What is haptoglobin and how is it used as a marker of intravascular hemolysis?

What is Haptoglobin and Its Role as a Hemolysis Marker

Haptoglobin is a plasma protein synthesized by the liver that binds free hemoglobin released during red blood cell destruction, and decreased haptoglobin levels serve as a key diagnostic marker for intravascular hemolysis. 1, 2

Biochemical Function

Haptoglobin binds to the globin portion of free hemoglobin with extraordinarily high affinity—once formed, this protein-protein association is essentially irreversible. 3 The primary physiological roles include:

• Prevention of oxidative damage: By binding free hemoglobin, haptoglobin prevents heme-catalyzed free radical production that would otherwise cause tissue injury 3, 4
• Prevention of iron loss: The haptoglobin-hemoglobin complex inhibits glomerular filtration of hemoglobin, preserving iron stores 3, 4
• Rapid clearance: The haptoglobin-hemoglobin complex is removed from circulation quickly by the reticuloendothelial system 2, 5

Diagnostic Application in Hemolysis

When to Order Haptoglobin

Haptoglobin should be measured immediately in any patient presenting with anemia plus thrombocytopenia, along with indirect bilirubin and LDH, to identify hemolytic conditions, atypical hemolytic uremic syndrome (aHUS), and delayed hemolytic transfusion reactions. 1

Haptoglobin is part of the first-level investigation panel for suspected hemolysis, which includes: 6, 1

• Complete blood count
• RBC morphology/peripheral smear
• Reticulocyte count
• LDH
• Unconjugated (indirect) bilirubin
• Haptoglobin

Interpretation of Results

Decreased or absent haptoglobin is a reliable marker of accelerated red cell destruction and chronic hemolytic anemia. 6, 1 The mechanism is straightforward: during hemolysis, free hemoglobin saturates and depletes circulating haptoglobin, causing levels to fall. 2

The combination of elevated LDH and decreased haptoglobin is specific for hemolysis, as LDH alone can be elevated in liver disease, myocardial infarction, kidney disease, and muscle damage. 7 This diagnostic triad should also include elevated indirect bilirubin for confirmation. 7, 8

Critical Pitfalls and Limitations

False Positive Results (Inappropriately Low Haptoglobin)

Be aware that decreased haptoglobin can occur without clinically significant hemolysis in: 7, 2

• Patients with mechanical heart valves (baseline low-grade hemolysis)
• Cirrhosis/advanced liver disease (decreased hepatic synthesis)
• Elevated estrogen states (pregnancy, oral contraceptives)
• Hemodilution
• Improper specimen preparation (in vitro hemolysis)

False Negative Results (Inappropriately Normal Haptoglobin)

Haptoglobin may remain normal despite hemolysis in: 2

• Acute inflammation or infection (haptoglobin is a positive acute phase reactant and may be elevated despite ongoing hemolysis)
• Hypersplenism
• Medications: androgens, corticosteroids

Age-Specific Considerations

In infants under 6 months of age, decreased haptoglobin has limited diagnostic utility and should be interpreted with extreme caution. 1 Alternative markers such as carboxyhemoglobin evaluation are more appropriate in this age group. 1

Effect of Blood Transfusion

Serum haptoglobin can be reliably measured to assess hemolysis even in recently transfused patients receiving multiple units of packed red blood cells. 9 Studies show no significant difference in haptoglobin levels before, immediately after, or 24-48 hours post-transfusion in patients transfused for non-hemolytic indications. 9

Haptoglobin Phenotypes

Three main phenotypes exist (Hp1-1, Hp2-1, Hp2-2) based on genetic polymorphisms, which may influence disease susceptibility and can cause characteristic splits in the alpha-2 globulin zone on protein electrophoresis. 5 However, phenotype determination is not routinely necessary for clinical diagnosis of hemolysis. 5

Integration into Diagnostic Algorithm

After confirming hemolysis with decreased haptoglobin, elevated LDH, and elevated indirect bilirubin: 1, 8

1. Perform direct antiglobulin test (DAT/Coombs) to differentiate immune from non-immune hemolysis 8
2. Examine peripheral blood smear for schistocytes (microangiopathic hemolysis), spherocytes (hereditary spherocytosis, autoimmune hemolytic anemia), or other morphologic abnormalities 6, 1, 8
3. If DAT positive: evaluate for autoimmune hemolytic anemia, drug-induced hemolysis, lymphoproliferative disorders 8
4. If DAT negative: consider hereditary causes (membranopathies, enzymopathies like pyruvate kinase or G6PD deficiency, hemoglobinopathies) or non-immune acquired causes (thrombotic microangiopathy, paroxysmal nocturnal hemoglobinuria) 8

Emerging Therapeutic Role

Plasma-derived haptoglobin has been identified as a potential therapeutic intervention to prevent deleterious consequences of excessive hemolysis, with evidence showing it reduces plasma-free hemoglobin levels and may preserve kidney function without adverse events. 1