What are the tests based on the intermediary metabolic functions of liver ? Explain the basis of each of these tests.

Tests Based on Carbohydrate Metabolism

The liver is central to glucose homeostasis. Tests reflecting carbohydrate metabolism include:

• Glucose Tolerance Test (GTT): This test assesses the body’s ability to clear glucose from the bloodstream. After a standardized glucose load, blood glucose levels are measured at intervals. Impaired glucose tolerance can indicate insulin resistance or impaired liver glycogen storage.
• Galactose Tolerance Test: Used to diagnose galactosemia, a genetic disorder where galactose cannot be metabolized. Elevated galactose levels in the blood indicate a deficiency in the enzymes required for galactose metabolism.
• Fructose Tolerance Test: Less commonly used, it assesses the liver’s ability to metabolize fructose.

Tests Based on Protein Metabolism

The liver is vital for protein synthesis, amino acid metabolism, and urea cycle function. Key tests include:

• Serum Albumin: Albumin is a major protein synthesized by the liver. Low albumin levels can indicate chronic liver disease, malnutrition, or nephrotic syndrome.
• Prothrombin Time (PT) / International Normalized Ratio (INR): The liver synthesizes several clotting factors (II, V, VII, X). Prolonged PT/INR indicates impaired liver synthesis of these factors, suggesting liver damage or vitamin K deficiency.
• Ammonia Levels: The liver converts toxic ammonia to urea for excretion. Elevated ammonia levels (hyperammonemia) can occur in severe liver disease, particularly cirrhosis, or in genetic defects of the urea cycle.
• Amino Acid Levels: Measuring specific amino acid levels can help diagnose inborn errors of metabolism affecting amino acid pathways. For example, elevated phenylalanine levels indicate phenylketonuria (PKU).

Tests Based on Lipid Metabolism

The liver plays a crucial role in lipid synthesis, transport, and metabolism. Relevant tests include:

• Serum Cholesterol & Triglycerides: While not specific to liver function, abnormal lipid profiles can be associated with liver disease. The liver synthesizes cholesterol and triglycerides, and impaired liver function can disrupt lipid metabolism.
• Lipoprotein Analysis (HDL, LDL, VLDL): The liver is involved in the synthesis and clearance of lipoproteins. Abnormal lipoprotein levels can indicate liver dysfunction.
• Bile Acid Levels: The liver synthesizes bile acids from cholesterol. Elevated bile acid levels in the blood can indicate impaired liver function or biliary obstruction.

Other Relevant Tests

• Urine Urobilinogen: Urobilinogen is formed from bilirubin in the intestine. Abnormal urine urobilinogen levels can indicate liver disease or hemolytic anemia.
• Serum Bilirubin (Total, Direct, Indirect): While often considered a marker of hepatocellular damage, bilirubin metabolism is a key intermediary function. Elevated bilirubin levels indicate impaired bilirubin processing by the liver.

Test	Metabolic Pathway	Basis of Test	Clinical Significance of Abnormal Results
Glucose Tolerance Test	Carbohydrate	Ability to clear glucose from blood	Diabetes, Insulin Resistance
Serum Albumin	Protein	Liver synthesis of albumin	Chronic Liver Disease, Malnutrition
Prothrombin Time (PT/INR)	Protein	Liver synthesis of clotting factors	Liver Damage, Vitamin K Deficiency
Ammonia Levels	Protein	Liver conversion of ammonia to urea	Cirrhosis, Urea Cycle Defects
Serum Cholesterol & Triglycerides	Lipid	Liver synthesis and metabolism of lipids	Liver Disease, Dyslipidemia