I agree with Vance2335, you should consult your doctor on this question. Your concern
is reasonable.
Biaxin is an antibiotic use to threat bacterial infections. And, yes syphilis is a bacterial infection. However, whether the medicine will affect Syphilis test result will depend on the medicine, when the medicine was taken.
You should check with your doctor or alternatively, you can post your question to the STDs Expert Forum, which runs by Dr. Handsfield and Dr. Hook.