What is the Widal Test?
The Widal test is a serology-based blood test that detects antibodies against Salmonella Typhi or Paratyphi bacteria in the body. This test was first introduced in 1896 by Georg Ferdinand Widal.It mainly detects O and H antibodies and indicates whether the patient is currently infected with typhoid.
What is typhoid fever and its symptoms ?
Typhoid fever is a type of enteric fever that is spread through contaminated food or water.
Common symptoms of typhoid fever
1.Persistent fever
2.Headache
3.Diarrhea or constipation
4.Stomach pain
5.Fatigue and weight loss
6.Rose spots on the skin
If left untreated, serious complications such as intestinal bleeding or even intestinal perforation can occur.
What is Principle of Widal Test ?
When you eat food contaminated with Salmonella bacteria, the germs enter your body carrying antigens. Once they reach your digestive system, your immune system responds by producing antibodies against those antigens. These antibodies then react with the antigens, causing agglutination (clumping). This reaction is the basis for detecting typhoid fever.
The Widal test works on this principle: if your blood serum contains specific antibodies against Salmonella, they will react with the corresponding antigens and form visible clumps on the test card.
After this initial reaction, the next step of the test is to measure the antibody titre — that is, the highest dilution of serum at which agglutination is still visible.
What is Preparation of Widal Test ?
Slide Widal Test – Requirements
To perform a Slide Widal Test, the following materials are needed:
1.Patient’s serum sample
2.Pipette (for transferring serum)
3.Antigen reagents:
O antigen (somatic/surface antigen)
H antigen (flagellar antigen)
AH antigen
BH antigen
4.Clean glass slide
5.Mixing sticks
6.Stopwatch (to note reaction time)
These reagents are essential to detect the presence of specific antibodies in the patient’s serum, which will react with the respective antigens (O, H, AH, BH) and produce visible agglutination.
What is the Widal Test Procedure?
Widal Test Procedure
The Widal test is carried out in two steps:
1. Qualitative Widal Test
This step is done using a slide with six reaction circles labeled: O, H, AH, BH, PC (positive control), and NC (negative control).
Procedure:
Place one drop of patient’s serum in the O, H, AH, and BH circles.
Add one drop of positive control serum in the PC circle and one drop of negative control in the NC circle.
Add one drop of antigen reagent to each respective circle:
O antigen → O circle
H antigen → H circle
AH antigen → AH circle
BH antigen → BH circle
Add the same antigen (O, H, AH, or BH) to the PC and NC circles.
Mix the serum and antigen gently within each circle using a mixing stick, making sure mixtures do not spill over or touch each other.
Slowly rotate the slide in a circular motion to ensure proper mixing.
Interpretation:
Positive: Agglutination appears (similar to the Positive Control).
Negative: No agglutination (similar to the Negative Control).
If the test is positive, proceed to the Quantitative Widal Test using the antigen(s) that showed reactivity. For example, if O antigen showed clumping, the O antigen will be used in the next step.
In most cases, O and H antigens (S. Typhi) are tested further, while AH or BH are rarely positive.
2. Quantitative Widal Test
This step helps to determine the antibody titre in the patient’s serum.
Procedure:
Take a clean slide with eight reaction circles – four for O antigen and four for H antigen.
If O antigen was positive in the qualitative test, place:
5 µl of serum in the 1st O circle
10 µl in the 2nd circle
20 µl in the 3rd circle
40 µl in the 4th circle (in a horizontal sequence).
Repeat the same steps for H antigen (if positive).
Add one drop of the corresponding antigen reagent to each circle.
Gently mix and rotate the slide for proper reaction.
Reporting:
Label the circles from right to left as 1:40, 1:80, 1:160, and 1:320.
The highest dilution of serum that still shows visible agglutination is reported as the antibody titre.
Summary:
Qualitative test → Detects presence/absence of antibodies.
Quantitative test → Determines the antibody titre, which helps confirm typhoid fever.
What are the limitations of the Widal test?
Previous infections or vaccinations can cause false results.
Dengue, malaria, liver disease, etc. can also cause false results.
Blood culture is the most reliable test to confirm this.
How much does the Widal test cost?
The cost of the Widal test usually ranges between ₹200 – ₹500 (may vary by region and lab).
