Chapter 20 - protein assays and hepatobiliary function

protein assays

• most plasma proteins are produced by the liver

total plasma protein

• includes fibrinogen

total serum protein

• does not include fibrinogen

what can affect total protein concentrations

• altered hepatic synthesis

• altered protein distribution

• altered protein breakdown or excretion

• dehydration

• overhydration

Albumin

• one of the most important proteins in plasma or serum

• makes up 35-50% of the total plasma in most animals

• synthesized by hepatocytes

• responsible for maintaining osmotic pressure of plasma

what can affect albumin levels

• renal disease, dietary intake, and intestinal protein absorption may also influence levels

globulins

• liver proteins

• direct chemical measurement - rarely done

• concentrations estimated by difference between total proteins and albumin concentrations

Albumin globulin ratio basics

• an alteration in the globulin/albumin ratio is often the first indication of a protein abnormality

• most pathologic conditions alter the ratio but if concentrations are reduced in equal amounts- no alternation of the ratio

what does increased or decreased albumin/globulin ratio mean

• albumin greater than globulins = A/G greater than 1.0

• albumin is equal to or lower than globulins = A/G less than 1.0

Bilirubin

• major component of bile

• measurements can help pinpoint cause of jaundice

total bilirubin

• liver damage

conjugated bilirubin

• bile duct obstruction

unconjugated bilirubin

• erythrocyte destruction or transport issue

  • can be caused by IMHA

Bile acids

• aid in fat absorption

• stored in the gallbladder until contraction associated with feeding (Except in horses)

• 90-95% actively reabsorbed in the ileum (the rest excreted as feces)

Cholesterol

• plasma lipoproetine produced primarily by the liver but also ingested in food

• not well characterized in veterinary species sometimes used to screen for hypothyroidism

• can be increased by cholestasis in some species

Cholestasis

• a condition where bile production from the liver slows or stops completely

Enzymes released from damaged hepatocytes

• As hepatocytes are damaged, enzymes leak into the blood

  • ALT

  • AST

  • ID and GLDH (not seen on normal blood tests)

Alanine transaminase (ALT)

• indicative of liver damage

• considered a liver specific test in dogs, cats, and primates

• screening test as it is not specific enough to identify specific liver disease

reasons for ALT elevation

• can be elevated by damage to renal cells, cardiac muscles, skeletal muscles and the pancreas

Aspartate transaminase (AST)

• present in hepatocytes more severe liver damage required to release the membrane-bound AST

• Levels raise slower than ALT but return to normal faster

• found in other tissues

  • RBCs, cardiac muscles, skeletal muscles, kidneys, and pancreas

common causes for AST elevation

• liver disease, muscle inflammation or necrosis, hemolysis

iditol dehydrogenase (sorbitol dehydrogenase)

• primary source is the hepatocytes

  • Also found in kidney, small intestine, skeletal muscles, and RBCs

• especially useful in evaluating liver disease in large animals

  • Large animals do not have diagnostic levels of ALT, so ID is the diagnostic test

• not a readily available veterinary test

glutamate dehydrogenase

• mitochondrial bound enzyme

  • high concentrations in cattle, sheep, and goats

• no standardized test method for the veterinary practice lab

enzymes associated with cholestasis

• gamma glutenmyltranferase (GT) (more specific to bile acids)

• Alkaline phosphate (AP)

glucose

• hypo or hyperglycemia

• liver maintains glucose

  • Consider liver disease, as the liver can no longer store glycogen