Acute-phase protein (Acute Phase Reactants)

• Acute-phase protein is raised in inflammatory conditions.
• When there is an increase in a protein called positive acute-phase protein.
  • In the case of a decrease in the acute phase protein, it is called the negative phase protein.

Haptoglobin

Sample for Haptoglobin

1. A blood sample of the patient is needed. The serum is needed to perform the test.
2. Can store for 2 weeks at -20 °C.

Indications for Haptoglobin

1. This test is done to detect intravascular hemolysis.
2. It may be advised in acute or chronic inflammation.

Precautions for Haptoglobin

1. Avoid the hemolyzed sample.
2. Estrogen and oral contraceptives lower the haptoglobin value.
3. Steroid therapy increases the haptoglobin level.
4. Regular vigorous exercise lowers the haptoglobin value.

Definition of Haptoglobin

1. Haptoglobin is a glycoprotein synthesized in the liver.
2. This is an α-2 glycoprotein (globulin) and binds irreversibly free hemoglobin.
   1. This is present in the α-2 zone on electrophoresis. 
      1. The α2-globulin band contains haptoglobin, α2-macroglobulin, and ceruloplasmin.
   2. This is a free Hb binding protein.
   3. Haptoglobin, also called transport glycoprotein synthesized in the liver. It is a carrier for free hemoglobin in the blood.

Characteristic features of Haptoglobin

1. Transport the free hemoglobin through the blood to the liver for degradation.
   1. Free hemoglobin is bound to the α-chain of the haptoglobin.
2. Haptoglobin increases from the mean value of 0.02 g/L at birth to an adult level within the first year of life.
   1. It increases with increasing age.
3. Haptoglobin is measured by two methods:
   1. Total haptoglobin. It is measured by immunologic techniques.
      1. Total haptoglobin level decreases more slowly than the binding capacity.
      2. After the hemolysis attack, its level decreases after 6 to 8 hours.
   2. Haptoglobin-binding capacity. It can be measured by electrophoresis or chemical method.
      1. HP-binding capacity decreases immediately after the hemolysis or when free hemoglobin is released into the blood circulation and remains low for 2 to 4 days.
4. Estimation of the haptoglobin for the possibility of hemolysis is controversial.
5. Congenital absence of haptoglobin occurs roughly in 3% of African-Americans and about 1% Europeans.
6. Newborns, 80% to 90%, lack haptoglobin after the first day of birth until 1 to 6 months of age.
7. Haptoglobin is an acute-phase protein, increased in severe infection, acute myocardial infarction,  burns, damage to the tissues, and some cancer patients.

Synthesis of haptoglobin

1. This is synthesized in the liver and consists of four peptide chains (αβ)2 linked by a disulfide bond.
   1. One of the references says there is three possible alpha (α) chains and only one β-chain.

Haptoglobin structure

2. It binds free hemoglobin and is rapidly cleared from the blood in case of intravascular hemolysis.
   1. In the initial stage of hemolysis, the liver can compensate for the synthesis of HP, so its level is decreased.

Functions of haptoglobin

1. Haptoglobin increases in acute and chronic inflammation.
2. Haptoglobin-free and its complex (HP+Hb) play a significant role in the inflammatory process.
3. Haptoglobin is a natural bacteriostatic agent.
4. Haptoglobin synthesis is stimulated by inflammation and not by hemolysis.

Haptoglobin (HP) and Hemoglobin (Hb) complex role in inflammation and hemolysis

5. It is a natural bacteriostatic agent for iron-requiring bacteria, e.g., E.coli.
6. This acute-phase protein increased in ulcerative colitis, acute rheumatic diseases, and heart attacks.
7. Haptoglobin’s physiologic function is to preserve the iron in the body.
8. Haptoglobin prevents damage to renal tubules by the excreted hemoglobin.
9. Haptoglobin binds the hemoglobin and carries that complex to the reticuloendothelial system.
10. This helps differentiate hemolytic anemia from the other causes of anemia.
    

    Clinical condition	Haptoglobin level	Reticulocytes count
    Intravascular hemolysis	Increased	Increased
    Extravascular hemolysis	Normal (or mild decrease)	Increased
    Bone marrow depression	Normal	Not increased

11. Haptoglobin depletion is a sensitive marker for hemolysis.
12. HP is decreased in hemoglobinuria.
13. HP is unchanged in myoglobinuria and rhabdomyolysis because there is no binding with HP.

Haptoglobin’s role as acute-phase protein:

1. Haptoglobin synthesis is increased in acute inflammation, tissue necrosis, and malignancy.
2. It increases after 4 to 6 days of the inflammatory process.
3. It takes around 2 weeks to become normal after removing the stimulating factor (infection or inflammation).
4. The cytokine, especially IL-6, initiates ita synthesis.

Normal haptoglobin

• Newborn          =  5 to 48 mg/dL (50 to 480 mg/L). It may be absent at birth.
  • 6 months to 16 years = 25 to 138 mg/dL
• 16 to 60 years  = 15 to 200 mg/dL
• >60 years          =  35 to 175 mg/dL
• Another source
  • Adult = 20 to 220 mg/dL
  • Newborn = 0 to 10 mg/dL
  • Children = Adult level reach by one year
• Critical value  = <40 mg /dL

The increased haptoglobin level + ESR (Acute phase reactant) is seen in the following:

1. By corticosteroid hormones and nonsteroidal anti-inflammatory drugs.
2. In nephrotic syndrome.
3. In biliary obstruction in the absence of hepatocellular disease.
4. Ulcerative colitis.
5. Peptic ulcer.
6. Acute rheumatic disease.
7. In cancers.
8. In acute and chronic inflammatory diseases and it acts as an acute-phase protein.
9. In acute myocardial infarction.
10. In tissue destruction like burn and cancers.
11. Drugs that may increase the level are:
    1. Steroids.
    2. Androgens.

Increased haptoglobin level is seen in:

1. Acute and chronic infections or inflammation.
2. Tissue destruction.
3. Advanced lymphomas.
4. Malignant tumors.
5. Biliary obstruction.
6. Collagen diseases.

The decreased haptoglobin level is seen in the following:

1. Genetic deficiency.
2. In hemolytic anemia.
3. Systemic lupus erythematosus
4. Erythroblastosis fetalis.
5. Estrogen will decrease its synthesis.
6. Decreased with severe liver diseases.
7. There is a slight decrease in blood transfusion, even in the compatible blood transfusion, due to a few RBC hemolysis.
8. This may be decreased in hematoma because of the binding of Hb with HP.
9. Drugs that may decrease the level are:
   1. Chlorpromazine.
   2. Isoniazid.
   3. Oral contraceptives.
   4. Streptomycin.
   5. Quinidine.
   6. Nitrofurantoin.

• The critical value is <40 mg/dL.

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