Antibodies against Salmonella can be detected in the patient's serum from the second week after the onset of infection. Information on antibody titers and whether or not they are rising or falling can be obtained by performing the Widal serological test using various antigens. These include S. typhi 'flagellar antigen H' and 'somatic antigen O' as well as the somatic O and flagellar H antigens of Salmonella paratyphi A, B and C serotypes.