intrahepatic cholestasis secondary to hypoxia. 2 In dogs, the most common cause of prehepatic hyperbilirubinemia is immune- mediated hemolytic anemia (IMHA), but that condition appears to be quite rare in cats. The list of prehepatic causes of hyperbilirubinemia in cats is extensive (Table 1, page 40). Hepatic Disease A significant decrease, or loss, of hepatocellular function effects bilirubin metabolism, and frequently results in intrahepatic cholestasis (Table 1, page 40). Unconjugated bilirubin from damaged hepatocytes is present, although the majority of bilirubin that appears in the cat's circulation is conjugated, having completed the metabolic step prior to encountering the cholestatic overflow into the vasculature. 2 Posthepatic Disease Extrahepatic biliary disease interferes with the normal flow of bile and the final steps in bilirubin excretion, resulting in extrahepatic cholestasis (Table 1, page 40). As with intrahepatic disease, both unconjugated and conjugated bilirubin appear in the serum, rendering the biochemical distinction between conjugated and unconjugated serum bilirubin of minimal diagnostic importance. The Yellow Cat: Diagnostic & Therapeutic Strategies Craig B. Webb, PhD, DVM, Diplomate ACVIM (Small Animal Internal Medicine) Colorado State University CE Ar TIC l E Figure 1. i cteric pinna of a cat in the critical care isolation unit; prehepatic hemolysis and anemia are a result of Cytauxzoon felis infection. Hyperbilirubinemia results when serum bilirubin concentrations reach 2 to 3 mg/dL (35–50 mcmol/L).