Syphilis can not be detected by blood tests because the syphilis spirochetes have not yet entered the blood system. Syphilis spirochetes can be detected by dark-field examination of secretions. If not treated, the syphilis spirochete will enter the bloodstream through the lymph, which is called stage 2 syphilis, and a blood test can detect syphilis spirochete antibodies. It takes about 2-3 months after unclean sex for a blood test to detect antibodies to syphilis spirochetes. If there is unclean sexual intercourse, it is recommended to seek medical attention to clarify the diagnosis and give timely treatment as soon as possible.