The most frequent laboratory abnormalities in patients with congenital portosystemic shunts (CPSS)are microcytosis, hypoglycemia, hypoalbuminemia, hypoproteinemia, increased liver enzyme activities,decreased blood urea nitrogen (BUN) and urate crystalluria. The microcytosis is probably due to impairediron transport. Hypoglycemia, hypoalbuminemia, hypoproteinemia and decreased BUN are partly dueto decreased hepatic synthesis. Prolonged coagulation times are also common. A presumptive diagnosisof CPSS is based on the combination of the history, clinical signs, clinicopathological abnormalities andabnormal liver function tests such as serum bile acid and venous ammonia concentrations. Nowadays,determination of pre- and postprandial serum bile acids is the most commonly performed liver functiontest because of the high sensitivity and greater convenience compared to plasma ammonia concentration.Confirmation of the diagnosis is only possible by visualizing the shunting vessel by medical imaging techniquesor during surgery.