Chapter 31: Common serological test, Antistreptolysin O (ASO), C-Reactive Protein (CRP)

Antistreptolysin O (ASO)

ASO titer is advised for the diagnosis of rheumatoid fever. The Streptolysin O(SO) is antigenic and cause hemolysis of RBCs. So there is antibody formation (Antistreptolysin-Ab) in the blood.

Principle:

Patient serum with dilution is mixed with a fixed amount of streptolysin O- Ag and  Abreaction take place.

Now add Ab-coated RBC. There will be hemolysis in the tube, where streptolysin O is free.

Antistreptolysin (ASO) leading to hemolysis

The result is reciprocal of the highest dilution where hemolysis starts. These are described as Todd units. Other units are international units (IU).

Antistreptolysin (ASO) titer procedure

Positive ASO indicates:

1. Rheumatic fever.
2. Acute glomerulonephritis.
3. Erythema nodosum.
4. Useful for differentiation of rheumatic fever/rheumatoid arthritis.

Normal

0-125 Todd units.

Definite value    =          400 or more.

Rising titer from 50-250 is significant.

While persistent low level rules out a rheumatic fever.

C – reactive protein (CRP)

This was recognized in 1930.

Definition

1. CRP is γ-globulin (found in the γ-region) is found in various inflammatory diseases. This is also called acute-phase protein.
2. CRP is produced in the liver.
3. CRP name is derived from the reaction with streptococcal capsular (C) polysaccharide.
   

   C-Reactive Protein (CRP)

Principle:

1. CRP is opsonin and it activates the complement system and ultimately leads to lysis.

C-reactive protein (CRP) leads to lysis by the activation of complement

C-reactive protein (CRP) role in Complement activation

2. Serum of the patient (CRP) + Somatic C polysaccharide of pneumococci mixed and gives rise to a precipitate.
3. Practically CRP is injected into the rabbit when the anti-CRP antibody is produced. Now take serum of the patient (CRP) + Mix anti-CRP. This will give precipitation.

                                                                           CRP + Anti CRP = Precipitation

C-Reactive Protein (CRP) formation

Causes of CRP:

1. Produced in various bacterial diseases.
2. Produced by injured myocardial muscle in myocardial infarction.
3. Positive in acute and chronic rheumatic fever. This is a reliable and sensitive indicator of rheumatic fever. Its absence rules out a rheumatic fever.

CRP is also positive in:

1. Sydenham’s Chorea.
2. Myocardial infarction and negative in angina.
3. Many malignancies.
4. Rheumatoid arthritis.
5. Gout
6. Viral infection like Viral Hepatitis.
7. Bacterial Pneumonia.
8. Active Tuberculosis.
9. Lepromatous Leprosy.
10. Acute Tonsillitis, Scarlet fever, and Mumps.

Advantage of CRP over ESR

1. Raised ESR may be seen even without the presence of fever in anemia, pregnancy, nephrotic syndrome, and hypogammaglobulinemia.
2. ESR may be normal in Frank’s active rheumatic fever.

CRP detection is valuable for:

1. Low grade and questionable rheumatic fever.
2. Follow-up and treatment of rheumatic fever.
3. The differential diagnosis of coronary insufficiency (angina) and myocardial infarction