TESTS FOR EVALUATION OF LIVER DISEASE

30 TESTS FOR EVALUATION OF LIVER DISEASE



1 What are the major categories of enzymes used to evaluate liver disease?


There are two major categories of liver enzymes: leakage enzymes and cholestatic enzymes. Leakage enzymes are enzymes that leak into the plasma when hepatocyte injury or death occurs. Therefore, high activities in serum are an indication of hepatocellular injury. Commonly measured leakage enzymes include the following:


• Alanine aminotransferase (ALT; also alanine transaminase)

• Aspartate aminotransferase (AST; also aspartate transaminase)

• Sorbitol dehydrogenase (SDH)

• Lactate dehydrogenase (LDH)

Cholestatic enzymes are enzymes for which synthesis is increased as a result of bile retention or administration of drugs. Bile retention usually results from intrahepatic or extrahepatic bile duct obstruction. Commonly measured cholestatic enzymes include the following:


• Alkaline phosphatase (ALP)

• Gamma-glutamyltransferase (GGT; also gamma-glutamyltranspeptidase)


2 What are isoenzymes?


Isoenzymes are molecules with similar function but variable molecular structure. In general, these isoenzymes are produced in different tissues. For example, LDH (also LD) has five isoenzymes. LD1 and LD2 are primarily located in cardiac muscle, whereas LD5 is primarily located in liver and skeletal muscle. By determining the specific isoenzyme increased in serum, the source of the cellular injury can be identified.



3 What determines the level of a liver enzyme in plasma?


Multiple factors are involved in determining the activity of an enzyme in plasma and include the following:


a. Amount of the enzyme in hepatocytes

b. Rate of synthesis of the enzyme

c. Number of hepatocytes injured

d. Molecular size

e. Intracellular location of the enzyme

f. Rate of clearance from the plasma

g. Rate of enzyme inactivation


4 Do liver enzyme tests evaluate liver function?


Enzyme tests provide information about hepatocellular injury or cholestasis but do not define how much functional liver is present. Therefore, specific liver function tests are needed to assess liver function.



5 What are the most clinically useful leakage enzymes?


Clinically useful leakage enzymes vary with the species tested. In dogs and cats, ALT is used as a liver-specific leakage enzyme. Horses, ruminants, and pigs have very little ALT in their liver, so serum ALT activity does not increase with liver disease; therefore SDH is used as a liver-specific leakage enzyme in these species. AST is used as a nonspecific indicator of tissue injury in all the species just listed.



6 How long do leakage enzymes remain elevated in blood of dogs and cats after transient hepatocellular injury?


ALT remains elevated for 1 to 3 weeks after a toxic insult in dogs and cats. AST remains elevated for 5 to 7 days (Figure 30-1).


image

Figure 30-1 Serum enzyme activity after acute transient liver injury in dogs and cats. Alkaline phosphatase (ALP) increases slightly in dogs but may remain normal in cats. AST, Aspartate transaminase; ALT, alanine transaminase; IU, international units.



7 How long do leakage enzymes remain elevated in blood of horses and ruminants after transient hepatocellular injury?


SDH remains high for only 3 to 4 days. AST stays elevated for 1 to 2 weeks (Figure 30-2).


image

Figure 30-2 Serum enzyme activity after acute transient liver injury in horses and ruminants. SDH, Sorbitol dehydrogenase; AST, aspartate transaminase, GGT, γ-glutamyltransferase.



8 Does the magnitude of increase in leakage enzymes predict the number of hepatocytes injured?


In general, the higher the activity in the serum, the greater is the number of hepatocytes injured.



9 Do leakage enzymes predict whether liver injury is reversible?


Leakage enzymes do not predict reversible liver injury. Acute reversible toxic or traumatic hepatocellular injury can result in very high serum activities of leakage enzymes, whereas chronic fibrotic liver disease, which is irreversible, may result in only slight increases in leakage enzymes.



10 Are there other factors in addition to hepatocellular injury that increase serum enzyme activities?


A mild to moderate increase in ALT can result from enzyme induction associated with administration of drugs such as anticonvulsants, thiacetarsemide, and glucocorticoids. AST is present in most tissue and increases with muscle injury as well as hepatocelluar injury. AST is also present in kidney, pancreas, and erythrocytes. Artifactual hemolysis of blood samples may result in increased serum ALT activity.

Related posts:

  1. ERYTHROCYTE DISORDERS
  2. OVERVIEW: HEMOSTATIC COMPONENTS AND DISORDERS
  3. INTERPRETATION OF THE LEUKOGRAM
  4. AVIAN BIOCHEMICAL ANALYSIS

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Aug 26, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on TESTS FOR EVALUATION OF LIVER DISEASE

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