Widal test

The Widal test is a presumptive serological test for enteric fever or
undulant fever whereby bacteria causing typhoid fever are mixed with
serum containing specific antibodies obtained from an infected
individual. 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 H 2S 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 fourfold 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.
A new serological test is called the Tubex test is not superior than
the Widal test and even the Widal test gives better performance
results, so the Tubex test is not recommended for diagnosis of
typhoid fever.