From the Guidelines

Syphilis diagnosis is typically made using a combination of nontreponemal and treponemal serologic tests.

Nontreponemal tests, such as the Venereal Disease Research Laboratory (VDRL) and Rapid Plasma Reagin (RPR) tests, are used to detect antiphospholipid antibodies, which are produced in response to Treponema pallidum infection 1.
Treponemal tests, such as the fluorescent treponemal antibody absorbed (FTA-ABS) and T. pallidum particle agglutination (TP-PA) tests, are used to detect treponemal-specific antibodies, which are more specific for syphilis infection 1.

A presumptive diagnosis of syphilis can be made using a combination of nontreponemal and treponemal tests, with the nontreponemal test used as a screening test and the treponemal test used as a confirmatory test 1.
Darkfield microscopy and direct fluorescent antibody tests can also be used to diagnose early syphilis, particularly in patients with mucocutaneous lesions 1.

Nontreponemal test titers can be used to monitor treatment response, with a fourfold change in titer considered necessary to demonstrate a clinically significant difference between two test results 1.
Treponemal test results are typically positive for life, regardless of treatment or disease activity, and should not be used to assess treatment response 1.

HIV-infected patients may have atypical responses to nontreponemal serologic tests, and false-negative serologic tests can occur in both HIV-infected and HIV-uninfected patients with documented T. pallidum infection 1.

From the Research

Syphilis diagnosis can be complex and time-consuming, requiring a combination of laboratory methods and clinical expertise 2
Traditional algorithms using nontreponemal and treponemal antibody tests remain the standard of care for syphilis diagnosis, but newer diagnostic modalities such as nucleic acid amplification assays (NAATs) and point-of-care testing (POCT) are being developed 2, 3
Serological testing is the most frequently used approach in the laboratory diagnosis of syphilis, with various tests available including non-treponemal tests (NTT) and treponemal tests (TT) 4, 5
Direct tests such as microscopy or PCR can be helpful in early syphilis, but are not widely used due to limitations in sensitivity and specificity 3, 6

Current diagnostic algorithms are unable to distinguish current from previously treated infections, leading to uncertainty in clinical management 5
The lack of a reliable gold standard for syphilis diagnostics makes it challenging to evaluate the performance of new tests 5
False-positive reactions can occur with both older and newer tests, highlighting the need for improved diagnostic methods 5

The development of new diagnostic modalities such as NAATs and POCT holds promise for increasing the identification of active syphilis, but more work is needed to improve their accuracy and reliability 2, 3
The use of molecular biology and whole-genome sequencing may lead to the development of more effective diagnostic tests, such as PCR 3, 6
Further research is needed to address the current challenges in syphilis diagnosis and to develop more accurate, reliable, and expeditious tests 2, 3, 6