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35 MAY/JUNE 2017 ■ TVPJOURNAL.COM CONTINUING EDUCATION should not be used to screen patients for hepatobiliary disease. However, the sensitivity of SBA measurement for portosystemic shunts (congenital and acquired) is high and in one study was reported to be 93% and 100% for dogs and cats with congenital portosystemic shunt, respectively. 11 Ammonia Blood ammonia concentration can be increased because of portosystemic shunting, severe hepatic insufficiency, or urea cycle enzyme deficiencies ( Box 4 ). 12 Potential indications for plasma ammonia measurement include: • Suspicion for portosystemic shunting (eg, seizures, other signs of encephalopathy) • Suspicion for urea cycle enzyme deficiency (eg, cat with feline hepatic lipidosis) • Unexplained ammonium urate urolithiasis Ammonia is mainly produced by catabolism of glutamine by enterocytes and bacterial degradation of urea and proteins in the large bowel. Therefore, blood coming from the splanchnic circulation is rich in ammonia. 13 The liver detoxifies ammonia through two pathways: (1) the urea cycle, which converts ammonia into urea and (2) consumption of ammonia during glutamine synthesis by hepatocytes. In animals with portosystemic shunting or severe hepatic dysfunction, the liver is unable to synthesize sufficient glutamine or urea, leading to hyperammonemia. Because ammonia freely passes across membranes, including the blood–brain barrier, hyperammonemia contributes to the development of clinical signs of hepatic encephalopathy. Fasting Ammonia Measurement Ammonia testing requires heparinized tubes, transfer of the sample on ice, and urgent separation of plasma and is ideally performed within 30 minutes of sample collection. These requirements can make this diagnostic assay difficult to perform in private practice. Increased serum ammonia is a sensitive marker for congenital and acquired portosystemic shunts, with a reported sensitivity of 83% to 98%. 11,14 However, in the absence of portosystemic shunting, ammonia is not a sensitive test of liver disease. Ammonia Tolerance Test When ammonia is administered orally or rectally to a normal dog, it should be efficiently extracted from the portal circulation by the liver. However, dogs with a portosystemic shunt or decreased BOX 3 Bile Acid Challenge Test Procedure 1. Withhold food for 12 hours and collect a blood sample in serum tube (preprandial sample). 2. Feed a meal containing protein and fat (canned food) to encourage gallbladder contraction. 3. Collect a blood sample 2 hours after the meal (postprandial sample) Interpretation of Results • Fasting bile acids should be <5 mcmol/L in normal dogs and cats. • Fasting bile acid concentrations >20 mcmol/L and postprandial bile acids >25 mcmol/L are considered specific for hepatobiliary disease. Preprandial values of 5 to 20 mcmol/L are considered equivocal. • Occasionally, the fasting SBA concentration may be higher than the postprandial concentration because of early gallbladder contraction, alterations in gastrointestinal transit, or decreased response to cholecystokinin. BOX 4 Causes of Hyperammonemia • Portosystemic shunts (congenital or acquired) • Diffuse hepatocellular disease • Urea cycle enzyme deficiencies (congenital or acquired) • Urinary infection with urease-containing bacteria and concurrent urethral obstruction • Renal failure • Ammonium chloride administration (per os or per rectum)