Widal Test (Slide Agglutination)

Description

What is Widal Test (Slide Agglutination)?

Widal Test (Slide Agglutination) test helps to detect typhoid fever and paratyphoid fever – collectively known as enteric fever, caused by the consumption of food or water contaminated by Salmonella bacteria. The symptoms include high fever, malaise, headache, constipation or diarrhoea, rose-colored spots on the chest, and enlarged spleen & liver.

This test helps to detect antibodies in a blood sample against two antigens (O & H) of bacteria. It should only be performed after a week or two for accurate results.

Please consult your health care practitioner to get an accurate diagnosis based on the symptoms, medical history and test results.

What does Widal Test (Slide Agglutination) Measure?

The Widal test measures the titres of antibody against the bacteria which cause Enteric fever.

Typhoid and paratyphoid fever are generally acquired when you consume food or water, contaminated by feces of an acutely infected or convalescent person (recovering from disease) or a chronic, asymptomatic carrier. The incubation period (the time interval between exposure to an infection and the appearance of the first symptoms) of Enteric fever is 6-30 days. Paratyphoid fever is similar but often less severe than typhoid fever.

Widal test is an agglutination test to detect antibodies (agglutinins) in a blood sample against two antigens (O & H) of bacteria, Salmonella enterica. Agglutination refers to the visible clumping of particles when a particulate antigen combines with its antibody in the presence of optimum conditions for antigen-antibody reaction. When this test is performed on a slide, it is called Slide agglutination and when it is carried out in a test tube,  it is called Tube agglutination. Widal test by Tube agglutination is recommended over Slide agglutination method. The antigens used in the test are “H” and “O” antigens of Salmonella Typhi and “H” antigen of S. Paratyphi.

Widal test should only be performed after the first week. The reason being the antibody against “O” and “H” antigens of Salmonella start appearing in serum at the end of the first week of fever. It is preferable to test two blood samples at an interval of 7 to 10 days to demonstrate rising antibody titres.

Interpreting Widal Test (Slide Agglutination) results

Interpretations

If the antibody against Salmonella is present in the patient’s serum, it will react with the respective antigen in the reagent and give visible agglutination (clumping) on the test slide.

The highest dilution of the patient’s serum sample that gives a visible agglutination with the Widal test antigen suspensions is the antibody titre.

The serum sample which shows the titre of 1:80 or more for O agglutination and 1:160 or more for H agglutination indicates active infection. A  rise in titres (4-fold or more) in the two blood samples taken 10 days apart confirms the diagnosis. Please note that different labs may have different reference values for the interpretation of the Widal test.

The interpretation of a Widal test is greatly influenced by the nature and extent of the patient’s previous contact with typhoid antigens. It can be either due to clinical or subclinical infection with typhoid or related organisms or from typhoid vaccination. The lesser the extent of the previous contact, the greater is the usefulness of the Widal test.