Can Syphilis Cause Rashes On The Arms And Legs As Per The Attached Images?
PCR test, but not necessary in my opinion.
I read your question carefully and I understand your concern.
Since those tests are negative the only remaining test would be the PCR (polymerase chain reaction) test which searches for genetic material of the bacteria in the blood (or cerebrospinal fluid when neurological involvement is suspected).
However I wouldn't deem it that necessary. While it is true that both RPR and FT-ABS can produce false negatives that depends on the stage. Widespread skin lesions are manifested in secondary syphilis stage. In that stage sensitivity of RPR and FT-ABS is almost 100%. So it is very unlikely to get a negative result. Also usually those skin lesions appear 6 weeks to 6 months after exposure, so since they have appeared after 4 years it's unlikely to be due to syphilis.
I hope to have been of help.