Widal Agglutination Test

The Widal test is a serological agglutination test used to detect antibodies produced against Salmonella antigens in a patient's serum. It mainly measures agglutinins against the O, somatic, and H, flagellar antigens of Salmonella Typhi, with additional antigens for Salmonella Paratyphi.

Principle of the test

The test is based on a specific antigen-antibody reaction. When serum containing Salmonella agglutinins is mixed with standardized suspensions of corresponding antigens, antibodies cross-link the antigen-bearing bacteria and produce visible clumping, or agglutination.

• The principal antigen suspensions are Typhi O, TO, and Typhi H, TH; Paratyphi A and B flagellar antigens may also be tested.
• O agglutination is generally granular and compact, whereas H agglutination forms larger, loose floccules.

Measurement and interpretation

The patient's serum is tested in serial dilutions. The antibody titre is the highest serum dilution that still shows visible agglutination.

• A fourfold or greater rise in titre between appropriately collected paired serum samples provides stronger evidence of recent infection than a single titre.
• A single titre must be interpreted against locally established baseline titres and the patient's clinical findings.

Diagnostic limitations

The Widal test does not independently confirm typhoid because antibodies may reflect previous infection, vaccination or cross-reacting organisms. Testing too early may also be negative because detectable antibodies have not yet developed.

• Background antibodies in endemic populations reduce the reliability of a single-sample Widal test.
• Isolation of the organism, especially through blood culture, provides more direct evidence of enteric fever.