DETERMINATION OF ANTIBODY FROM TYPHOID PATIENTS AGAINST LIPOPOLYSACCHARIDE AND PROTEIN ANTIGENS OF SALMONELLA-TYPHI

Although the Widal test is simple, inexpensive and the most widely used for serodiagnosis of typhoid fever, the sensitivity and specificity of the test is sometimes doubtful. In this study, an enzyme-linked immunosorbent assay (ELISA) was developed for the detection of serum IgG and IgM antibodies to protein and lipopolysaccharide (LPS) antigens of Salmonella typhi which was compared with the Widal test in various groups of subjects. In typhoid patients with hemocultures positive for S. typhi (TP group), ELISA positively was found on 100% for IgG antiprotein, 94.44% for IgG anti-LPS and 88.89% for IgM to both the protein and LPS antigens. In contrast, the Widal test was positive in only 61.11% for anti-O and 83.33% for anti-H antibodies. In healthy control subjects (HC group), only 5% of serum samples were positive for IgG anti-protein and none was positive for IgG anti-LPS or IgM to either the protein or LPS. In contrast, the Widal test was positive in 7.5% of HC group for anti-O and 17.5% for anti-H antibodies. In blood bank donors (BB group), both ELISA and Widal tests were positive in 23-40% of sera. Since the hospital records of BB group were incomplete. It might be possible that some of these subjects had recently been infected with S. typhi. Our data indicate that the standard Widal test was associated with false negative reactions in 16-39% of blood culture positive subjects. In non-typhoid patients with blood culture positive for bacteria other than S. typhi (OP group), most of the sera were negative by ELISA; 5% were positive for IgG anti-protein and 2.5% for IgM anti-protein and IgG anti-LPS, whereas 15% were positive on the Widal test for anti-O and 40% for anti-H antibodies. The final two groups of patients included in this study were those with a diagnosis of typhoid fever based on clinical signs and symptoms, in whom hemocultures were not done (WP group) or were negative (NP group). All of these subjects were positive for anti-O antibody by Widal test since this was the antigen for their diagnosis. The NP group had about 70% positive tests for IgG and IgM anti-protein and anti-LPS antigens by ELISA. The WP group showed lower percentage (28-44%) of sera positive for antibodies against protein and LPS antigens. Our data suggest that ELISA assays with S. typhi protein or LPS antigens have superior sensitivity and specificity to the Widal test in the diagnosis of typhoid fever and that the majority of individuals who are Widal seropositive but blood culture negative may not have typhoid fever. Further evaluation of the ELISA test is indicated.