Diagnosis of enteric fever is usually performed by the use of stool and urine cultures as
well as febrile agglutinins to specific lipopolysaccharides and to somatic antigens.
In the case of Salmonella typhi and S. paratyphi, the primary antigens tested for are
the "O" antigens, the "H" flagellar antigens, and the Vi antigens. The febrile agglutinin
test commonly referred to as the Widal test is widely used because of its simplicity.
A four-fold rise in agglutinin titer is considered significant and diagnostic. Generally
the antibodies are slow to develop, rise to a peak, and then recede. If the patient has
been previously infected the antibody response is more rapid. The ImmunoDOT Salmonella
typhi reactions for the detection of typhoid antibodies are set to represent a 1:160 tube
agglutination titer. The antigens include of the primary antigens listed above.
Recent reports suggest that a titer of 1:160 or greater for "O" and "H" antigens
has specificity of 97%, sensitivity of 30%, and an accuracy of 83.1% in the diagnosis
of typhoid fever. The number of antibody positive subjects in a population depends on
two factors: disease prevalence and clinical criteria used to select the tested population.
Serology tests are most useful to test patients in an endemic region with signs and
symptoms consistent with the disease.