From the Guidelines
A RPR titer of 1:8 or less is generally considered indicative of a low disease activity or a serofast state after treatment for syphilis, while a fourfold increase in titer is usually considered necessary to demonstrate a clinically significant difference between two nontreponemal test results. When interpreting RPR titers, it is essential to consider the clinical context and the patient's treatment history. According to the guidelines from the CDC, the National Institutes of Health, and the HIV Medicine Association of the Infectious Diseases Society of America 1, after successful treatment for syphilis, 15%--20% of persons might remain "serofast," meaning that serum nontreponemal test titers remain reactive at low and unchanging titers, usually <1:8, for prolonged periods. Some key points to consider when interpreting RPR titers include:
- A fourfold change in titer, equivalent to a change of two dilutions, is usually considered necessary to demonstrate a clinically significant difference between two nontreponemal test results 1
- The use of only one type of test is insufficient for diagnosis because false-positive nontreponemal test results occasionally occur secondary to various medical conditions 1
- Nontreponemal test antibody titers usually correlate with disease activity, and results should be reported quantitatively 1
- Serologic detection of potential reinfection should be based on an at least a fourfold increase in titer above the established serofast baseline 1
- Retreatment of persons with early-stage syphilis should be considered for those who have a sustained fourfold increase in serum nontreponemal titers after an initial reduction after treatment, or have persistent or recurring clinical signs or symptoms of disease 1
From the Research
RPR Titer Indicative of Syphilis Infection
- The RPR (Rapid Plasma Reagin) titer is used to indicate the level of syphilis infection in a patient 2, 3, 4.
- A decrease in RPR titer is expected after treatment, with a 4-fold decrease indicating a successful treatment outcome 2, 3.
- The RPR titer can be used to distinguish between early and late latent syphilis, with higher titers (>1:8) more commonly associated with early latent syphilis 4.
- However, there is considerable overlap in RPR titer distributions between different stages of syphilis, making it difficult to rely solely on titer for staging 4.
Factors Affecting RPR Titer Response
- Baseline RPR titer ≤1:16 is a predictive factor for serological failure after early syphilis treatment 2.
- A previous history of syphilis and a CD4 T-cell count below 350 cells/ml are also associated with serological failure 2.
- The type of syphilis treatment (1 dose versus 3 doses of benzathine penicillin G) does not appear to affect the proportion of serological failure 2.
Interpretation of RPR Titer Results
- A ≥ 4-fold decline in RPR titers 6-12 months after adequate therapy is considered a successful treatment outcome 3.
- Patients with early syphilis who do not achieve a ≥ 4-fold decline in RPR titers 6-12 months after treatment may require further evaluation, including CSF examination 3.
- RPR titers ≥ 1:32 and/or CD4 cell counts <350 cells/mm^3 may indicate an increased risk of asymptomatic neurosyphilis in HIV-infected patients 3, 5.