L91a Laboratory Diagnosis of Hepatobiliary Disease - Specific Liver Enzymes

Alanine Aminotransferase (ALT): Fairly liver specific only in

cats, dogs, rabbits, rats and primates

ALT also occurs with muscle damage

Skeletal muscle - 5% of liver ALT activity

Cardiac muscle -25% of liver ALT activity

Cause of mild to moderate ALT elevation ($\le 5\times$): Endocrine/metabolic condition

Diabetes mellitus

Cause of mild to moderate ALT elevation ($\le 5\times$): Feline hyperthyroidism mechanism

High circulating thyroid hormone levels exert direct hepatotoxic effects on liver tissue.

Cause of mild to moderate ALT elevation ($\le 5\times$): Age-related factor

Aged animals

Cause of mild to moderate ALT elevation ($\le 5\times$): Cardiac and GI secondary mechanisms

Secondary to heart disease (due to passive hepatic congestion) and gastrointestinal disease (released by the liver in association with GI disease).

Cause of mild to moderate ALT elevation ($\le 5\times$): Pharmacologic agents and mechanisms

Anticonvulsants (phenobarbital can induce liver disease), corticosteroids (dogs only), and NSAIDs (attributed to increased hepatocellular membrane permeability).

Marked Alanine Aminotransferase(ALT) (>10x top of RI) due to

Toxic, hypoxic or traumatic conditions

Acute severe liver injury / necrosis or cell death

Alanine Aminotransferase (ALT) not useful in large animals

ALT present only in very low concentrations in hepatocytes of large animals

• Use GLDH or SDH

Aspartate Aminotransferase (AST): In liver & myocardial / skeletal muscle

Concurrent estimation of CK required to exclude muscle damage

AST has a longer T½ in serum than CK

If no muscle damage then AST increases similar to ALT

Aspartate Aminotransferase (AST)

Induction by anticonvulsants

Generally not increased with corticosteroids

Can increase with exercise by 30%

In early training (muscle component) - Resting levels > 50-100 % than horses

not in training(GGT may be ↑ in race horses)

Glutamate Dehydrogenase(GLDH / GDH / GMD)

Primarily used in large animals as an indicator of acute hepatocellular necrosis instead of ALT (tiny quantities)

Sorbitol Dehydrogenase (SDH)[Iditol Dehydrogenase (ID)]

very liver specific - large animals

Unstable so impractical to use unless in research setting or at teaching hospitals(Eq up to 5 hr at 21 °C)

ALP: Isoforms found in high concentrations in

Liver (L-ALP) – induced by glucocorticoids, bile acids, phenobarb(hepatocyte membranes, biliary epithelium)

Bone (B-ALP) (osteoblasts)

Corticosteroid induced (dogs) (C-ALP) (hepatocytes)

ALP other isoforms in high concentration with short half-lives

Intestinal mucosa, Kidney, Placenta, Some leukocytes

ALP = indicator of cholestasis (intra- or post hepatic)

→ ↓ excretion bile acids (BA) via bile, ↑ BA in hepatocytes & plasma

Induction of L-ALP (↑ total ALP) in dogs and cats

Cholestatic sensitivity varies with species

ALP in dogs

high diagnostic sensitivity; usually ↑ALP before icterus

ALP in horses and cats

very low diagnostic sensitivity; usually icteric before ↑ALP

ALP in cattle

moderate diag. sensitivity (cholestasis uncommon)

• ALP not used much in large animals

Liver ALP in Cats: LALP has short T½ in cats < 8 hrs

Any increase in ALP in a cat is significant → Usually icteric before ↑ALP

↑ ALP cats - usually primary liver disease or hyperthyroidism (liver & bone isoenzyme)

GGT can be used as alternative / adjunctive to ALP in small animals

Liver Alkaline Phosphatase (L-ALP) Increases 12 - 24 hours after use of:

Volatile anesthetic agents, Anticonvulsants (barbiturates)

L-ALP in Scottish terriers (adults)

high ALPs(up to 4-5 x)(normal finding)

± Corresponding abnormal liver histopathology

• Some have vacuolar hepatopathy

Corticosteroid induced Alkaline Phosphatase (CiALP) Dogs only

Endo / exogenous corticosteroids induce hepatic ALP synthesis → Marked elevations in Ci-ALP → Steroid / Vacuolar hepatopathy with hepatocyte swelling → impairs bile flow in caniculi = cholestasis

● Different isoenzyme to that induced by cholestasis (L-ALP)

● Methods of isoenzyme differentiation not routinely used

Vacuolar hepatopathy cytology:

1. Glycogen deposition

2. Hydropic change “feathery” vacuolation & cells appear outlined

Vacuolar hepatopathy histology:

1. Glycogen deposition

2. Hydropic change

3. Lipid accumulation Liver: indistinct holes

Bone Alkaline Phosphatase (B-ALP): Produced by osteoblasts

2-3x increases normal in young growing animals - within adult ranges by ± 15 months - Siberian huskies have transient familial increases at 6 weeks of age

Bone disease – tumour / infection / fractures

B-ALP: hyperT4 cats

Increase in bone and liver isoenzyme

Alkaline Phosphatase (ALP): suckling differences

Up to 30 x increase in post suckling pups, due to colostral ALP

• values decrease within 10 days

Ru, Eq, Pig high colostral ALP

• Not cats

• Narrow window +/- < 36 hrs

ALP: Pancreatitis (& Foreign Bodies)

± increases ALP levels - Swelling / fibrosis around bile duct

Horses: Most likely liver origin (cholestasis) or osteoblast production

Other things to consider with ALP

Chronically ill (stressed) dogs may have steroid induced -ALP isoezymes - secondary to glucocorticoid release

50% of dogs with glycogen-associated vacuolar hepatopathy

• have no exposure to steroids

Check drug history - especially alternative herbal medications

Gamma Glutamyltransferase (GGT)

Increased by similar causes as ALP

Marker of cholestasis / biliary hyperplasia(intrahepatic or post hepatic - impaired bile flow)

GGT – species differences

Dogs: diag. sensitivity similar to ALP; magnitude of ↑ is less

Cats: diag. sensitivity similar to ALP (for all liver diseases except hepatic lipidosis)

Horses & Cattle: good diag. sensitivity (better than ALP)

Gamma Glutamyltransferase (GGT) Also induced by:

Corticosteroids, Anticonvulsants (barbiturates) in dogs

• Not increased by pancreatic or GIT disease

If GGT elevated >> ALP

compared to ALP which is in hepatocellular membranes

along bile canaliculi = biliary tree disease

GGT is a better indicator of?

cholestasis than ALP in large animals

Gamma Glutamyltransferase (GGT) chronic fascioliasis

bile duct damage

• increase not primarily cholestatic

GGT levels sometimes normal in chronically infected animals

GGT and colostrum

GGT is excreted in colostrum of bovines & dogs (not horses)

Can be used as indicator of colostrum intake (passive transfer) in calves < 2 weeks of age

GGT: donkeys

have 2-3 x higher GGT levels than horses

High-Yield Exam Pearls

Dog + high ALP >> ALT = think steroid induction

Cat + high ALP = significant disease (short half-life, poor induction)

AST without CK increase = think liver

AST + CK both high = muscle

Horse liver disease = look at GGT, SDH, GLDH

Hemolyzed sample + high AST = interpret cautiously

Summary: Pathogenesis of Increased Liver Enzyme Activities:

Liver enzyme elevations (ALP, ALT, AST, GGT, LDH, GDH) occur due to hepatocellular damage, biliary stasis, or enzyme induction

ALT and AST increase with hepatocellular injury.

ALP and GGT rise with cholestasis.

Bilirubin, excretory product of heme metabolism, accumulates with impaired hepatic uptake, conjugation, or excretion, indicating hepatobiliary disease.

Pattern analysis of enzyme changes helps distinguish between hepatocellular injury, cholestasis, or secondary hepatic involvement