Canlit Clinic
Kitgum Municipality, Uganda
(Rev. Canlit Memorial Medical Clinic)
Phone / Whatsapp: +256783096734

Widal test

The Widal test is an indirect agglutination test for enteric fever or undulant fever whereby bacteria causing typhoid fever is mixed with a serum containing specific antibodies obtained from an infected individual. In cases of Salmonella infection, it is a demonstration of the presence of O-soma false-positive result. Test results need to be interpreted carefully to account for any history of enteric fever, typhoid vaccination, and the general level of antibodies in the populations in endemic areas of the world.

Widal Test is an agglutination test which detects the presence of serum agglutinins (H and O) in patients serum with typhoid and paratyphoid fever. When facilities for culturing are not available, the Widal test is the reliable and can be of value in the diagnosis of typhoid fevers in endemic areas.

Typhoid fever and paratyphoid fever cause similar symptoms. People with these diseases usually have a fever that can be as high as 103 to 104°F (39 to 40°C). They also may have weakness, stomach pain, headache, diarrhea or constipation, cough, and loss of appetite. Some people have a rash of flat, rose-colored spots.

Widal test for Typhoid fever and paratyphoid fever
(Typhoid Fever Salmonella Typhi)

It's caused by a bacterium called Salmonella typhi, which is related to the bacteria that cause salmonella food poisoning. Typhoid fever is highly contagious. An infected person can pass the bacteria out of their body in their poo or, less commonly, in their pee.

Principle of Widal Test

Bacterial suspension which carry antigen will agglutinate on exposure to antibodies to Salmonella organisms. Patients’ suffering from enteric fever would possess antibodies in their sera which can react and agglutinate serial doubling dilutions of killed, colored Salmonella antigens in a agglutination test.

(Interpretation of Widal Test Slide Method)
(Interpretation of Widal Test Slide Method)

The main principle of widal test is that if homologous antibody is present in patients serum, it will react with respective antigen in the reagent and gives visible clumping on the test card and agglutination in the tube. The antigens used in the test are "H" and "O" antigens of Salmonella Typhi and "H" antigen of S. Paratyphi. The paratyphoid "O" antigen are not employed as they cross react with typhoid "O" antigen due to the sharing of factor 12. "O" antigen is a somatic antigen and "H" antigen is flagellar antigen.

(Interpretation of Widal Test Tube Method)
(Interpretation of Widal Test Tube Method)

The patient’s serum is tested for O and H antibodies (agglutinins) against the following antigen suspensions (usually stained suspensions):

Salmonella antibody starts appearing in serum at the end of first week and rise sharply during the 3rd week of endemic fever. In acute typhoid fever, O agglutinins can usually be detected 6–8 days after the onset of fever and H agglutinins after 10-12 days.

It is preferable to test two specimens of sera at an interval of 7 to 10 days to demonstrate a rising antibody titre. Salmonella antigen suspensions can be used as slide and tube techniques.


Canlit Clinic
(Rev. Canlit Memorial Medical Clinic)
Kitgum Municipality, Uganda
Last modified on Wednesday, 22-Feb-2023 06:24:46 CET
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