A nation-wide survey was conducted to see the prevalence of serosensitivity to Pseudomonas pseudomallei antigens by indirect hemagglutination (IRA) and indirect immunofluorescent assay (IFA) for IgG and IgM. Serum samples were collected from blood donors in eight selected areas and bacteriologically confirmed melioidosis patients in Ubon Ratchathani province. The distribution patterns of antibody titers were compared among the survey areas with cut-off points set at 1: 160 for IRA, 1: 4 for IFA-IgM and 1: 32 for IFA-IgG. These cut-off points were decided by ROC (Receiver Operating Characteristics) analysis. The specificity (% true negative reactions) of each serological test in the general population differed significantly among survey areas, possibly reflecting the extent of inapparent infection in each community. IFA was more successful than IRA in differentiating between negative from positive reactions. The survey classified the areas into endemic (Khon Kaen, Ubon Ratchathani), transported (Bangkok), and non-endemic (other provinces) types.
