Hi when a person tests positive for syphilis a second more sensitive test is performed to confirm an active infection.
Since syphilis is detected by antibodies showing in your system, antibodies will never go away so in a sence it will never go back to a negative. Its easily treated.
That's a 4-fold drop in your titer, or test result, which means that the antibiotic worked. When they did the follow up test, they should have done a different test with it to see if that was positive or negative.
Many people will never test negative on an RPR or VDRL after having syphilis. If those numbers ever go up, it will be because you are newly infected.
When you test from now on, they will probably automatically do an RPR or VDRL first, and then confirm it with the other type of test. If the other test is negative, you don't have the infection.