How long does it take for syphilis to be detectable in a blood test?

Weeks to months: Testing for syphillis relies on both non-specific tests that can be falsley +or - and more specific germ tests as backup. A + or rising level of a screening test is confirmed by a germ test. The most specific test is finding the germ in material from a lesion. Since transmission ~30% per contact with an infected source, a single dose of penicillin is often given as early as exposure is suspected.

Usually about 2 wks: From the time of infection, blood tests become positive in about two weeks in most people.