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.

Additional recommended knowledge

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Often 2-marceptoethanol is added. This agent binds to 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 antibdoy classes is important; as it allows for the distinction of a recent (IgM) from an old infection (IgG).

See also

Georges-Fernand Widal

References

• Olopoenia LA, King AL. Widal agglutination test - 100 years later: still plagued by controversy. Postgrad Med J 2000; 76(892): 80-4. PMID 10644383