Overview of Widal Test (Slide Agglutination)
What is Widal Test (Slide Agglutination)?
Preparation for Widal Test (Slide Agglutination)
- No special preparation required
Why Get Tested for Widal Test (Slide Agglutination)?
To diagnose Enteric fever (Typhoid and Paratyphoid fever).
Understand more about Widal Test (Slide Agglutination)
Enteric fever is caused by bacterial infection and it is of two types:
Typhoid fever - It is caused by Salmonella enterica serotype Typhi (S. Typhi).
Paratyphoid fever - It is caused by Salmonella enterica serotypes Paratyphi A (S. Paratyphi A), Paratyphi B (S. Paratyphi B) or Paratyphi C (S. Paratyphi C).
Typhoid and paratyphoid fever are most often acquired through consumption of food or water that has been 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.
Typhoid fever has a gradual onset, with increasing fatigue and a fever that increases daily from low-grade to as high as 102°F–104°F (38°C–40°C) by the third to fourth day of illness (Stepladder fever pattern). This hallmark Step ladder fever pattern is seen in a few cases these days. Headache, weakness, and loss of appetite are seen in nearly all cases, and abdominal pain, diarrhea, or constipation are common. A transient rash of rose-colored spots can occasionally be seen on the trunk (Rose spots). The disease can last for a month, if untreated. Early diagnosis and treatment is important as life-threatening complications of typhoid fever generally occur after 2–3 weeks of illness and may include intestinal bleeding or perforation.
Paratyphoid fever is similar but often less severe disease than typhoid fever.
Widal test is an agglutination test that detects 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 the test is called Tube agglutination when it is carried out in a test tube. 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.
As antibody against “O” and “H” antigens of Salmonella starts appearing in serum at the end of the first week of fever, Widal test should only be performed after the first week. It is preferable to test two blood samples at an interval of 7 to 10 days to demonstrate rising antibody titres.
What Results of Widal Test (Slide Agglutination) mean?
If the antibody against Salmonella is present in patients serum, it will react with the respective antigen in the reagent and gives visible agglutination (clumping) on the test slide.
The highest dilution of the patient 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. Demonstration of a rise in titres (4-fold or more) in the two blood samples taken 10 days apart confirms the diagnosis (Different labs may have different reference values for the interpretation of 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 a previous contact, the greater is the usefulness of the Widal test.
Patient Concerns about Widal Test (Slide Agglutination)
Frequently Asked Questions about Widal Test (Slide Agglutination)