Chapter 30: Common Serological Tests, Widal test, RA factor, ASO titer, and C-Reactive Protein
Some of the common serological tests are discussed.
Principle: This test is advised for antibodies in patients serum against Enteric bacilli for the diagnosis of enteric or typhoid fever.
The serum of patients is needed.
- Qualitative or
This can be done by:
- Slide method or
- Tube method
Salmonella groups A, B, C, D, and E and Paratyphi A, B, C.
Typhi H antigens are:
- O-Antigen (Somatic Antigen):- These are 0-17 groups. 95% fall in group A, B, C, D, and E.
- O-Ag rises in 50% of the cases by the first week. It disappears in 6-12 months. This is diagnostic for acute infection.
- H-Antigen (Flagellar Ag).
- H-Ag rises slowly and disappears after many years.
- V1-Virulence antigen. This is also a surface antigen. this is used to find carrier state.
- No value if done before 7 days of the onset of fever. Two or preferably more tests are done every 3-5 days to see rising titer.
- Many people without the disease have agglutinin in low titer.
- The immunized person will also show agglutinin (antibodies).
- There will be a significant rise after 7-10 days.
- The anamnestic reaction will show the negative results in low dilution and will be positive in high dilution.
- The test has to be done with a battery of antigens.
- The slide test is good for screening but confirmation should be done by tube method.
- The history of patients and discussions with physicians will be helpful.
- A single test is not diagnostic.
- In a vaccinated case antibody may be present. So a significant level is a four-fold rise in O-Ag or at least O Ag is > 1:80.
- Antibiotic use prevents the rise in antibody levels.
- When there is no agglutination
- Blood was drawn early before 7 days.
- A negative test does not rule out an enteric fever.
- Indicate infection.
- In-person with a history of vaccination.
- A high titer of H-Ag indicates disease or vaccine.
Rheumatoid Factor (RA Factor)
Rheumatoid factor is used to diagnose Rheumatoid Arthritis.
This factor consists of a group of immunoglobulin reacting with the Fc region of IgG. This may be IgM, or IgG, or IgA.
RA factor is present in:
- Joint fluid
It first appears in the joint fluid than in the blood.
These antibodies “RA” factor is identified by agglutination reaction.
- Haemagglutination: When sheep RBC are coated with IgG-Ab are used. Now add serum of patients and this will give agglutination.
- Latex particles are coated with IgG and then serum is added and this will show agglutination.
- 70-90% of the cases are positive for rheumatoid arthritis..
- A negative result does not rule out rheumatoid arthritis.
- The false-positive test may be seen in:
- 71% Rheumatic fever.
- 30-40% SLE.
- 12% Gout
- RA factor may be positive in tuberculosis & syphilis.
- False Positive may also be seen in old age, liver disease, SBE, chronic lung disease & syphilis.
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.
Patient serum with dilution is mixed with a fixed amount of streptolysin O- Ag & Ab reaction takes place.
Now add Ab-coated RBC. There will be hemolysis in the tube, where streptolysin O is free.
The result is reciprocal of the highest dilution where hemolysis starts. This is described as Todd units. Other units are international units (IU).
Positive ASO indicates:
- Rheumatic fever.
- Acute glomerulonephritis.
- Erythema nodosum.
- Useful for differentiation of rheumatic fever/rheumatoid arthritis.
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.
CRP is α-globulin and found in various inflammatory diseases. This is also called as acute-phase protein.
Serum of the patient (CRP) + Somatic C polysaccharide of pneumococci mixed and gives rise to a precipitate.
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
Causes of CRP:
- Produced in various bacterial diseases.
- Produced by injured myocardial muscle in myocardial infarction.
- 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:
- Sydenham’s Chorea.
- Myocardial infarction and negative in angina.
- A number of malignancies.
- Rheumatoid arthritis.
- Viral infection like Viral Hepatitis.
- Bacterial Pneumonia.
- Active Tuberculosis.
- Lepromatous Leprosy.
- Acute Tonsillitis, Scarlet fever, and Mumps.
Advantage of CRP over ESR
- Raised ESR may be seen even without the presence of fever in anemia, pregnancy, nephrotic syndrome, and hypogammaglobulinemia.
- ESR may be normal in Frank’s active rheumatic fever.
CRP detection is valuable for:
- Low grade and questionable rheumatic fever.
- Follow-up and treatment of rheumatic fever.
- The differential diagnosis of coronary insufficiency (angina) and myocardial infarction