Widal Test- Principle, Procedure, Types, Results, Uses


Widal test is a serological test that is used to diagnose enteric fever, also known as typhoid or paratyphoid fever, caused by the bacteria Salmonella Typhi and Paratyphi. Enteric fever is a systemic infection that can cause high fever, abdominal pain, headache, diarrhea, and sometimes complications such as intestinal perforation or hemorrhage. It is transmitted by ingestion of contaminated food or water.

Widal test detects the presence of antibodies against specific antigens of Salmonella Typhi and Paratyphi in the blood serum of the patient. Antibodies are proteins produced by the immune system in response to foreign substances (antigens) that enter the body. Salmonella Typhi and Paratyphi have two types of antigens: O (somatic) and H (flagellar). O antigens are present on the cell wall of the bacteria, while H antigens are present on the flagella, which are hair-like structures that help the bacteria move.

Widal test is based on the principle of agglutination, which is a visible clumping of antigens and antibodies. When a serum sample containing antibodies against Salmonella Typhi or Paratyphi is mixed with a suspension of killed and stained antigens of these bacteria, an agglutination reaction occurs if there is a match between the antibodies and the antigens. The degree of agglutination indicates the level of antibodies in the serum.

Widal test was developed in 1896 by Georges-Fernand Widal, a French physician and bacteriologist. He originally described this test to diagnose Salmonella Paratyphi B infection. Later it was modified and used to diagnose all types of enteric fever.

Widal test is widely used throughout the world as a major diagnostic tool for detecting enteric fever, especially in areas where culture facilities are not available or reliable. However, Widal test has some limitations, such as false-positive or false-negative results, cross-reactivity with other infections or vaccinations, and variation in antibody levels depending on the stage and duration of infection. Therefore, Widal test results need to be interpreted carefully in conjunction with clinical symptoms, history of exposure, and other laboratory tests.