Clinical usefulness of 16S ribosomal RNA real-time PCR for the diagnosis of scrub typhus

Scrub typhus is a major acute febrile disease in the Asia–Pacific region. The purpose of the present study is to investigate the clinical usefulness of real-time PCR (Q-PCR) of 16S rRNA for the diagnosis of scrub typhus. We examined blood specimens from 148 adult patients who were confirmed to have scrub typhus from September 2008 to December 2009. Among the 148 scrub typhus patients, 36 patients were treated with antibiotics before admission. To evaluate the clinical usefulness of 16S rRNA Q-PCR, we compared its diagnostic accuracy to the accuracy of the following methods: nested PCR (N-PCR) targeting the gene encoding the 56-kDa protein, Q-PCR targeting the gene encoding the 47-kDa protein, and conventional PCR (C-PCR), targeting the 16S rRNA gene. According to 16S rRNA Q-PCR and 47-kDa Q-PCR, the mild group had copy numbers of 234.4 ± 261.9 and 130.5 ± 128.3, whereas the severe group had copy numbers of 584.4 ± 911.4 and 244.7 ± 210.9, respectively. In both tests, the mean copy numbers were significantly greater in the severe group (P = 0.037 and P = 0.035). 16S rRNA Q-PCR detected Orientia tsutsugamushi infections with a sensitivity of 91.9% (95% CI 86.3–95.7), and 56-kDa N-PCR, 47-kDa Q-PCR, and 16S rRNA C-PCR exhibited lower sensitivities of 81.1% (95% CI 73.8–87.0), 74.3% (95% CI 66.5–81.1), and 87.8% (95% CI 81.5–92.6), respectively, for all 148 patients. In addition, 16S rRNA Q-PCR exhibited a sensitivity of 99.1% (95% CI 95.1–100.0) in the 112 patients who were not treated with antibiotics before admission. 16S rRNA Q-PCR is clinically useful for the rapid diagnosis of scrub typhus and is more accurate than the 56-kDa N-PCR, 47-kDa Q-PCR, and 16S C-PCR methods.