What does haptoglobin measure?

Haptoglobin Measurement and Clinical Significance

Haptoglobin is a blood plasma glycoprotein that primarily functions to bind free hemoglobin released during hemolysis, making it a valuable marker for detecting hemolytic conditions. 1, 2

Function and Mechanism

Haptoglobin serves several critical functions in the body:

• Primary function: Binds free hemoglobin released from lysed red blood cells, preventing its toxic effects 1
• Tissue protection: Prevents oxidative damage by removing free hemoglobin from circulation 2
• Prevents renal injury: By binding free hemoglobin, it prevents kidney damage and iron loss following hemolysis 3
• Immunomodulation: Acts as an immunosuppressor of lymphocyte function and modulates helper T-cell balance 3

Clinical Utility

Haptoglobin measurement is primarily used for:

1. Detection of hemolysis: Decreased haptoglobin levels indicate intravascular hemolysis as haptoglobin becomes depleted when binding large amounts of free hemoglobin 1
2. Acute phase reactant: Haptoglobin increases during inflammatory conditions 2
3. Assessment of hemolytic anemias: Helps differentiate hemolytic from non-hemolytic causes of anemia 4

Interpretation of Results

• Low or undetectable levels: Strongly suggestive of hemolysis
• Normal or elevated levels: May indicate absence of hemolysis or could represent an acute phase response masking underlying hemolysis 1
• Reference range: Typically 30-200 mg/dL (varies by laboratory method)

Important Considerations

Factors Affecting Measurement

• Recent transfusions: Evidence suggests that serum haptoglobin can still be used to diagnose hemolysis in patients receiving multiple units of packed red blood cells 5

• Age considerations: Decreased haptoglobin is only useful as a marker of hemolysis after 6 months of age 4

• False positives (falsely low haptoglobin without hemolysis):

  • Improper specimen preparation
  • Cirrhosis
  • Elevated estrogen states
  • Hemodilution 1

• False negatives (normal haptoglobin despite hemolysis):

  • Hypersplenism
  • Medications (androgens, corticosteroids)
  • Concurrent inflammatory conditions 1

Haptoglobin Phenotypes

Three major haptoglobin phenotypes exist with different biological activities:

• Hp 1-1: Most effective in binding free hemoglobin and suppressing inflammatory responses
• Hp 2-2: Least biologically active
• Hp 2-1: Moderately active 3

These phenotypes may be associated with predisposition to various diseases, including cardiovascular conditions 3, 6.

Alternative or Complementary Tests

When evaluating for hemolysis, consider additional tests:

• Complete blood count
• Peripheral blood smear
• Reticulocyte count
• Bilirubin (indirect)
• LDH (lactate dehydrogenase)
• Urinary hemosiderin

Summary

Haptoglobin measurement is a valuable tool in the diagnostic workup of suspected hemolytic conditions. Its primary clinical utility is in detecting intravascular hemolysis, where levels become depleted as haptoglobin binds to free hemoglobin released from lysed red blood cells. While several factors can affect interpretation, it remains an important component of hemolysis evaluation when considered alongside clinical context and other laboratory parameters.