Prospective evaluation of non-invasive saliva specimens for the diagnosis of syphilis and molecular surveillance of Treponema pallidum

The promising diagnostic performance of molecular testing for syphilis using saliva and urine samples has been reported; however, further evaluation of its possible application for diagnosis and molecular surveillance is required. In addition, the development of a rapid and easy-to-perform molecular test for syphilis is important for its use in the clinical setting. We comprehensively evaluated the diagnostic and surveillance performance of two novel loop-mediated isothermal amplification (LAMP) assays using saliva and urine samples. Saliva, urine, and whole blood were collected from patients who underwent serological testing for syphilis at outpatient clinics. Treponema pallidum DNA in specimens was detected using quantitative PCR (qPCR), nested PCR, and novel LAMP assays. T. pallidum genotyping was conducted by multi-locus sequence typing (MLST). Of the 163 patients recruited, 98 were diagnosed with syphilis (primary: n = 35; secondary: n = 40; latent: n = 23). qPCR showed the highest sensitivity among the molecular tests performed with a sensitivity of 54.1% and 30.3% for all syphilis patients using saliva and urine samples, respectively. A novel method of LAMP combined with dry reagents and crude DNA extraction (Dry-LAMP) showed a probit detection limit of 37.4 copies/reaction within 45 min. The agreement rate between Dry-LAMP and qPCR for saliva was 95.7% (κ coefficient 0.90). The T. pallidum genotype was identified in 48 patients by MLST using saliva samples. Molecular analysis of saliva could be used as a supplementary diagnostic test for syphilis and molecular surveillance of the T. pallidum genotype. Dry-LAMP is expected to be helpful in the clinical diagnosis of syphilis.