BACKGROUND: Laboratory studies as diagnostic tools of scrub typhus have shown PCR to be sensitive and specific, but there have been few clinical studies. The aims of this study were to determine the diagnostic accuracy and clinical usefulness of the nested PCR through a prospective comparison of the nested PCR and indirect immunofluorescence assay (IFA). MATERIALS AND METHODS: We conducted a multicenter prospective study of patients with possible scrub typhus. Adult patients who have had fever together with eschar or a maculopapular skin rash and more or equal to two of the following symptoms: headache, malaise, myalgia, coughing, nausea, and abdominal discomfort were enrolled. Each patient was admitted between September, 2004 and December, 2004 to Chosun University Hospital and one of its three community branch hospitals (Haenam General Hospital, Jangheung Hospital, Muan Hospital), which are all located in the southwest of Korea. Whole blood samples were collected for PCR testing and sera were obtained for serology evaluation using the IFA and passive hemagglutination assay (PHA). RESULTS: We enrolled 135 possible scrub typhus patients, and 118 scrub typhus patients were confirmed on the basis of either a single indirect immunofluorescent specific IgM titer against O. tsutsugamushi of > or =1:10 or 4-fold or greater rise in IFA follow up titer. One hundred eighteen patients were confirmed as scrub typhus, 7 patients were undetermined and 10 patients were confirmed as the other diseases. The result of nested PCR assay demonstrated a sensitivity of 82.2% (95% confidence interval 0.74-0.88), a specificity of 100% (95% confidence interval 0.66-1), positive predictive value of 1 (95% confidence interval 0.95-1) and negative predictive value of 0.32 (95% confidence interval 0.17-0.51). 96 patients out of 118 patients were positive for IgM on the admission day. Of 22 patients with negative for IgM antibody at admission, 19 had positive results for nested PCR using buffy coat. CONCLUSION: The nested PCR assay of buffy coat is useful for rapid and reliable test for confirmation of the diagnosis of scrub typhus.