Widal Test | Purpose & Procedure

Developed more than a century ago, Widal test is still one of the most popular test in use today. The Widal tube and slide agglutination test helps to diagnose typhoid fever by detecting the agglutinating antibodies against the specific antigens of Salmonella typhi and Salmonella paratyphi A and B (bacteria causing typhoid). With an estimated 12 to 33 million cases per year, typhoid fever is a major cause of illnesses in many parts of the world. The bacteria are usually transmitted by five ‘F’: flies, food, fingers, faeces and fomites

Your doctor may suspect typhoid fever based on your travel and medical history, as well as your symptoms. Confirmation of typhoid fever is done by culturing Salmonella typhi and/or Salmonella paratyphi A or B in your blood sample.

However, knowing the symptoms and getting tested as early as possible can lead to positive treatment outcomes and early recovery. You can know more about symptoms of typhoid and Widal test in this article.

What is the Widal Test?

The Widal test is a diagnostic test used to determine the presence of antibodies against the salmonella bacteria that causes typhoid fever. It detects the presence of antibodies against O and H antibodies in patient’s serum. Thus, the test determines the exposure of a person to the bacteria that causes typhoid fever by looking for the presence of these antibodies.

What is the Purpose of the Widal Test?

Your travel history, medical history, and symptoms may lead your doctor to suspect typhoid fever. The common symptoms of typhoid fever are:

Fever
Headache
Loss of appetite
Chills
Abdominal pain
Rose spots on your chest or stomach
Muscle aches
Nausea
Vomiting
Diarrhea
Constipation

Principle Behind Widal Test

Salmonella bacteria have antigens that enter your body when infected food is consumed. Once those antigens enter your digestive tract, your immune system produces antibodies to fight them.

The purpose of this agglutination test is to identify these antibodies produced in reaction to a specific antigen or bacteria. In an agglutination test, your sera (blood) will react and agglutinate salmonella antigens if you're suffering from typhoid fever. The basic concept behind the Widal test is that if a certain antibody is in the serum, it will react with a particular antigen and seen as a clump in a slide test.

What Widal Test Measures?

Widal test detects the antibodies against O and H antigens of S. Typhi and paratyphi A or B. However, it has certain shortcomings in terms of reliability, sensitivity, and specificity. Similar to most serologic tests, a false-positive Widal test could be the consequence of a previous infection, exposure to cross-reactive antigens in the past, or vaccination. Therefore, mere presence of antibodies in the Widal test should not be taken as proof of Typhoid fever. A false-negative Widal test could also happen early in the course of an illness. Demonstration of four-fold rising titre by testing paired sera at 1 week interval has a significance than a single test.

Widal Test Preparation

No specific preparation is required. Your doctor will ask to collect the blood sample to test the presence of antibodies against Salmonella typhi and/or Salmonella paratyphi A or B.

Widal Test Procedure

Take a sample of the patient's blood and allow it to clot.
Centrifuge the sample to extract the serum
Write the patient's name and the date on a sterile glass slide.
Put a drop of serum in four circles and add one drop of individual antigen solution to the serum drop (Salmonella typhi O antigen and H antigens of Salmonella typhi and Salmonella paratyphi A & B).
Make sure to thoroughly mix the two drops with a clean toothpick or glass rod.
For one minute, rotate the slide slowly to allow agglutination to take place. If agglutination is visible, the test is positive. If no agglutination is seen, test is negative.

Widal Test Result Interpretation

Agglutinin titre greater than 1:80 is considered significant and can suggest infection as titre below1:80 is found in normal individuals.

Conclusion:

Widal test is still the widely accepted serodiagnostic test for the diagnosis of typhoid fever. Since major consequences, such as severe intestinal bleeding or perforation, can develop within a few weeks, early diagnosis and treatment of typhoid fever is crucial. Getting vaccinated is the best defense against contracting typhoid disease. Consult your healthcare provider immediately if you suspect you may have typhoid disease. If you've recovered from typhoid, get tested to make sure you can't unintentionally infect others (carrier state).

FAQs

Q1: What is the Widal test's normal range?

A: When the titres of the O and H antigens are equal to or less than 1:80, they are considered as negative. Positive titres of O and H antigens greater than 1:80 can be considered significant and suggests infection.

Q2: Are ESR levels high in typhoid?

A: Typhoid fever patients typically present with thrombocytopenia, relative lymphopenia, elevated erythrocyte sedimentation rate (ESR), and mild anemia. ESR can increase in other conditions also. ESR is not a specific marker for typhoid fever.

Q3: Is typhoid fever detectable by blood culture?

A: During the first week of the fever, S typhi and/or S. paratyphi A or B bacteria can be detected in a blood culture (90% positivity).

Q4: Do typhoid patients have low platelet levels?

A: Thrombocytopenia is relatively common in typhoid fever. It can be a marker of severity and indicates high risk of development of complications.