WIDAL TEST (SLIDE AGGLUTINATION)
WIDAL AGGLUTINATION - SLIDE
1)Sera from normal individuals may show agglutination in dilutions up to 1:40
2)Agglutinin titre greater than 1:80 is considered significant and suggests infection, whereas low titres are found in normal individuals.
3)There should be a four fold rise in titre between two serum samples collected at the acute phase and the convalescent phase.
Note :
1. Individuals vaccinated with typhoid vaccine (TAB) may show moderately elevated titre of all three 'H' antibodies.
2. Repeated subclinical infection may give high titres due to previous antibodies.
3. Treatment with antibiotic such as chloramphenicol before the test gives false negative result for 'O' agglutinin. In that case diagnosis should be based on the significant elevation of 'H' agglutinin in the paired sera.
4. Patients of chronic active liver disease may give high titre due to failure of antigens in discriminating the specific antibodies from the dysglobulinaemia of chronic active liver disease.
5. Infection with many non-Salmonella organisms like Malaria, Dengue, Miliary Tuberculosis, Endocarditis, Brucellosis, Influenza etc. may give anamnestic response.
6. Potential carriers of the disease exhibit negative result due to high antibody concentration.
7. Immunological disorders such as Rheumatoid Arthritis, Rheumatic fever or Nephritic Syndrome demonstrate high titre of 'O' and 'H'agglutinins.
8. Narcotic addicts demonstrate non-specific activity to the Widal test.
9. VI antigen may block the 'O' antigen binding to 'O' antibody, leading to false negative results.
10. In endemic areas people usually show moderately elevated level 'O' and 'H' agglutinins
