Evaluation of the response of human humoral antibodies to Salmonella typhi lipopolysaccharide in an area of endemic typhoid fever.

Because of the limited value of Widal's test in the diagnosis of typhoid fever in areas of endemicity, individual serum levels of IgM, IgA, IgG, and IgG subclass antibodies to Salmonella typhi lipopolysaccharide were evaluated in samples collected in Egypt. The study involved 106 febrile patients, including 40 patients for whom cultures were positive for S. typhi and 66 patients for whom diseases other than typhoid were diagnosed. Multivariate regression modeling revealed that detection of the combination of IgA, IgG, and IgG2 correlated best, although not perfectly (adjusted r(2) = 68), with a positive culture; the sensitivity and specificity of testing for IgA, IgG, and IgG2 (i.e., all three tests positive vs. all three tests negative) were 91.7% and 98.1%, respectively. These results suggested that testing for IgA, IgG, and IgG2 in combination is of diagnostic value for S. typhi infection.