Liver Function Tests (LFTs)

Major Metabolic Functions of the Liver

• Synthetic Functions:
  • Synthesis of plasma proteins, specifically albumin and globulins.
  • Synthesis of lipids, including cholesterol, triglycerides, and lipoproteins.
• Detoxification and Excretion Functions:
  • Conversion of toxic ammonia into urea via the urea cycle.
  • Detoxification and excretion of bilirubin, cholesterol, and drug metabolites.
• Storage Functions:
  • Storage of fat-soluble vitamins: Vitamin A, Vitamin D, Vitamin E, and Vitamin K.
  • Storage of water-soluble Vitamin $B_{12}$.
• Digestive Functions:
  • Production of bile salts to assist in lipid digestion and absorption.

Overview, Utility, and Limitations of Liver Function Tests

• Definition and Clinical Purpose:
  • Noninvasive screening methods utilized to detect liver dysfunction.
  • Essential for identifying broad categories of hepatic disorders.
  • Assess disease severity and enable the prediction of clinical outcomes.
  • Provide ongoing evaluation for disease progression and treatment monitoring.
• Clinical Spectrum of Liver Dysfunction:
  • Hepatocellular disease.
  • Cholestasis (obstruction of bile flow).
  • Cirrhosis.
  • Hepatitis.
  • Jaundice.
  • Primary liver cancer.
  • Steatosis (fatty liver).
  • Genetic disorders, such as Hemochromatosis (pathological iron storage).
• Limitations of Liver Function Tests:
  • Normal LFT results do not rule out liver disease due to the organ's substantial reserve capacity.
  • Asymptomatic individuals can exhibit abnormal LFT results.
  • Clinical diagnosis must always be integrated with physical clinical examination.

Systematic Classification of Liver Function Tests

• Broad Functional Classification:
  • Tests to detect hepatic injury: Evaluate mild vs. severe injury, acute vs. chronic pathology, and the nature of damage (hepatocellular vs. cholestatic).
  • Tests to assess hepatic function: Quantify synthetic and excretory capabilities.
• Group I: Markers of Liver Dysfunction:
  • Serum bilirubin (total bilirubin and conjugated bilirubin).
  • Urinary markers (bile salts and urobilinogen).
  • Protein markers (total protein, serum albumin, and the albumin/globulin ratio).
  • Coagulation profile (Prothrombin Time).
• Group II: Markers of Hepatocellular Injury:
  • Alanine aminotransferase (ALT).
  • Aspartate aminotransferase (AST).
• Group III: Markers of Cholestasis:
  • Alkaline phosphatase (ALP).
  • $\gamma$-glutamyltransferase (GGT).
• Summary of Clinical Implications for Serum Chemistry Abnormalities:
  • Alanine aminotransferase elevation: Indicates hepatocellular damage.
  • Aspartate aminotransferase elevation: Indicates hepatocellular damage.
  • Bilirubin elevation: Indicates cholestasis, impaired conjugation, or biliary obstruction.
  • Alkaline phosphatase elevation: Indicates cholestasis, infiltrative disease, or biliary obstruction.
  • Prothrombin time prolongation: Indicates altered synthetic function.
  • Serum albumin decrease: Indicates altered synthetic function.
  • $\gamma$-glutamyltransferase elevation: Indicates cholestasis or biliary obstruction.
  • Bile acids elevation: Indicates cholestasis or biliary obstruction.

Bilirubin Metabolism and Excretory Pathway

• Primary Origin of Bilirubin:
  • Senescent red blood cells represent the primary source of hemeproteins and hemoglobin.
• Detailed Step-by-Step Pathway of Bilirubin Metabolism:
  • Step 1: Breakdown of hemoglobin and heme to unconjugated bilirubin occurs inside macrophages of the reticuloendothelial system (tissue macrophages, spleen, and liver).
  • Step 2: Unconjugated bilirubin is released into the blood and transported bound to serum albumin.
  • Step 3: Unconjugated bilirubin is transported to the liver via systemic circulation.
  • Step 4: Bilirubin is taken up into liver parenchymal cells via facilitated diffusion.
  • Step 5: Inside hepatocytes, bilirubin is conjugated with glucuronic acid to form bilirubin diglucuronide (conjugated bilirubin).
  • Step 6: Conjugated bilirubin is actively secreted into the bile canaliculi, stored in the gallbladder, and delivered into the small intestine.
  • Step 7: In the intestine, bacterial enzymes remove glucuronic acid and reduce bilirubin into urobilinogen.
  • Step 8: A portion of urobilinogen is reabsorbed from the intestinal lumen into the portal vein blood.
  • Step 9: Reabsorbed urobilinogen enters the enterohepatic urobilinogen cycle to return to the liver.
  • Step 10: The remainder of systemic urobilinogen is transported via blood to the kidneys, converted into yellow urobilin, and excreted in urine, giving urine its characteristic color.
  • Step 11: Unabsorbed urobilinogen remaining in the large intestine is oxidized by anaerobic intestinal bacteria into brown stercobilin, which is excreted in feces.

Clinical Assessment of Serum Bilirubin and Jaundice

• Bilirubin Characteristics:
  • Yellowish pigment generated as a byproduct of red blood cell breakdown.
  • Primary pigment responsible for the physical yellowish discoloration observed in jaundice.
  • Elevated serum levels occur in gallstones, acute hepatitis, and chronic hepatitis.
• Reference Ranges for Serum Bilirubin:
  • Normal Total Bilirubin: $0.2 - 0.8\,\text{mg/dL}$
  • Unconjugated (Indirect) Bilirubin: $0.2 - 0.7\,\text{mg/dL}$
  • Conjugated (Direct) Bilirubin: $0.1 - 0.4\,\text{mg/dL}$
  • Latent Jaundice Threshold: Above $1.0\,\text{mg/dL}$
  • Overt Clinical Jaundice Threshold: Above $2.0\,\text{mg/dL}$
• Diagnostic Etiology and Classification of Jaundice:
  • Pre-hepatic (Hemolytic) Jaundice: Caused by abnormal red blood cells, antibodies, drugs, toxins, thalassemia, hemoglobinopathies, Gilbert's syndrome, and Crigler-Najjar syndrome.
  • Hepatic (Hepatocellular) Jaundice: Caused by viral hepatitis, toxic hepatitis, and intrahepatic cholestasis.
  • Post-hepatic Jaundice: Caused by extrahepatic cholestasis, gallstones, bile duct tumors, and carcinoma of the pancreas.
• Diagnostic Interpretation of Urobilinogen (UBG) and Bile Salts:
  • Most urobilinogen is metabolized within the large intestine, with a minor fraction excreted in urine at levels less than $4\,\text{mg/day}$.
  • Bile salts are normally absent from urine.
  • Obstruction of biliary passages causes leakage of bile salts into systemic circulation, leading to their direct excretion in urine.

Diagnostic Markers of Hepatic Synthetic Capacity

• Serum Albumin:
  • Most abundant serum protein synthesized exclusively by hepatocytes.
  • Normal Serum Reference Range: $3.5 - 5.0\,\text{g/dL}$
  • Synthesis capacity depends directly on the functional liver cell mass.
  • Circulatory half-life: $20\,\text{days}$.
  • Serum concentration drops progressively in all forms of chronic liver disease.
• Serum Globulins and Immunoglobulins:
  • Normal Serum Reference Range: $2.5 - 3.5\,\text{g/dL}$
  • $\alpha$-globulins and $\beta$-globulins are predominantly synthesized by hepatocytes.
  • $\gamma$-globulins constitute immunoglobulins (antibodies) produced by plasma cells.
  • Elevated serum $\gamma$-globulin levels occur in chronic hepatitis and cirrhosis:
  • Elevated $\text{IgG}$ is characteristic of autoimmune hepatitis.
  • Elevated $\text{IgA}$ is characteristic of alcoholic liver disease.
• Albumin to Globulin (A/G) Ratio:
  • Normal Reference Ratio: $1.2/1 - 1.5/1$
  • Globulin synthesis increases compensation for low osmotic pressure during hypoalbuminemia.
• Prothrombin Time (PT):
  • Prothrombin is a blood coagulation protein synthesized by the liver, serving as an acute indicator of hepatic synthetic function.
  • Circulatory half-life: $6\,\text{hours}$ (provides a real-time assessment of hepatic functional capacity).
  • PT becomes prolonged only when the liver loses greater than $80\%$ of its functional reserve capacity.
  • Vitamin K deficiency also produces a prolonged PT.
  • Exogenous vitamin K administration does not correct or affect a prolonged PT caused by intrinsic liver parenchymal disease.

Diagnostic Markers of Hepatocellular Injury

• Aspartate Aminotransferase (AST):
  • Normal Serum Reference Range: $8 - 20\,\text{U/L}$
  • Marker indicating hepatocellular destruction and damage.
  • Marked serum elevations occur in chronic hepatitis, cirrhosis, and primary liver cancer.
• Alanine Aminotransferase (ALT):
  • Demonstrates significantly higher specificity for liver tissue than AST.
  • Normal Serum Reference Range by Sex:
  • Male: $13 - 35\,\text{U/L}$
  • Female: $10 - 30\,\text{U/L}$
  • Pathological Serum Elevation Levels:
  • Acute Hepatitis: High elevation ranging between $300 - 1000\,\text{U/L}$.
  • Alcoholic Hepatitis: Moderate elevation ranging between $100 - 300\,\text{U/L}$.
  • Cirrhosis, Hepatitis C, and Non-Alcoholic Steatohepatitis (NASH): Minor elevation ranging between $50 - 100\,\text{U/L}$.
  • Clinical Utility Details:
  • Rises in plasma many days prior to the clinical emergence of overt symptoms.
  • Normal serum levels do not exclude ongoing underlying liver damage.
  • Obese individuals without overt pathology may demonstrate elevated baseline ALT levels.

Diagnostic Markers of Cholestasis and Biliary Tract Pathology

• Alkaline Phosphatase (ALP):
  • Non-specific enzyme marker for liver pathology.
  • Synthesized by bone osteoblasts (to facilitate bone calcification) and localized on hepatocyte cell membranes.
  • Normal Serum Reference Range: $40 - 125\,\text{U/L}$
  • Diagnostic Elevation Levels:
  • Moderate Elevation: Infective hepatitis, alcoholic hepatitis, and hepatocellular carcinoma.
  • High Elevation: Extrahepatic biliary obstruction (obstructive jaundice) and intrahepatic cholestasis.
  • Very High Elevation: Primary metabolic or neoplastic bone diseases.
• $\gamma$-Glutamyltransferase (GGT):
  • Enzyme essential for cellular glutathione synthesis.
  • Normal Serum Reference Range: $10 - 30\,\text{U/L}$
  • Moderate Elevation: Infective hepatitis and prostate malignancies.
  • Frequently elevated in chronic alcohol users even when all other standard LFT results are normal, making it a highly sensitive diagnostic marker for identifying alcohol abuse.

Additional Physiological and Defensive Functions of the Liver

• Cholesterol and Bile Acid Metabolism:
  • Synthesizes cholesterol de novo.
  • Removes circulating cholesterol from blood plasma.
  • Converts cholesterol into functional bile acids.
  • Excretes excess cholesterol via biliary secretion to maintain body cholesterol homeostasis.
• Vitamin and Mineral Handling:
  • Converts and stores Vitamin A.
  • Stores Vitamin $B_{12}$.
  • Stores elemental iron bound to ferritin.
  • Produces specialized plasma transport proteins for iron circulation.
  • Participates in the metabolic hydroxylations of Vitamin D.
• Immune and Defensive Functions:
  • Houses specialized resident tissue macrophages known as Kupffer cells.
  • Kupffer Cell Functions:
  • Removes bacteria from portal blood.
  • Phagocytoses senescent and damaged blood cells.
  • Clears cellular debris from systemic circulation.
  • Participates in general antigen presentation and immune responses.
  • Synthesizes immune system proteins, including components of the complement cascade system.
• Thermoregulation:
  • Contributes significantly to endogenous heat production and core body temperature maintenance due to high metabolic activity.
• Nutrient Deposition and Mobilization:
  • Serves as a primary storage depot for Glycogen, Iron, Copper, Vitamin A, Vitamin D, Vitamin $B_{12}$, and Folate.
  • Mobilizes and releases stored nutrients into systemic circulation during metabolic demand.

Academic References

• Lippincott's Illustrated Reviews Biochemistry: 6th edition, Unit IV, Chapter 21, Pages 282–285.
• Lecture Notes: Clinical Biochemistry: 9th edition, Chapter 13, Pages 174–187.
• Clinical Chemistry - Techniques, Principles and Correlations: 6th edition, Chapter 24, Pages 520–521.