Doxycycline effectively treats and cures most bacterial STDs, including chlamydia, syphilis,
and most cases of
gonorrhea (depending in part on the dose). Accordingly, it will make
gonorrhea and chlamydia tests negative, and may result in lower strength in RPR and VDRL, commonly used to diagnose syphilis. But this isn't really the same as "interference": that is, the tests themselves work just fine after or during doxycycline treatment.