EQUINE | CPD Article
Serum protein electrophoresis:
This identifies elevations in specific globulin fractions
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Alpha 2 globulin - acute-phase inflammatory
protein responses.
Beta 1 globulin - Strongylus vulgarisand mixed
strongyle larval activity.
Beta 2 globulin – hepatopathy*.
Gamma globulin - antibody responses to bacterial
or viral infections.
*Serum samples should be used for protein
electrophoresis, as raised fibrinogen levels in
heparinised plasma samples will cause confusing rises
in beta 2 globulins.
Aspartate Aminotransferase (AST)
Elevations of AST are documented with both acute
and chronic hepatopathy. Although AST is mostly
cytosolic, significant concentrations are also found in
mitochondria. However, more severe cell injury or cell
death is required for the release of mitochondrial AST.
AST is found in many tissues including liver, striated
muscle, intestine, kidney and erythrocytes. There is
currently no method available to differentiate between
the various AST tissue sources. Therefore, AST has low
specificity for liver disease, although in the majority of
liver disease cases it will be increased. The half-life has
been reported at 7 to 8 days in horses.
Increased serum AST activity is observed with both
reversible and irreversible injury to hepatocytes and can
be seen following necrosis, ischemia, enzyme induction
(phenobarbitone,
corticosteroids),
drug-induced
hepatotoxicity, cholestasis or trauma.
Likewise, serum AST is increased following myocyte
injury. In either case, the definitive disease process
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cannot be identified, only that cellular injury in muscle
or liver has occurred. Because serum AST activity cannot
differentiate between hepatocellular or myocyte injury,
further testing using organ-specific enzymes such
as SDH or creatine kinase (CK) is indicated. Markedly
increased serum AST and SDH suggest acute or active
hepatocellular injury, and markedly increased serum
AST with modest to moderate SDH activity suggests
chronic hepatic injury or recovery from acute liver injury.
An increased serum AST together with CK is a clear
indication of muscle damage. The sensitivity of serum
AST activity in horses has been reported as 72% for
hepatic necrosis and 100% for hepatic lipidosis. For a
correct diagnosis, determination of AST activity should
be complemented with other hepatic indicators, such
as SDH, total bile acids and GGT among others.
Careful sample collection and handling is important
as hemolysis leads to increased AST activity, while
improper handling leading to enzyme degradation
results in artefactual reduction in AST levels. Serum
samples can be stored for up to 24 hours in a refrigerator
without appreciable loss of AST activity. The AST activity
in frozen serum samples is also fairly stable, as long as
the serum is not frozen and thawed repeatedly.
Sorbitol Dehydrogenase (SDH)*
This is an enzyme found almost exclusively in the
cytoplasm of hepatocytes and considered liver-specific,
although in rare instances elevations can be seen
in horses with skin conditions and enteropathy. It is
considered a good indicator of active hepatocellular
disease plus moderate to severe cholestasis.
Unfortunately, SDH is highly labile and activity declines
rapidly after blood collection. Analysis needs to be
performed within 4 to 6 hours post collection and so
this limits the usefulness of SDH as a diagnostic test.
• Equine Health Update •