Liver Function Tests

LIVER FUNCTION TESTS

LFTs are used for mainly 2 reasons:

  1. To confirm clinical suspicion of potential liver injury or disease.
  2. To distinguish between hepatocellular injury (hepatic jaundice) and cholestasis (post hepatic or obstructive jaundice).

LFTs are classified according the functions of liver:

  1. Tests based on excretory function- Measurement of bile pigments, bile salts, bromosulphthalein.
  2. Tests based on serum enzymes derived from liver- Determination of transaminases, alkaline phosphatase, 5’-nucleotidase, γ-glutamyl transpeptidase.
  3. Tests based on metabolic capacity- galactose intolerance, antipyrine clearance.
  4. Tests based on synthetic functions- Prothrombin time, serum albumin.
  5. Tests based on detoxification- Hippuric acid synthesis.

AST(SGOT)- low specificity for liver, seen in periportal hepatocytes.

ALT(SGPT)- high specificity.

Both these enzymes go up for all forms of liver injury.

ALBUMIN: It is made in the liver, decreases in chronic liver injury because albumin has 20-day half-life.

Prothrombin Time: It is reliable for both acute and chronic liver diseases. Clotting requires many enzymes produced in the liver. PT increases when liver is not able to synthesis these enzymes.

Prothrombin time also decreases when there is deficiency of vitamin K, coumadin, haemophilia.

  • RBCs last for about 120 days, after which they are broken down and gives unconjugated or indirect bilirubin that goes to liver, where it is converted and excreted as conjugated or direct bilirubin.
  • Conjugated bilirubin can be excreted through kidneys and not unconjugated bilirubin, because former is water soluble. If there is a blockage in the intrahepatic or extrahepatic duct, conjugated bilirubin builds up in the blood and can be checked through the blood test. Therefore, if there is bilirubinuria, it means there is conjugated bilirubin in the blood, that implies either there is intrahepatic or extrahepatic obstruction.
    • Alkaline phosphatase: It has low specificity for cholestasis. Three conditions increase the level of alkaline phosphatase:
      • Cholestasis
      • Pregnancy
      • Bone growth (Paget disease)
    • GGT (γ-glutamyl transpeptidase):  It’s increase is not seen in bone disease, just cholestasis. Alcohol also elevates it.
    • If there is increased breakdown of blood, levels of unconjugated bilirubin will increase, it can be checked by finding values of total bilirubin and conjugated bilirubin and the difference is unconjugated bilirubin. If it is high, it means either there is a lot of breakdown of blood products (DIC, intravascular haemolysis), or inability to convert unconjugated bilirubin to conjugated bilirubin (Gilbert’s disease).  If there is blockage in the excretion of conjugated bilirubin into the ducts, it’s levels will increase and this will back up in the blood.

– Ariba Ali

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