The Widal test is a diagnostic test used to confirm the presence of certain bacteria, such as Salmonella typhi and other related species that cause typhoid fever. The test detects antibodies (agglutinins) that the body produces in response to an infection by these bacteria.
During the Widal test, a small sample of blood is collected from the patient and mixed with a solution containing antigens derived from the bacteria. If the patient has been previously infected, the antibodies present in the blood will bind to the antigens and cause clumping, or agglutination, of the bacteria. The degree of agglutination is measured and compared to a standard to determine the level of antibodies present in the blood.
The Widal test is useful in diagnosing typhoid fever, but it has limitations, such as false-positive and false-negative results. It should always be interpreted in conjunction with other clinical and laboratory findings. In addition, the test does not distinguish between a current or past infection, and it can take up to two weeks for antibodies to appear in the blood after infection, so it is not useful for early diagnosis.