“For most persons with HIV infection, serologic tests are accurate and reliable for diagnosing syphilis and following a patient’s response to treatment.
However, atypical nontreponemal serologic test results (i.e., unusually high, unusually low, or fluctuating titers) might occur regardless of HIV-infection status.
When serologic tests do not correspond with clinical findings suggestive of early syphilis, presumptive treatment is recommended for persons with risk factors for syphilis, and use of other tests (e.g., biopsy and PCR) should be considered.”
https://www.cdc.gov/std/tg2015/syphilis.htm