Understanding a Reactive Anti-TP (Treponema pallidum) Syphilis Test
A reactive anti-TP test indicates the presence of antibodies against Treponema pallidum, the bacterium that causes syphilis, and confirms either current infection or past exposure to syphilis. 1
What Anti-TP Tests Detect
Anti-TP tests are treponemal tests that detect specific antibodies against Treponema pallidum antigens, including tests like FTA-ABS (fluorescent treponemal antibody absorbed), MHA-TP (microhemagglutination assay), TP-PA (T. pallidum particle agglutination), and treponemal EIA/CLIA (enzyme/chemiluminescent immunoassays). 1
These tests use specific treponemal antigens and are more specific than nontreponemal tests (RPR/VDRL), which detect antibodies to lipoidal antigens that can produce false-positives in various medical conditions. 1, 2
Critical Interpretation: Reactive Anti-TP Does NOT Mean Active Infection
Most patients with reactive treponemal tests will remain reactive for life, regardless of treatment or disease activity. 1, 3
Only 15-25% of patients treated during primary syphilis may revert to serologically nonreactive after 2-3 years. 1, 3
Treponemal test titers correlate poorly with disease activity and should NEVER be used to assess treatment response or determine if infection is active. 1, 3
What You Must Do Next: The Two-Test Algorithm
A reactive anti-TP test alone is insufficient for diagnosis—you must also perform a nontreponemal test (RPR or VDRL) to distinguish between active infection and past treated infection. 3
If Anti-TP Reactive + RPR/VDRL Reactive:
- This pattern indicates either active syphilis requiring treatment OR past treated syphilis with persistent low-level antibodies (serofast state). 3, 4
- Compare current RPR titer to any previous titers—a fourfold increase indicates new infection or treatment failure. 3, 4
- If no prior treatment documented, treat as late latent syphilis with benzathine penicillin G 2.4 million units IM weekly for 3 weeks. 3, 5
If Anti-TP Reactive + RPR/VDRL Non-Reactive:
- This pattern represents the expected "serologic scar" following successful treatment of syphilis. 4
- Active syphilis is unlikely if there are no clinical signs/symptoms, no new sexual exposure, and no documented recent seroconversion. 4
- However, this pattern can also represent late latent or tertiary syphilis where nontreponemal antibodies have waned (RPR sensitivity drops to 61-75% in late latent and 47-64% in tertiary syphilis). 3, 5
Common Clinical Pitfalls
Never use treponemal tests to monitor treatment response—they remain positive regardless of cure and do not correlate with disease activity. 1, 3
Do not assume a reactive anti-TP test means active infection requiring treatment—you must correlate with nontreponemal test results, clinical history, and prior treatment documentation. 3, 4
In HIV-infected patients, serologic responses may be atypical with unusually high, low, or fluctuating titers, though standard tests remain accurate for most HIV patients. 1, 3
False-positive treponemal tests are rare but can occur—if clinical suspicion is low and RPR is non-reactive, consider the possibility of false-positive treponemal testing. 5
Essential Concurrent Actions
Test for HIV infection in all patients with reactive syphilis serology, as HIV status affects management, monitoring frequency, and risk of neurosyphilis. 3
Obtain detailed sexual history and treatment history to determine if this represents new infection, reinfection, or past treated disease. 3
Screen for symptoms of neurosyphilis (headache, vision changes, hearing loss, confusion) or tertiary syphilis (cardiovascular or gummatous manifestations), especially if RPR is non-reactive. 3