Test whereby bacteria causing typhoid fever are mixed with serum containing specific antibodies obtained from an infected individual. It is a presumptive serological test for enteric fever or undulant fever. In case of Salmonella infections, it is a demonstration of the presence of O-soma 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 perform the diagnosis takes 7-14 days, which limits it applicability in early diagnosis. Other means of diagnosing Salmonella typhi (and paratyphi) include cultures of blood, urine and faeces. These organisms produce H₂S from thiosulfate and can be easily identified on differential media such as Bismuth sulfite agar.

Often 2-mercaptoethanol is added to the Widal test. 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.

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