Widal Test for Typhoid and Paratyphoid Fever | Salmonella Agglutination Test

Описание: The Widal test, also known as the Salmonella agglutination test, is a serological test used to diagnose infections caused by the bacteria Salmonella, specifically Salmonella typhi and Salmonella paratyphi. These bacteria are responsible for causing typhoid and paratyphoid fevers, respectively. The test is based on the principle of agglutination, which involves the clumping together of antigens and antibodies in the presence of specific pathogens.

Here's an introduction to the Widal test:

Principle:
The Widal test is a type of agglutination test that detects the presence of antibodies in a patient's blood serum that are specifically reactive with antigens present on the surface of Salmonella bacteria. In this test, patient serum is mixed with a suspension of killed Salmonella cells that contain specific antigens. If the patient has been exposed to the bacteria and has developed antibodies against them, these antibodies will react with the antigens on the bacterial surface, leading to visible clumping or agglutination of the bacterial cells.

Procedure:

Preparation of Antigens: Bacterial cultures of Salmonella typhi and Salmonella paratyphi are grown and killed, preserving their surface antigens. These antigens are then collected and suspended in a solution.

Preparation of Patient Serum: A blood sample is taken from the patient, and the serum is separated. This serum contains antibodies that the patient's immune system may have produced in response to a Salmonella infection.

Mixing of Antigens and Serum: Different tubes are prepared, each containing a specific dilution of patient serum. The diluted serum is mixed with a standardized suspension of the killed Salmonella antigens (typhi and paratyphi).

Incubation: The tubes are incubated at a specific temperature for a designated period. During this time, if the patient's serum contains antibodies that can react with the bacterial antigens, agglutination will occur, resulting in visible clumping.

Observation: After incubation, the tubes are observed for agglutination. The highest dilution of serum that shows agglutination gives the "titre," which represents the level of antibodies present in the patient's serum.

Interpretation:
A significant rise in antibody titers (a fourfold or greater increase) between paired serum samples taken during the acute and convalescent phases of the illness indicates an active infection. For example, if the initial sample shows a titre of 1:40 and the convalescent sample shows a titre of 1:160 or higher, it suggests a recent or ongoing Salmonella infection. However, the Widal test has limitations and can produce false positives or negatives due to cross-reactivity with other infections or past vaccinations.

While the Widal test has been historically used for diagnosing typhoid and paratyphoid fevers, it's worth noting that more modern and accurate diagnostic methods, such as PCR-based techniques and blood culture, are now preferred for confirming these infections. The Widal test's reliability can be influenced by factors such as vaccination history, geographical location, and prevalence of similar infections.
Serological test
Salmonella
Typhoid fever
Paratyphoid fever
Agglutination
Antibodies
Antigens
Serum
Bacterial cultures
Immune response
Diagnosis
Paired serum samples
Acute phase
Convalescent phase
Titre
Cross-reactivity
False positives
False negatives
PCR-based techniques
Blood culture
Vaccination history
Geographical prevalence