EFFECTIVENESS OF A DOT-BLOT IMMUNOASSAY OF ANTI-RICKETTSIA TSUTSUGAMUSHI ANTIBODIES FOR SEROLOGIC ANALYSIS OF SCRUB TYPHUS

We compared a commercially available dot-blot immunoassay system with the indirect immunofluorescence assay (IFA) in tests of known negative and known positive sera from scrub typhus cases. Using a panel of 100 sera from patients with various rickettsial and nonrickettsial infect ions, we observed that the IFA was 99% specific and the dipstick assay was 98% specific. In tests of 91 sera (30 negative and 61 positive fo r scrub typhus antibodies) from a study of febrile patients in Malaysi a, using the standard of an IFA titer < 1:64 as negative, an IFA titer > 1:128 as positive, and an IFA titer = 1:64 as either positive or ne gative (supported by clinical records), dipsticks were 83% specific an d 90% sensitive. The quantitative correlation of the dipsticks to IFA titers was confirmed by significant differences in geometric means of inverse IFA titers corresponding to the number of positive dipstick sp ots (no dots = 8.5, one dot = 43.3, two dots = 206.7, and three dots = 676.9). The assay would enable physicians and public health workers w ho deal with patients to quickly diagnose and appropriately treat most cases of the disease, especially in areas of high prevalence where th e proportion of false-positve results to true-positive results would b e low.