Sunday, November 11, 2012

Liver function tests (Serum Bilirubin estimation)


LIVER FUNCTION TEST (MADE)

1.      Establish liver disease
2.      Diagnose the disease
3.      Access severity
4.      Monitor progression of disease.

The standard liver function test consists of:

a.      Plasma bilirubin

b.      Activities of enzymes in plasma (aminotransferases, alkaline phosphatase, γ-GT, 5’-NT)

c.       Plasma proteins, albumin and globulin concentrations.

These tests will correctly indicate liver disease in about 75% of cases.

a.      Bilirubin

Elevated level indicates jaundice. It is formed in liver, spleen and bone marrow from haem released after red blood cell ages (80%), hepatic heme proteins (15%) like catalase, myoglobins, cytochromes, etc. and ineffective erythropoiesis (5%).

The rate limiting step is the oxidation of haem to biliverdin by haem oxygenase; followed by reduction (biliverdin reductase) to bilirubin, with production of equimolar amount of –CO and Fe3+ iron.
  
This unconjugated bilirubin is tightly bound to albumin in 1:1 molar ratio, but additional binding sites of low affinity are recruited in hyperbilirubinaemic states. This binding prevents extrahepatic uptake and thus facilitates transport to liver. Other molecule like thyroxin and certain drugs compete for albumin binding site and displace bilirubin.

Bilirubin actively transported to sinusoid of liver and binds to ligandin (glutathione transferase B). It is then conjugated with glucuronic acid by UDP-glucuronyl transferase to form mono and diglucuronides rendering water soluble.  This bilirubin is transported to bile and hence to gut. In gut some bilirubin is deconjugated by bacterial glucuronidases and is reabsorbed, but most is oxidized to urobilinogen and further oxidized to stercobilin and urobilin, and excreted in faeces. Conjugated bilirubin can circulated binding loosely to album and this can be filtered off the plasma and excreted in urine.

Hyperbilirubinaemia:

Jaundice occurs when plasma bilirubin level exceeds 50 µmol/L. Its measurement is important during biliary disorder, hepatic disorder, neonatal jaundice and sometimes renal failure.

Methods of measurement:

Current method for bilirubin measurements are based on diazo coupling of the pigment first described by Ehrlich in 1883. In 1996 van den Berg and Muller noted bilirubin from patients with obstructive jaundice reacted directly whereas bilirubin (conjugated bilirubin) from patient with hemolytic jaundice reacted indirectly i.e., accelerator like alcohol was required (unconjugated bilirubin).

Bilirubin bound to albumin probably covalently is now recognized as 3rd form of bilirubin. It may account for upto 90% of total bilirubin in both hepatocellular and cholestatic jaundice. It is not detectable in normal subjects or in unconjugated byperbilirubinemia including Gilbert’s syndrome. This form persists in plasma during recovery from jaundice. 
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