Liver function tests-tests for evaluation of liver enzymatic activity
The liver function tests are
important to determine
1. Presence of liver diseases
2. Type of liver diseases
3. Extent and progression of liver
diseases
Tests to assess liver
function:
The liver function tests can be
divided into:
A. Tests for evaluation of liver enzymatic
activity
B. Tests for evaluation of
hepatic synthetic function
C. Tests for evaluation of
hepatic excretory function
A. Tests for evaluation of liver enzymatic activity
Evaluation of liver enzymes
involve measurement of serum enzymes derived from liver such as alanine
transaminase(ALT), aspartate transaminase(AST), alkaline phosphatase(ALP),
gamma glutamyl transpeptidase(GGT), 5' -nucleotidase and lactate
dehydrogenase(LDH).
The liver diseases result in
damage of liver and elevation of serum enzymes.
1. Alanine
transaminase(ALT)
ALT belong to transaminase
enzymes which are part of transferase enzymes. The transferase enzymes catalyze
transfer functional group. Alanine transaminase(ALT) is a recent name for serum
glutamate pyruvate transaminase (SGPT). ALT is found in the liver, skeletal
muscle Kidney and heart (minor activity). Transaminases (AST and ALT) catalyze
transfer of amino group from amino acid to keto acid.
Raised ALT activities occur in
the following cases:
a-Viral hepatitis.
b-Circulatory failure with
shock and hypoxia.
c- Liver cirrhosis.
d-Obstructive jaundice.
e- Skeletal muscle disease
F- Infectious mononucleosis (due
to liver involvement). Note: ALT is more specific for liver disease than
AST.
2-Apatate transaminase(AST)
AST belong to transaminase
enzymes. Aspartate transaminase(AST) is also called serum glutamate
oxaloacetate transaminase (SGOT). Aspartate transaminase(AST) is a recent name.
The ALT is found in heart, liver, skeletal muscle, Kidney and
erythrocytes. AST
Raised AST activities occur in
the following cases:
a. Viral hepatitis
b. Myocardial infarction
c. Toxic hepatitis
d. Circulatory failure with shock
and hypoxia.
e. Liver cirrhosis.
F. Cholestatic jaundice.
g. Skeletal muscle disease and
cardiac surgery.
h. Malignant infiltration of
liver.
i. Infectious mononucleosis (due
to liver involvement). Note: AST is not specific for hepatic disease.
3.
ALP
(Alkaline phosphatase)
The ALP family of
enzymes which catalyze hydrolysis of phosphate group at alkaline pH. Alp has
highest activity liver, bone, intestine, kidney, and placenta. The clinical
utility of ALP lies in its ability to differentiate hepatobiliary disease from
osteogenic bone disease.
ALP
isoenzymes
The major ALP
isoenzymes which are found in the serum are those derived from the liver, bone,
intestine, and placenta which are separated by electrophoresis technique. In
electrophoresis technique, the liver fraction migrates the fastest, followed by
bone, placental, and intestinal fractions.
Serum
ALP increases in:
1. In the liver, the enzyme ALP is
localized to the microvilli of the bile canaliculi, and therefore it increases
in:
Ø Extrahepatic
biliary obstruction, such as a stone in bile duct.
Ø Intrahepatic
cholestasis, such as drug cholestasis.
Ø Primary
biliary cirrhosis.
Ø Hepatocellular
disorders such as hepatitis and cirrhosis.
2. Bone-related disorders.
Ø Paget’s
disease.
Ø Bony
metastases, diseases associated with an increase in osteoblastic activity.
Ø Osteomalcia.
Ø Rickets.
Ø Osteogenic
sarcoma.
Ø In
healing bone fracture
3. Hyperparathyroidism.
4. Physiologically
Ø Rapid
bone growth during puberty.
Ø In
pregnancy due to its release from the placenta.
4. GGT (gamma glutamyl transferase, gamma glutamyl
transpeptidase)
γ-Glutamyltransferase
(GGT) is involved in the transfer of the γ-glutamyl residue from γ-glutamyl
peptides to amino acids and small peptides. In most biologic systems,
glutathione serves as the γ-glutamyl donor.
Tissue Source
GGT activity is found in the kidney, brain, prostate,
pancreas, and liver. Serum GGT activity determination is used mainly
to evaluation of liver and biliary system disorders. Elevated serum level of
GGT is indication of liver disease and biliary tract. In the liver, GGT is
located in the canaliculi of the hepatic cells and particularly in the
epithelial cells lining the biliary ductules. In conjunction with ALP, GGT
differentiates between bone and liver disease.
The GGT levels are increased in
following cases:
a. Enzyme-inducing
drugs such as warfarin, phenobarbital, and phenytoin.
b. Alcoholism,
particularly chronic alcoholism.
c. Diabetes
mellitus.
d. Cholesteric
liver disease
e. Infectious
hepatitis.
f. Alcoholic
hepatitis
g. Acute
pancreatitis.
h. Bile
duct cancer.
i.
Biliary tract
Obstruction.
5.
Lactate dehydrogenase
LDH
consists two different polypeptide chains H (heart) and M (muscle) which
combine to yield the five major isoenzymes. Each isoenzyme consists of four
polypeptide chains.
1.
LDH-1 (HHHH)
2.
LDH-2 (HHHM)
3.
LDH-3 (HHMM)
4.
LDH-4 (HMMM)
5.
LDH-5 (MMMM)
Tissue Source
LDH
is found in the heart, liver, skeletal muscle, kidney, and erythrocytes; lesser
amounts are found in the lung, smooth muscle, and brain.
LDH
enzyme consists of four polypeptide subunits. There are two types of subunits
namely M (for muscle) and H (for heart).
The highest levels of total LDH are seen in
a.
Pernicious
anemia and hemolytic disorders.
b.
Liver
disorders, such as viral hepatitis and cirrhosis.
c.
Myocardial
infarction and pulmonary infarct.
d.
Skeletal
muscle disorders.
Some leukemias e.g. acute lymphoblastic leukemia.
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