Plasma Bilirubin Concentrations Quiz

Urine Bilirubin

• Presence of urine bilirubin indicates hepatobiliary disease
• Unconjugated bilirubin is tightly bound to albumin and not filtered by the glomerulus, thus not present in urine
• Measurable amounts of conjugated bilirubin in serum are found only in hepatobiliary disease
• Conjugated bilirubin may be found in urine when serum bilirubin levels are normal, particularly in early acute viral hepatitis
• Test strips impregnated with diazo reagent can detect as little as 1-2 mmol bilirubin/L

Urobilinogen

• Increase in urobilinogen in urine is a sensitive indicator of hepatocellular dysfunction
• Good indication of alcoholic liver damage, well-compensated cirrhosis, or malignant disease of the liver
• Appears early in viral hepatitis and is markedly increased in hemolysis

Bilirubin

• Derived from hemoglobin and cytochrome
• Normally conjugated with glucuronic acid
• Both conjugated and unconjugated bilirubin may be present in plasma
• Conjugated bilirubin is water-soluble, while unconjugated bilirubin is not and binds to albumin
• Neurotoxic and can cause permanent brain damage in neonates if levels rise too high
• Bilirubin metabolites are responsible for the brown coloration of feces
• Bilirubin in the gut is metabolized by bacteria to produce stercobilinogen, which is partly reabsorbed and re-excreted in the urine as urobilinogen

Causes of High Bilirubin Level

• Hemolysis (Haemolytic): Increased hemoglobin breakdown produces bilirubin, overloading the conjugating mechanism
• Liver disorder (Hepatocellular): Failure of the conjugating mechanism within the hepatocyte
• Obstruction in the biliary system (Cholestatic)

Liver Function Tests (LFTs)

• Helpful in follow-up of certain liver diseases and evaluating response to therapy, such as autoimmune hepatitis
• Limitations: Lack sensitivity and specificity
• Lack specificity and are not specific for any particular disease
• Serum albumin may be decreased in chronic disease and nephritic syndrome
• Aminotransferases may be raised in cardiac diseases and hepatic diseases

Classification of Liver Function Tests

• Tests of the liver's capacity to transport organic anions and to metabolize drugs: Serum bilirubin, urine bilirubin, urobilinogen
• Tests that detect injury to hepatocytes (serum enzyme tests): Aminotransferases, alkaline phosphatase, α-glutamyl transpeptidase, 5-nucleotidase, leucine aminopeptidase
• Tests of the liver's biosynthetic capacity: Serum proteins, albumin, prealbumin, serum ceruloplasmin, procollagen III peptide, α1-antitrypsin, alpha-fetoprotein, prothrombin time

Test your knowledge on the reference range for normal plasma bilirubin concentrations and the differences between conjugated and unconjugated bilirubin. Learn about the clinical significance of bilirubin in biochemistry.