Introduction

Clinical signs of liver disease are mostly non-specific and can be gastrointestinal (vomiting, diarrhoea, discoloured faeces), haematological (pallor, jaundice), renal (polyuria, polydipsia) and neurological (behaviour change, ataxia, seizure). Findings from physical examinations differ widely and may include hepatomegaly or microhepatia, abdominal fluid accumulation and/or pain, weight loss and fever. Hepatobiliary and vascular disorders can originate primarily from the liver but can also be secondary to multiple extrahepatic disorders such as gastrointestinal, haematological and circulatory diseases, endocrinopathies, neoplasm and systemic infections (including sepsis).

In searching for primary or secondary liver disorders, screening laboratory parameters include serum activities of hepatobiliary enzymes and concentrations of liver metabolites. Urinalysis looks for bilirubinuria and urate crystals, which are associated with parenchymal and vascular hepatopathies. Laboratory results from screening tests influence decisions about the need for liver function testing and invasive diagnostic or therapeutic procedures such as liver biopsy, abdominal surgery or endoscopic retrograde cholangiography. Laboratory tests aim to rule out extrahepatic disorders and assess disease severity. They also complement imaging procedures (radiography, ultrasonography) used to reveal primary and secondary disorders of the hepatic parenchyma, the biliary tract system, or the liver vessels. Liver parameters can roughly be grouped into cholestatic and parenchymal parameters.

Liver Parameters Associated with Cholestasis

Serum parameters of cholestasis are alkaline phosphatase (ALP), gamma-glutamyltransferase (GGT), bilirubin and cholesterol.

Serum ALP and GGT

Serum ALP and GGT activities rise in hepatobiliary disorders due to the membrane localisation of both enzymes. Highest serum activities of ALP are measured with cholestasis, followed by chronic hepatitis, corticoid-induced hepatopathy and hepatic necrosis. However, due to the existence of liver-, glucocorticoid- and bone-associated isoenzymes, not every ALP elevation is caused by a hepatobiliary disorder. High serum ALP activities can also be found in healthy Scottish Terriers, hypothyroidism, neoplasia and gastrointestinal diseases such as pancreatitis and chronic enteropathies. When detecting increases in ALP it is important to look for such extrahepatic causes and to assess the likelihood of an increase due to glucocorticoid application, hyperadrenocorticism or bone diseases before starting a more intensive search for a primary hepatobiliary (cholestatic) disease.

Serum Bilirubin

Serum bilirubin originates in its non-water-soluble form from the metabolism of haem by the reticuloendothelial cell system. In the liver, it needs to be filtered out of the plasma and conjugated to glucuronic acid for final excretion of its water-soluble form into the bile. Prehepatic, hepatic and posthepatic disturbance in the processing of bilirubin can lead to hyperbilirubinaemia, bilirubinuria and icterus. In the case of hyperbilirubinaemia, prehepatic icterus due to haemolytic anaemia can be easily differentiated by measuring a markedly reduced haematocrit, whereas hepatobiliary diseases cause only a very mild decrease of the haematocrit. Intrahepatic icterus occurs in primary hepatic disorders or in sepsis with intrahepatic cholestasis leading to impaired bilirubin processing by the liver. Extrahepatic icterus is caused by extrahepatic biliary tract obstructions. For differentiation of intra- and extrahepatic icterus, abdominal ultrasonography is indicated to examine the liver parenchyma, hepatobiliary system and pancreas.

Cholesterol

Cholesterol is mainly extracted from plasma and partly synthesised in the liver. The liver can esterify, store and secrete cholesterol. Free cholesterol is excreted with bile. Therefore, increased hepatic synthesis or extrahepatic biliary bile duct obstruction can lead to hypercholesterolaemia.

The combination of increases in ALP, hyperbilirubinaemia and hypercholesterolaemia is a good indicator for intra- or extrahepatic cholestasis indicating the need for a more intensive diagnostic work-up of primary hepatobiliary diseases including abdominal ultrasonography, liver biopsy or exploratory laparotomy.

Liver Parameters Associated with Parenchymal Disorders

Serum enzyme parameters of parenchymal disorders are alanine aminotransferase (ALT) and aspartate aminotransferase (AST). Parenchymal disorders also affect liver metabolism and can therefore change serum concentrations of its products such as plasma proteins, ammonia, urea, glucose, cholesterol and bile acids.

ALT and AST

ALT and AST are cytosolic enzymes and leak into serum with hepatocellular necrosis and inflammation. ALT is the more liver-specific enzyme and the magnitude of its serum activity is roughly proportional to the amount of leaking hepatocytes. In contrast, AST is also produced in muscles and therefore a less specific liver parameter than ALT despite its high sensitivity for parenchymal liver diseases.

When assessing liver enzyme activity in serum, it needs to be taken into consideration that their increase parallels the severity of hepatocyte damage to only some degree. The level of increase does not allow conclusions about the functional capacity of the liver nor on the prognosis of liver damage. In dogs with end-stage chronic liver disease the enzyme activities can be only slightly increased or even normal despite severe liver functional disorders, due to a decreased absolute amount of hepatocytes in a cirrhotic or fibrotic liver. Repeated enzyme determinations allow a better prognostic assessment than single measurements. In an acute insult, a decrease in sequential ALT measurements of 50% within 2.5 days (serum half-life of ALT) indicates a good prognosis. In contrast, repeated measurements of serum ALT activities which are relatively higher than ALP when compared to the reference ranges of both enzymes are highly indicative of a chronic parenchymal disease, especially when the increase can be documented for about 3–4 months. Such cases require liver biopsies for the pathohistological differentiation of parenchymal liver diseases.

Plasma Proteins

Plasma proteins of diagnostic importance are albumin and coagulation proteins. Hypoalbuminaemia is mainly caused by chronic parenchymal disorders such as cirrhosis but also by portosystemic vascular anomalies (PSVAs). Since the differential diagnosis of low serum albumin is quite wide, the parameter is not diagnostic for a liver disease but is of importance for its severity assessment and prognosis. Altered haemostasis such as disseminated intravascular coagulation is common in dogs with hepatopathies and can complicate invasive diagnostic methods such as liver biopsy or exploratory surgery. The clinical evidence of bleeding, or the planning of a liver biopsy, indicates the use of coagulation tests such as prothrombin time, activated partial thromboplastin time, fibrinogen and D-dimers.

Ammonia

Ammonia originates from the gastrointestinal tract, where it is produced by bacterial degradation of amino acids and purines, effects of bacterial urease and catabolism of glutamine. It is detoxified in the liver by conversation to urea or synthesis of glutamine. In dogs, ammonia is increased in hepatic failure, PSVA, urea-cycle enzyme deficiency and decreased availability of urea-cycle substrates such as argininosuccinate synthetase or vitamin B12. Hyperammonaemia is one major factor for the development of hepatoencephalopathy indicating the importance of its determination in patients with suspected parenchymal or vascular liver disease and neurological signs. Ammonia needs to be measured from EDTA plasma within 1 hour after sampling.

Serum Glucose

Serum glucose homeostasis is maintained by the liver. Hypoglycaemia can be associated with fulminant acute hepatic failure, PSVA in small dogs, and paraneoplastic syndrome due to hepatic neoplasia. It is not diagnostic for liver diseases but is an important indicator of disease severity and of the need for an intensified diagnostic work-up.

Serum Cholesterol

Serum cholesterol can be decreased in parenchymal and vascular liver diseases. Hypocholesterolaemia is reported in dogs with late-stage chronic liver disease and PSVA. Its determination has more importance for the assessment of severity than for the diagnosis.

Bile Acids

Bile acids are produced in the liver only and undergo a very efficient enterohepatic circulation. In parenchymal and vascular liver disorders, the hepatic clearance of bile acids out of the serum is often insufficient, which leads to an accumulation of bile acids in the blood. Since the abnormal clearance cannot always be found in single serum samples, a bile acid stimulation test is used to start the enterohepatic circulation by stimulation of the gallbladder contraction with intraduodenal bile secretion using food, or the synthetic cholecystokinin analogue ceruletide. Increased bile acids before and/or after stimulation are seen as a sign of decreased hepatic clearance. Increased serum bile acid concentrations have to be assessed in connection with clinical signs, other laboratory test results and findings of diagnostic imaging, since the concentration of the bile acids in the serum does not allow the differentiation of parenchymal from vascular liver diseases. There is also no correlation between serum bile acid concentration and histological severity of a parenchymal disorder or the degree of a PSVA.

References

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