Laboratory tests play a central role in
the diagnosis and management of liver disease in children. In this article the
usefulness Liver function tests in the diagnosis of liver disease has been
dealt with. Though imaging, histology and endoscopy are also important in the
diagnosis of liver diseases covering these tests are beyond the scope of this
article
Sometimes abnormal laboratory
tests may be the only indication of liver dysfunction. Although the term liver function tests
is in common usage, most of the lab test used routinely do not measure the
function of the liver except serum albumin and prothrombin time and the rest
are indirect tests and not specific to liver diseases alone.
Tests to evaluate liver disease:
Tests of biochemical activity
Alanine aminoTransferase (ALT)
· Aspartate aminoTransferase (AST)
Lactic Dehydrogenase (LDH)
Tests of cholestasis
·
Gamma Glutamyltranspeptidase (GGT)
·
Alkaline phoshatase (AP)
·
5’nucleotidase
·
Bilirubin (total and fractionation)
·
Urinary urobilinogen
·
Serum and urine bile acids
Tests for synthetic
function
·
Albumin and other serum proteins
·
Prothrombin time
·
Ammonia
·
Serum lipids and lipoproteins, cholesterol and
triglycerides
Miscellaneous
specific serum tests (a1-antitrypsin, ceruloplasmin, a-fetoprotein, auto
antibodies)
Tests for
quantitative function – difficult to perform and not practically useful
LIVER ENZYMES
A liver enzyme is a protein that helps to speed up a chemical reaction in the liver. Liver enzymes have been useful as a screening test for liver diseases, to determine the pattern of liver disease,assess severity and also in the follow up of the patients with liver disease
Enzymes that detect hepatocellular necrosis Aminotransferases Aspartate aminotransferase (AST,
formerly SGOT) and Alanine aminotransferase (ALT, formerly SGPT) are the most frequently utilized and
specific indicators of hepatocellular necrosis and usually catalyze the
transfer of a-amino group to a-keto group.
Aspartate aminotransferase (AST)is a
mitochondrial enzyme present in liver, cardiac muscle, skeletal muscle, kidneys,
brain, pancreas, lungs leukocytes and erythrocytes whereas Alanine aminotransferase (ALT) is a
cytosolic enzyme which is more specific for liver damage and present in higher
concentration in liver than in heart/skeletal muscle and normal values of these
enzymes range from 5-35 IU/l.
The rise in liver enzymes may be severe ( >
20 times, 1000 U/L) in viral hepatitis, drug or toxin induced hepatic necrosis
and circulatory shock, moderate (3-20 times) in acute & chronic hepatitis,
neonatal hepatitis, and acute biliary tract obstructions or mild (1-3 times)
in sepsis, Extra Hepatic Biliary Atresia
(EHBA), Myositis, Duchenne muscular dystrophy and even after vigorous exercise.
Level of enzyme elevation shows poor correlation regards the extent
of necrosis and is of little
value in predicting the outcome. Similarly decreasing enzyme level though may indicate
recovery can also be due to massive destruction when it indicates poor
prognosis.
The ratio of AST and ALT (AST/ALT) may be useful
to differentiate certain conditions. The normal ratio is less than or equal to
1 and a ratio of more than 2 is seen commonly in alcoholic hepatitis probably due to pyridoxine
deficiency and in acute Wilsons the ratio is usually more than 4 and one should
remember that a higher ratio may also be seen incirrhosis due to any cause
because of reduced plasma clearance of AST.
Low levels of amino transferases have been seen in patients on
long term haemodialysis and also in uremia and the cause probably secondary to
either dialysate or pyridoxine deficiency.
Other enzymes like glutamate dehydrogenase, isocitrate
dehydrogenase, lactate dehydrogenase and sorbitol dehydrogenase have also been
found to be raised in hepatocellular necrosis but have not found to be
clinically significant.
Enzymes that detect cholestasis
Alkaline phosphatase is found histochemically
in the microvilli of bile canaliculi and on the sinusoidal surface of
hepatocytes and normally serum values range between 45-115 U/L and the increase in serum levels
is usually due to overproduction
and leakage in blood
secondary to bile ductular damage the increase in serum levels may be due
to disorders of bone, kidney,
intestine, placenta and tumours in addition to cholestatic liver disease and
most circulating alkaline phosphatase usually originates from either liver or
bone. Physiologically high values are seen in childhood and puberty due
to bone growth. Though highest levels of alkaline phosphatase occur in
cholestatic disorders, other causes for increase may be infiltrative liver diseases, abscesses, granulomatose
liver disease and amyloidosis. Low levels of alkaline phosphatase can
occur in hypothyroidism, pernicious anaemia, zinc deficiency and congenital
hypophosphatasia.
GGlutamyltranspeptidase
(GGT) is a membrane
bound glycoprotein and normal value ranges between 0-30 U/L and increasing serum level is due to
overproduction and leakage in
blood. In addition to
liver, the enzyme is also expressed in the kidney, spleen, pancreas, heart,
lung and brain except bones and therefore useful in the exclusion of bone
diseases. The new borns may have high levels of GGT 5 to 8 times the
upper limit of normal. One should also remember that this enzyme is inducible
by certain drugs like anticonvulsants and depressed by female sex hormones
Serum GGT is high in all
cholestatic disorders and cannot differentiate between intahepatic and extra
hepatic causes. Low levels of GGT is found in certain cholestatic disorders
like Proressive familial intrahepatic cholestasis (PFIC) types I and II and
Inborn errors of bile acid metabolism where GGT has been useful to
differentiate these conditions from other causes of cholestasis.
Other enzymes to detect cholestasis are 5’Nucleotidase and Leucineaminopeptidase
but not put used commonly.
Bilirubuin
Bilirubin is a yellow tetrapyrrole pigment
formed from the degradation of the heme found in hemoglobin in the red cells of
the blood which is also found in myoglobin and certain enzymes including
catalase, peroxidase and the cytochromes. About 75% of daily production of
bilirubin is from the breakdown of red cells and 22% is produced from heme
containing enzymes and a small amount 3% from ineffective erythropoeisis and
normally less than 1mg/dl beyond neonatal period. Hyperbilirubinemia or
increased serum bilirubin may result from overproduction, impaired uptake,
conjugation or excretion or regurgitation.
Increased unconjugated bilirubin: This results from overproduction/impaired
uptake, conjugation
Increased conjugated bilirubin: Impaired intrahepatic excretion /
regurgitation of unconjugated or conjugated bilirubin from hepatocytes into bile
ducts.
Serum bilirubin could be lowered by drugs like salicylates,
sulphonamides, free fatty acids which displace bilirubin from its attachment to
plasma albumin. On the contrary it could be elevated if the serum
albumin increases and the bilirubin may shift from tissue sites to circulation.
Level of bilirubin rise may be mild in liver diseases, physiological
jaundice and inherited hyperbilirubinemias. Moderate elevations of serum
bilirubin is seen with biliary atresia, drugs, viral hepatitis, inherited
hyperbilirubinemias
Other causes of hyperbilirubinemia may be due
to extra hepatic sources either hemolysis, ineffective erythropoiesis, hematoma
or myoglobinemia
But you should remember that bilirubin
fractionation is very important to recognize conjugated
bilirubinemia/cholestasis which is always pathological.
Urine Bilirubin & UBG
Conjugated bilirubin is water soluble and
excreted in the urine and therefore the presence of urine bilirubin indicates
hepatobiliary disease. Laboratory methods can detect low levels of bilirubin in
urine. Tests strips
impregnated with diazo reagent are easy to use to detect bilirubin in urine
Urobilinogen is formed from the
degradation of conjugated bilirubin by bacteria in the intestinal lumen and
upto 20% undergoes enterohepatic circulation. A small amount is excreted in
urine. Increase in the
urobilinogen in urine is a sensitive indicator of hepatocellular dysfunction. In
cholestatic jaundice with complete biliary obstruction urobilinogen disappears
from urine.
Tests of liver synthetic function:
Albumin and other serum proteins
Albumin is the principal serum protein and
increases from a mean of 3.41g/dl in the first month of life to a mean of
4.25g/dl by adolescence. It is synthesized only in the rough endoplasmic
reticulum of hepatocytes at a rate of 150mg/kg/day and has a half life of
approximately 20 days. The major functions of albumin are to maintain
intravascular colloid osmotic pressure and to bind and serve as a carrier
protein. Decreases in serum levels may result from decreased production by the liver
due to significant parenchymal liver disease. Because of the long half-life low
serum albumin is taken as a sign of chronic liver disease. Hypoalbuminemia
is not specific for liver disease and can occur in other non hepatic causes
like poor nutrition, nephritic syndrome, and protein-losing enteropathies.
Serum globulins are often
elevated in chronic liver disease including cirrhosis due to any cause and
particularly in autoimmune hepatitis and this is due to increase in gamma
globulins usually in the setting of low albumin
Abnormalities of
coagulation – Prothrombin time (PT)
The liver plays three roles in
the control of coagulation (a) the production partly or exclusively of all
coagulation factors with the exception of von Willebrand factor, (b) the production
and breakdown of factors integral to fibrinolysis such as plasminogen and
plasminogen activator, and (c) the clearance of activated clotting factors from
the circulation. Synthesis of factors II, VII, IX and X is dependent on
adequate supply of vitamin K.
Prothrombin time (PT) is a
measure of the time it takes for prothrombin to be converted into thrombin in
the presence of other factors. The result is expressed in seconds or as a ratio
of the plasma PT to a control PT. Normal range - 9-11 seconds. A prolongation of more than 2 seconds is
considered abnormal.The prolonged PT is not specific for liver diseases. In
acute and chronic hepatocellular disease the PT may serve as a prognostic
indicator. Prothrombin time
corrected by inj. Vitamin K indicates cholestasis causing vitamin k deficiency
Other tests for biosynthetic
function of the liver are Serum
prealbumin, Serum Ceruloplasmin, Procollagen III peptide, A 1
antitrypsin and Alpha feto protein
Prealbumin: The serum prealbumin level is 0.2- 0.3 g/L. these levels fall
in liver disease presumably due to reduced synthesis. Determination of prealbumin
has been considered particularly useful indrug-induced hepatotoxicity.
Serum Ceruloplasmin: Normal plasma levels are
0.2-0.4g/L. It is synthesized in the liver and is an acute phase protein. The
plasma concentration rise in infections, rheumatoid arthritis, pregnancy, non
Wilson liver disease and obstructive jaundice. This is an important diagnostic
marker in Wilson disease, in which the plasma level is usually low. Low levels
may also be seen in neonates, Menke’s disease, kwashiorkor, marasmus, protein
losing enteropathy, copper deficiency and aceruloplasminemia.
Procollagen III Peptide: The serum concentration of this peptide appears to increase
not only with hepatic fibrosis but also with inflammation and necrosis. Serial
measurement of procollagen III may be helpful in the follow up of chronic liver
disease.
a1 Antitrypsin: a1
antitrypsin is a glycoprotein synthesized by the liver and is an inhibitor of
serine proteinases, especially elastase. Its normal concentration is 1- 1.6g/L.
it is an acute phase protein, serum levels increase with inflammatory disorders,
pregnancy and after oral contraceptive pills (OCP). Liver disease is usually
seen with deficiency of a 1 antitrypsin, an inherited
disorder. Deficiency should be confirmed by quantitative measurement.
Afeto Protein: This protein, the principal one in fetal plasma in early gestation
is subsequently present at very low levels (<25mg/L) It is increased in Hepato Cellular Carcinoma (HCC) and
more than 90% of such patients have raised levels. Raised values are also found
in other liver diseases like chronic hepatitis, in regeneration phase of
acute hepatitis and in hepatic metastasis. This is also raised in adenomas
associated with tyrosinemia.
Dr. D. Nirmala
Professor and Head
Department of Paediatric Gastroenterology
Institute of Child Health and Hospital for children
