Widal test is used for the diagnosis of typhoid or enteric fever. It is a cheap and simple test. It is basically an indirect agglutination test. It has its advantages and limitations too.
Typhoid or enteric fever was a major disease in the history of humanity that has caused a lot of deaths. Currently due to improved quality of life and hygiene, the incidences are reduced. Still it continues to be a major health concern in developing parts of the world.
Enteric fever or typhoid is caused by Salmonella typhi and Salmonella paratyphi A and B. It is generally caused by drinking contaminated water and food.
Culture tests are gold standards for the diagnosis of typhoid or enteric fever. They are difficult to perform and not available everywhere; still the Widal test is used in many countries.
Today we will discuss the Widal test in detail. We will go through the principle of test, instruments needed, procedures. Then we will discuss the benefits and limitations of the test.
Some of you may not be interested in technical details of this test and may skip the information about instruments and procedure.
What is the Widal test?
The Widal test was first discovered by a French scientist Georges-Fernand Widal. It was discovered nearly 125 years ago.
This test is used to detect typhoid fever or enteric fever infection. It is easy to perform and a cheap test. This test is done on patients serum obtained from the blood from peripheral venous blood.
It detects the antibodies in the patient’s serum that acts against Salmonella typhi and paratyphi. It is an indirect agglutination test.
What is the principle of the Widal test?
Widal test is an indirect agglutination test. To know everything about it we first should know about agglutination and indirect agglutination.
Agglutination is the phenomenon seen in antigen-antibody reaction. When antigen and antibody suspended in a fluid react the combine with each other and form a network like pattern.
This combination gets settled at the bottom of the fluid level. This is called flocculation. It looks like clumps accumulated at the bottom of the fluid. This flocculation is called an agglutination reaction.
Agglutination reactions can be difficult to be seen by naked eyes. So the antigen or antibody is attached to latex particles. Those latex particles are coloured so easily visible with naked eyes.
Here for the Widal test the antigen preparation which is commercially available is attached to latex particles that are colored. When they combine with antibodies in the serum of the patient they get settled down forming colorful clumps visible to naked eyes.
Thus the antibodies in the patient’s serum are detected. That suggests the patient is probably infected with Salmonella.
What are types of Widal tests?
This test can be done as a rapid test, which is generally a slide test. Second way is that this test is done as a tube test. Tube test needs more time and an incubator. It takes at least a night to get the results.
Slide tests are qualitative and quantitative. Qualitative test is just used to detect the type of bacteria infecting or the types of antibodies against that bacteria.
Slight test is also used to detect the level of antibodies against bacteria. It is called an antibody titre. It is a quantitative test.
Tube test is a quantitative test used to detect the antibody levels in the patient’s serum.
What is the slide Widal test?
Slide Widal test is used to do the indirect agglutination test. In this test special types of glass slides are used. They have wide and shallow circular wells made on glass slides.
These wells are used to mix the patient’s serum with latex particles mounted colored antigen liquid of different types of Salmonella typhi and paratyphi bacteria.
What is a tube widal test?
In tube tests as its name implies test tubes made up of glass are used instead of shallow wells on glass slides. In these tubes various dilution of the patient’s serum is used and mixed with latex particles mounted color mixed antigen solution.
Tube agglutination test is comparatively more difficult and needs an incubator and more time to get the results.
What precautions are needed while performing the test?
Some precautions are needed while doing this test. They are as follows:
- All the universal safety precautions are to be followed as the patient’s serum can be potentially infectious to lab workers.
- Caution is prescribed while handling the instruments as they can easily break and can cause injury to lab workers.
- The materials should be discarded following all the biological waste management protocols as it has potential to pollute the environment.
- The standard protocols and procedures of tests are needed as not following those may alter the test results causing mistakes in reading.
What are the benefits of the Widal test?
Widal test being used nearly for the last 100 years there are some benefits of widal test. Those benefits are listed below.
- Availability: This test is available everywhere in the world where at least basic lab facilities are available.
- Price: This test is very cheap in price. Compared to other tests like blood culture it is costly.
- Simplicity: Compared to other tests this test is simple to do and can be performed at any lab where basic lab facilities are available.
- Technical skill: This test does not need much technical skill to perform this test.
What are the limitations of the Widal test?
There are some benefits and some limitations of the widal test. Some limitations are as follows:
- This is not the confirmatory test for the diagnosis of typhoid fever.
- The test may get false positive results in other viral diseases and infections.
- The test does not distinguish current infections from the previous infections.
- There may be cross reactivity and false positive tests in case of other salmonella species bacteria infection.
- Vaccinated people may get false positive tests.
- This test detects elevated antibody titres. It takes time to form antibodies against infection in the body. The diagnosis may be missed in early disease with this test.
- False positive and false negative results are possible in various conditions with this test.
Conditions causing a false positive widal test.
False positive is detection of a positive widal test even when the patient does not have typhoid fever. Following conditions can cause false positive tests.
- Viral fever
- Non typhoid salmonella infections
- Typhoid vaccination
- In region where typhoid is endemic