Widal test

Widal test

The Widal test is a presumptive serological test for Enteric fever or Undulant fever. In case of Salmonella infections, it is a demonstration of agglutinating antibodies against antigens O-somatic and H-flagellar in the blood. For brucellosis, only O-somatic antigen is used. It's not a very accurate method, since patients are often exposed to other bacteria (e.g. Salmonella enteritidis, Salmonella typhimurium) in this species that induce cross-reactivity; many people have antibodies against these enteric pathogens, which also react with the antigens in the Widal test, causing a false-positive result. Test results need to be interpreted carefully in the light of past history of enteric fever, typhoid vaccination, general level of antibodies in the populations in endemic areas of the world. Typhidot is the other test used to ascertain the diagnosis of typhoid fever. As with all serological tests, the rise in antibody levels needed to make the diagnosis takes 7-14 days, which limits their use. Other means of diagnosing Salmonella typhi (and paratyphi) include cultures of blood, urine and feces. The organism also produces H2S from thiosulfate.

Often 2-mercaptoethanol is added. This agent more easily denatures the IgM class of antibodies, so if a decrease in the titer is seen after using this agent, it means that it's IgM that's high but not IgG. This differentation of antibody classes is important; as it allows for the distinction of a recent (IgM) from an old infection (Ig E)

So we can define this test as " a test involving agglutination of typhoid bacilli when they are mixed with serum containing typhoid antibodies from an individual having typhoid fever; used to detect the presence of Salmonella typhi and S. paratyphi."

See also

Georges-Fernand Widal widal test if TO antigen more than 1.160 in active infection widal test if TH antigen more than 1.160 in past infection or in immunized

References

wida test interpation

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