What is the WIDAL test? How is it done?

Have you ever been to a doctor with a fever and he writes a test, something like a WIDAL test? May be he is suspecting Typhoid fever.Don't worry; it's a very simple test and easy to understand. 

But it is not that specific, if you have malaria, or Systemic Lupus Erythematosus (SLE), or a disease called infectious mononucleosis (IM), which is a fever caused by the Epstein-Barr virus (EBV), then also it can show positive.So the doctor may ask you to do this test in a gap of 2 weeks again to confirm the diagnosis.

The Widal test is primarily used for diagnosing typhoid fever caused by Salmonella enterica serotype Typhi and Paratyphi.

What is the WIDAL test?

The Widal test is a laboratory test used to detect antibodies against Salmonella enterica serotypes Typhi and Paratyphi, which cause typhoid fever and paratyphoid fever, respectively. It is basically an antibody-antigen reaction where we collect antigen from the patient and mix it with the antibodies. If the reaction occurs, then it agglutinates, thus the test is positive.

WIDAL test is of two types:

• Slide WIDAL test
• Tube WIDAL test(Kahn tubes)

WIDAL test

Principles of the WIDAL test:

If we take the cases of typhoid and paratyphoid typhoid fever,which are caused by Salmonella typhi and Salmonella paratyphi, respectively,. 

 

To understand the principle, let's 

Antigens present in bacteria:

There are three types of antigens present in the bacteria: Salmonella typhi and Salmonella paratyphi.

• O (somatic) antigens (To)- present in both Salmonella typhi and Paratyphi. 
• H (flagella) antigens (TH,AH, and BH) are present differently in Salmonella typhi and Paratyphi A and B.
• Vi(vaccination) Antigens are used in the formation of vaccines.

For diagnostic tests, we generally see H antigens, because by this antigen we can differentiate the bacteria. I am going to show you a chart and explain the process.

Chart of presence of anitigens in Salmonella Typhi and Paratyphi. 

In the chart you can see, To is present in both Typhi and Paratyphi, so it will be positive in both cases.

TH, which is present only in S. typhi, so it will be positive or agglutinate with TH antibodies in cases of typhoid fever.

AH,which is present in S. paratyphi A, so it will be positive or agglutinate with AH antibodies in the case of paratyphoid fever type A.

BH,which is present in S. paratyphi B, so it will be positive or agglutinate with BH antibodies in the case of paratyphoid fever type B.

The pathologist will take the antigens and test them with antibodies, either on a slide or in a test tube. As per this principle, the results will be shown, i.e., if we take the blood sample of the patient and we see the blood agglutinates in the TH antibody sample, he or she can conclude it is typhoid fever.

If you have taken the vaccine, you can be TH,AH, and BH positive.

A WIDAL test is generally done in the second week of a fever; if it is in the first week, then a blood culture may be enough for a diagnosis. But if it is the third or fourth week, then a sample of stool and urine may be required. Take proper consultation with the doctor to diagnose the disease.

How it is done?

To perform the Widal test:

1. Collect a blood sample from the patient.

2. Centrifuge the blood sample to separate the serum (liquid portion).

3. Dilute the serum using saline solution in a series of test tubes.

4. Add standardized antigens derived from Salmonella typhi and paratyphi A and B to each tube.

5. Incubate the tubes at a specific temperature for a set period.

6. Examine the tubes for agglutination, which indicates the presence of antibodies reacting with the antigens.

7. Interpret the results based on the degree of agglutination and the dilution factor to determine the antibody titer.

 These are proper procedures used by the pathologist to perform WIDAL test.

The WIDAL test is very popular due to its easy availability and sensitivity but it is not that specific, so there are some new tests present in the market that can specify the disease, i.e.

• Card-based tests
• Typhidot-IgM/IgG against outer membrane proteins. 
• Dot blot IgG against flagellar membrane protein antigens.

If you want to know more about this type of tests,and diseases,  please check out my other blogs and follow me. Do let me know, if you have any suggestions or complaints on my contact us page. 

Thank you