Initial investigations for syphilis

Syphilis is diagnosed by serological testing. Identification of Treponema pallidum using nucleic acid amplification testing (NAAT) (eg polymerase chain reaction [PCR]) can be performed if lesions are present, but must be accompanied by serologyOng, 2023.

Diagnosis of active syphilis requires positive serological test results for bothOng, 2023Workowski, 2021:

  • nontreponemal testing (eg rapid plasma reagin [RPR] test)
  • treponemal testing (eg treponemal enzyme immunoassay [EIA], T. pallidum particle agglutination test [TPPA], chemiluminescent microparticle immunoassays [CMIAs], T. pallidum haemagglutination assay [TPHA]).

Neither test alone is adequate to confirm a diagnosis of active syphilis.

In early syphilis, even if a chancre is present, the results of serological testing may be negative. Treponemal tests may not become reactive until up to 2 weeks after the chancre developsBritish Association for Sexual Health and HIV (BASHH), 2019, and nontreponemal tests may take up to 4 weeks to become reactiveWHO 2016 Sy. A nonreactive nontreponemal test but reactive treponemal test may indicate early syphilis.

Nontreponemal tests may be nonreactive in late latent or tertiary syphilis.

Syphilis serology can be complex to interpret; see the ASHM Decision Making in Syphilis tool, which is also available as an interactive tool. If serological results are difficult to interpret, seek expert advice.

False-positive test results are common with nontreponemal tests and can also occur with treponemal enzyme immunoassay tests; they are rare with the T. pallidum particle agglutination test and the T. pallidum haemagglutination assay.

Note: Check for pregnancy in females of childbearing potential who have syphilis.

Check for pregnancy in females of childbearing potential who have syphilis, to ensure prompt treatment and avoid complications including spontaneous miscarriage, stillbirth and congenital syphilis; for more information, see Syphilis in pregnancy and congenital syphilisAustralian Living Evidence Collaboration, 2024.

Perform a lumbar puncture for cerebrospinal fluid (CSF) analysis to investigate for neurosyphilis in patients with clinical neurological findings (see Clinical presentation of neurosyphilis, ocular syphilis and otosyphilis)Workowski, 2021. For people with HIV infection who have a CD4 cell count of less than 350 cells/microlitre, seek expert advice about the need for and interpretation of CSF analysisOng, 2023.
For patients presenting with isolated ocular or otological symptoms (see Clinical presentation of neurosyphilis, ocular syphilis and otosyphilis), perform a full ocular examination (in collaboration with an ophthalmologist) and cranial nerve evaluation to investigate for ocular syphilis, otosyphilis and neurosyphilisWorkowski, 2021. CSF examination is required for patients with cranial nerve dysfunctionWorkowski, 2021. Manage patients with isolated otological symptoms in collaboration with an otolaryngologistWorkowski, 2021.

Perform investigations for other sexually transmissible infections (STIs) (including HIV, Chlamydia trachomatis and Neisseria gonorrhoeae). Patients with undiagnosed HIV infection may present initially with syphilis; for patients who need to be started on antiretroviral therapy for HIV, see Syphilis in adults with HIV infection.