This article will discuss the widal test principle, procedure and results interpretation.
The Widal test procedure helps identify typhoid fever caused by salmonella enterica serotype typhi. Developed by Georges Fernand Isidore Widal in the late 19th century, provided insight into antibodies indicative of exposure to the typhoid-causing bacterium. Typhoid fever, a potentially severe and life-threatening illness, necessitates accurate and timely diagnosis for effective treatment. The widal test has been a longstanding method for assessing the immune response of individuals to Salmonella typhi
Widal Test Principle
The Widal test exposes the patient's serum to killed Salmonella typhi bacteria carrying specific antigens. If the patient has been previously exposed to the bacterium, their immune system would have generated antibodies that specifically bind to this antigen. The ensuing antigen-antibody reaction results in visible clumping or agglutination, forming the basis of the test. The presence or absence of agglutination becomes a crucial indicator of the patient's immune response to Salmonella typhi
Types of Antigen Used “H”, “O”, “AH”, and “BH”
The Widal test employs various antigens to assess the immune responses comprehensively. The “O” antigen, a somatic antigen, and the “H” antigen, a flagellar antigen derived from Salmonella typhi, are basic to the test, further “AH” and “BH” antigens are utilized in certain test interpretations. This antigen provides specificity, allowing for the identification of contributes to the accuracy and sensitivity of the Widal test, enabling healthcare professionals to distinguish between various stages of infection and tailor appropriate intervention
Who Should Get the Widal Test?
The Widal test indicated for patients who have a :
- History of fever for more than five days
- Gastrointestinal symptoms
- Have no response to empirical antibiotic treatment
- Have clinical signs and symptoms suggestive of treatment
- Have clinical signs and symptoms suggestive of typhoid fever (fever, diarrhoea, red
- spot on the body) etc.
- Have no other obvious focus of infection
Principle:
The patient's immune system reacts to Salmonella Bacterial infection by generating antibiotics against the bacteria. These antibodies aim to neutralise the infection, typically becoming detectable to the bacteria. These antibodies aim to neutralise the infection, typically becoming detectable in the blood approximately 6 years after the
Infection Starts:
- Antibiotics against a flagellar antigen of Salmonella typhi (H)
- Antibodies against a flagellar antigen of S. Paratyphi A (AH)
- Antibodies against flagellar antigen S. Paratyphi B (BH)
- Antibiotic against a somatic antigen of S. typhi (o)
- The Widal test works by checking these specific salmonella antibiotics in the patient's serum using prepared salmonella antigen
- A positive result is indicated by the presence of agglutination
- Absence of agglutination indicates a negative result
Note:
- Getting a negative result doesn’t rule out the presence of salmonellosis.
- The paratyphoid O antigen is not employed as they cross-react with the typhoid O antigen
Reagents
The test kit contains the following reagents:
- Reagents 1. S. Typhi (h), contains the H antigen from Salmonella type
- Reagents 2. S. typhi contains the o antigen from Salmonella typhi
- Reagent 3. S. PARATYHPI A(AH), Contains the H antigen from salmonella paratyphi A
- Reagent 4. S. PARATYHPI B (BH), Contains the H antigen from salmonella paratyphi B
- Negative control (NC): This control sample does not contain antibodies to salmonella typhi and is used as a reference to validate the test result. It should not produce a positive reaction
- Positive control (PC): it is used to ensure that the test is functionally correct and should produce a positive reaction.
What Are the Specific Preparations Needed for the Widal Test?
There is no specific preparation needed for the widal test. However, our healthcare provider may advise you to avoid certain medications, certain foods or strenuous activity In the hours leading up to the test
Types and Methods of The Widal Test
The Widal test can be conducted either on a slide or in a tube and can be either qualitative or quantitate, depending on the specific testing protocol and the medical requirements
- Widal Slide Method: A drop of the patient serum is mixed with the antigen on the slide, and the reaction is observed for agglutination (clumping) (Book Now)
- Widal tube Method: the tube agglutination test is conducted in small test tubes and offers more accuracy compared to the slide agglutination technique (Book Now)
- Qualitative test: it is conducted on a slide for screening purposes, determining whether antibodies against specific salmonella antigens are present or absent in the patient serum and does not provide specific antibody concentration levels
- Quantitative testing: this is performed for sample that shows positive agglutination in the qualitative test. The antibody concentration is determined in the patient’s serum.
Interpretation of the Results
The interpretation of the widal test can vary based on the type of test (qualitative or quantitate), antibody levels, eases progression, the patient's symptoms and the result of other tests
Antibody titer
S.typhi interpretation
- Negative result: S typhi titer is smaller than or equal to 1:80
- Positive result: S. typhi titer I more than equal to 1:160
S. Paraatyphi interpretation
- Negative result: S paratyphi titer is smaller than or equal to 1:80
- Positive result: S. paratyphi titer is more than equal to 1:160
Disease Progression
- The O antibodies emerge before and vanish quicker than the H antibodies. The H antibodies occur after and stay longer than the O antibodies
- If anti-o antibodies are detected at a high level without the presence of anti-H antibodies this indicates recent infection
- An isolated increase in anti-H antibodies suggests a longstanding Salmonella infection.
- The presence of anti-H antibodies suggests a long-standing salmonella infection
- The presence of anti-H antibodies directed against S.Typh. Paratyphi A and B indicate previous vaccination
Conclusion
In summary, the Widal test is a serological test used to detect antibodies against salmonella typhi and salmonella paratype in the blood. It helps in the diagnosis and monitoring of typhoid and paratyphoid fever patients, completing other clinical results and conclusions.
