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Chemistry

Result Interpretation

Interpreting heme derivative test results requires integrating lab values with the patient’s clinical presentation and medical history. Deviations from reference intervals can indicate various conditions

  • Key Tests
    • Bilirubin (Total, Direct, Indirect)
    • Urine Bilirubin
    • Urine Urobilinogen
    • Iron Studies (Serum Iron, TIBC, Transferrin Saturation, Ferritin)
    • Porphyrin Analysis (Urine, Blood, Feces)

Bilirubin

  • Total Bilirubin
    • Reference Interval: 0.3-1.0 mg/dL (5.1-17.1 μmol/L)
    • Interpretation
      • Elevated levels indicate hyperbilirubinemia
  • Direct (Conjugated) Bilirubin
    • Reference Interval: 0.0-0.3 mg/dL (0-5.1 μmol/L)
    • Interpretation
      • Elevated levels suggest biliary obstruction or liver disease
  • Indirect (Unconjugated) Bilirubin
    • Reference Interval: 0.2-0.8 mg/dL (3.4-13.7 μmol/L)
    • Interpretation
      • Elevated levels suggest increased red blood cell breakdown (hemolysis) or impaired liver uptake/conjugation
  • Clinical Significance
    • Elevated total bilirubin with increased indirect bilirubin suggests pre-hepatic (hemolytic) jaundice
    • Elevated total bilirubin with increased direct bilirubin suggests hepatic or post-hepatic (obstructive) jaundice
    • Isolated elevation of indirect bilirubin can be seen in Gilbert’s syndrome

Urine Bilirubin

  • Reference Interval: Negative
  • Interpretation
    • Positive result indicates conjugated hyperbilirubinemia
  • Clinical Significance
    • Suggests biliary obstruction or liver disease (conjugated bilirubin is water-soluble and can be excreted in urine)
    • Absent in pre-hepatic jaundice (unconjugated bilirubin is not water-soluble)

Urine Urobilinogen

  • Reference Interval: 0.2-1.0 mg/dL (3.4-16.9 μmol/L)
  • Interpretation
    • Increased levels suggest increased red blood cell breakdown or liver disease
    • Decreased or absent levels suggest biliary obstruction
  • Clinical Significance
    • Elevated in hemolytic anemia, hepatitis, and early stages of biliary obstruction
    • Decreased or absent in complete biliary obstruction (e.g., gallstones, tumors)

Iron Studies

  • Serum Iron
    • Reference Interval: 60-170 μg/dL (10.7-30.4 μmol/L)
    • Interpretation
      • Decreased levels suggest iron deficiency
      • Elevated levels suggest iron overload
  • Total Iron-Binding Capacity (TIBC)
    • Reference Interval: 240-450 μg/dL (42.9-80.6 μmol/L)
    • Interpretation
      • Elevated levels suggest iron deficiency
      • Decreased levels suggest iron overload or chronic inflammation
  • Transferrin Saturation
    • Reference Interval: 20-50%
    • Interpretation
      • Decreased levels suggest iron deficiency
      • Elevated levels suggest iron overload
  • Ferritin
    • Reference Interval: 20-200 ng/mL (women), 20-500 ng/mL (men)
    • Interpretation
      • Decreased levels are a sensitive indicator of iron deficiency
      • Elevated levels suggest iron overload, inflammation, or malignancy
  • Clinical Significance
    • Iron deficiency anemia: Decreased serum iron, ferritin, and transferrin saturation; increased TIBC
    • Iron overload (Hemochromatosis): Increased serum iron, ferritin, and transferrin saturation; decreased TIBC
    • Anemia of chronic disease: Decreased serum iron and TIBC; normal or increased ferritin

Porphyrin Analysis

  • Interpretation
    • Elevated levels of specific porphyrins and porphyrin precursors in urine, blood, or feces indicate porphyria
  • Clinical Significance
    • Different types of porphyria are associated with specific enzyme deficiencies and accumulation of specific porphyrins
    • Accurate identification of the specific porphyrins is essential for diagnosis and management

Fecal Occult Blood Test (FOBT)

  • Interpretation
    • Positive result indicates the presence of blood in the stool
  • Clinical Significance
    • Suggests gastrointestinal bleeding, which may be due to:
      • Colorectal cancer
      • Polyps
      • Ulcers
      • Inflammatory bowel disease
      • Other gastrointestinal conditions
    • Positive FOBT requires further investigation with colonoscopy or other diagnostic procedures

Factors Affecting Interpretation

  • Medications: Certain medications can affect bilirubin and iron levels (e.g., rifampin, phenytoin, oral contraceptives, iron supplements)
  • Hemolysis: Can falsely elevate serum iron levels and affect bilirubin assays
  • Diet: Iron-rich foods can affect serum iron levels
  • Liver and Kidney Function: Can affect bilirubin metabolism and excretion
  • Genetic Factors: Can influence iron absorption and storage
  • Age and Gender: Reference intervals may vary with age and gender

Key Terms

  • Bilirubin: A yellow pigment produced from the breakdown of heme
  • Hyperbilirubinemia: Elevated bilirubin levels in the blood
  • Urobilinogen: A colorless compound produced in the intestine by bacterial reduction of bilirubin
  • Serum Iron: A measure of iron in the blood
  • Total Iron-Binding Capacity (TIBC): A measure of the blood’s capacity to bind iron
  • Transferrin Saturation: The percentage of transferrin that is bound to iron
  • Ferritin: A protein that stores iron
  • Porphyrins: Intermediates in heme synthesis
  • Fecal Occult Blood Test (FOBT): A test to detect blood in stool
  • Jaundice: Yellowing of the skin and eyes due to elevated bilirubin
  • Anemia: A condition characterized by a deficiency of red blood cells or hemoglobin
  • Porphyria: A genetic disorder caused by deficiencies in enzymes of the heme synthesis pathway
  • Hemolysis: The breakdown of red blood cells
  • Pre-hepatic Jaundice: Jaundice due to increased red blood cell breakdown
  • Hepatic Jaundice: Jaundice due to liver dysfunction
  • Post-hepatic Jaundice: Jaundice due to obstruction of bile flow
  • Gilbert’s Syndrome: A common genetic disorder affecting bilirubin metabolism