Introduction:
Widal test is a serological test used for the detection of Salmonella enterica antigens in the blood of a patient. This test helps in the diagnosis of typhoid fever, which is a severe bacterial infection caused by Salmonella enterica. The test is based on the principle of agglutination, which means that the antigen-antibody reaction leads to the formation of clumps or aggregates. In this blog post, we will discuss the principle of Widal test, procedure, requirements, specimen, and observation.
What Is Widal Test?
A Widal test is a quick and simple blood test that confirms or rules out enteric fever, also known as typhoid fever, even when the symptoms are mild. Usually, it takes anywhere between 6 to 30 days for the initial symptoms of fever to appear after being exposed to an infection. There are multiple reasons doctors prescribe a Widal test.
In biological terms, a Widal test is used for detecting antibodies against flagellar (H) and somatic (O) antigens that cause infection and typhoid fever. On infection, these antigens stimulate the body to produce specific antibodies. The Widal test analyses the interaction between these antigens and antibodies found in the blood samples of the patients.
Principle of Widal test:
The principle of the Widal test is based on the fact that when a patient is infected with Salmonella enterica, the body produces specific antibodies against the bacteria. These antibodies are known as O antibodies and H antibodies. The O antibodies are produced against the lipopolysaccharide (LPS) present on the outer membrane of the bacteria, while the H antibodies are produced against the flagellar protein (H antigen) present on the bacterial flagella.
The test involves mixing the patient’s serum with a suspension of Salmonella enterica antigens. If the patient has been infected with Salmonella enterica, the antibodies in their serum will react with the bacterial antigens and cause clumping or agglutination. The agglutination reaction can be observed under a microscope, and the titer or concentration of antibodies can be determined.
Procedure of Widal test:
The Widal test is performed using a slide agglutination technique. The procedure involves the following steps:
Step 1: Collect the patient’s blood sample and allow it to clot. Centrifuge the sample to obtain serum.
Step 2: Prepare a suspension of Salmonella enterica antigens in a test tube.
Step 3: Add a small amount of the patient’s serum to the first well on the slide.
Step 4: Add a small amount of the Salmonella enterica antigen suspension to the same well.
Step 5: Mix the contents of the well using a toothpick or a pipette tip.
Step 6: Repeat steps 3-5 for the remaining wells on the slide, using different dilutions of the patient’s serum.
Step 7: Incubate the slide at 37°C for 18-24 hours.
Step 8: Examine the slide under a microscope for agglutination.
Requirements for Widal test:
The following are the requirements for performing the Widal test:
- Clean and sterile glassware and equipment
- Salmonella enterica antigens
- Patient’s serum sample
- Microscope
- Incubator
- Reagents such as saline and distilled water
Specimen for Widal test:
The specimen required for the Widal test is the patient’s serum. Blood is collected from the patient, and the serum is separated by centrifugation. The serum is then used for the test.
Observation of Widal test:
The Widal test is observed under a microscope for agglutination. The presence of clumps or aggregates indicates a positive reaction. The titer or concentration of antibodies can be determined by using different dilutions of the serum.
Conclusion:
The Widal test is a serological test used for the detection of Salmonella enterica antigens in the blood of a patient. The test is based on the principle of agglutination and involves mixing the patient’s serum with a suspension of Salmonella enterica antigens. The test helps in the diagnosis of typhoid fever, which is a severe bacterial infection caused by Salmonella enterica. The Widal test is a simple and cost-effective method for the diagnosis of typhoid fever.
What is a positive Widal test?
A positive Widal test result means there is a presence of salmonella typhi in the blood.
What is normal Widal test results?
The normal range for the widal test is that the titres of O and H antigens are less than 1:160 and are considered negative. The titres of O and H antigens above 1:160 to 1:320 or above are considered positive.
Who Should Get Tested?
If a person is showing symptoms of typhoid fever, the doctor may want to look into his/her Widal test reports for a better diagnosis. These symptoms include:
- High fever with head and body aches
- Abdominal pain, constipation, and possibly diarrhoea.
- Red Spots on the body
- Weakness and a loss of appetite
- Agitation, confusion, and, in the worst-case, hallucinations.
Methods Of Widal Testing:
- Qualitative Method
One drop of undiluted blood serum is placed on the slides, followed by one drop of O, H, AH and BH antigen. After mixing the contents with a separate applicator stick, the slide is gently rocked for a minute. If no agglutination is observed the test is considered negative.
- Quantitative Method
Followed after a positive qualitative test, it is a semi-procedural Widal tube test that includes testing the positive antigens with the blood sample to confirm the slide agglutination.
Because the slide Widal test has higher sensitivity but lower specificity, it should be used for screening, but positive slide test results should be confirmed by the tube Widal test.
Understanding Test Results And Ranges:
A Widal test needs thorough and precise interpretation and can be analysed by understanding the titre value. For a proper diagnosis, titres ranging from 1:20, 1:40, 1:60, 1:80, 1:160, and 1:200 need to be included in the diagnosis to obtain the typhoid test report.
- Negative Result – A negative, commonly considered a normal range for a Widal test result is when the value of O and H antigens are less than 1:160.
- Positive Result – A titre of O and H antigens greater than 1:60 to 1:320 is considered positive. A positive Widal test result means there is a presence of salmonella typhi in the blood.
