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 and some types of E. coli) 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, and the 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 faeces. 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 the contribution of IgM has been removed leaving the IgG component. This differentiation of antibody classes is important; as it allows for the distinction of a recent (IgM) from an old infection (IgG)

The Widal test is positive if TO antigen titer is more than 1:160 in an active infection, or if TH antigen titer is more than 1:160 in past infection or in immunized persons. A single Widal test is of little clinical relevance due to the number of cross reacting infections, including malaria. If no other tests (either bacteriologic culture or more specific serology) are available, a four fold increase in the titer (e.g., from 1:40 to 1:160) in the course of the infection, or a conversion from an IgM reaction to an IgG reaction of at least the same titer, would be consistent with a typhoid infection.

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; which may be used to detect the presence of Salmonella typhi and S. paratyphi."

See also

Georges-Fernand Widal

Kauffman-White classification

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

wida test interpation