Objective To investigate the contribution of a semi - nested polA PCR assay for the detection of extremely low numbers of treponema pallidum (Tp) in whole blood of persons with syphilis. Methods Routine PCR and semi - nested PCR assays were performed to amplify specific fragments of DNA polymerase Ⅰ gene (polA) of Tp from 165 whole - blood samples of persons with suspected syphilis or non -syphilis. Compared with serology, the semi -nested PCR method was evaluated in the detection of Tp in whole blood. Results Of 165 tests performed, direc...