What Is the Role of Paired Rapid Plasma Reagin Testing (Simultaneous Testing of Acute and Convalescent Samples) in the Diagnosis of Repeat Syphilis and the Follow-up of Syphilis?

Evaluation of the AIX1000 Automated Rapid Plasma Reagin Assay in a High-Prevalence Setting

BACKGROUND

Manually performed nontreponemal assays, such as rapid plasma reagin (RPR), are labor intensive and time consuming. Recently, commercial automated RPR assays gained attention. The aim of this study was to compare the qualitative and quantitative performance of the AIX1000 (RPR-A; Gold Standard Diagnostics) to a manual RPR test (RPR-M; Becton Dickinson Macrovue) within a high-prevalence setting.

METHODS

A retrospective panel of 223 samples was selected for comparison between RPR-A and RPR-M, including 24 samples from patients with known syphilis stages and 57 samples from 11 patients in follow-up. In addition, 127 samples obtained during routine syphilis diagnosis with RPR-M were analyzed prospectively with AIX1000.

RESULTS

Overall qualitative concordance (percent agreement) between both assays was 92.0% in the retrospective and 89.0% in the prospective panel. Of 32 discordances, 28 were explained by a treated syphilis infection still positive in one assay and already negative in the other. One sample was false positive with RPR-A, 1 infection remained undetected by RPR-M, and 2 remained undetected by RPR-A. A hook effect was apparent on the AIX1000 at RPR-A titers from 1:32 onward; however, no infections were missed. Accepting a ±1 titer difference, quantitative concordance between both assays reached 73.1% and 98.4% for the retrospective and prospective panels, respectively, with an upper limit of reactivity for RPR-A at 1:256.

CONCLUSIONS

The AIX1000 showed a similar performance to Macrovue RPR with the exception of a negative deviation for high-titer samples. Within the reverse algorithm used in our high-prevalence setting, AIX1000's main advantage is automation.

Management of Adult Syphilis: Key Questions to Inform the 2021 Centers for Disease Control and Prevention Sexually Transmitted Infections Treatment Guidelines

A panel of experts generated 5 "key questions" in the management of adult syphilis. A systematic literature review was conducted and tables of evidence were constructed to answer these questions. Available data suggest no clinical benefit to >1 dose of benzathine penicillin G for early syphilis in human immunodeficiency virus (HIV)-infected patients. While penicillin remains the drug of choice to treat syphilis, doxycycline to treat early and late latent syphilis is an acceptable alternate option if penicillin cannot be used. There are very limited data regarding the impact of additional antibiotic doses on serologic responses in serofast patients and no data on the impact of additional antibiotic courses on long-term clinical outcomes. In patients with isolated ocular or otic signs and symptoms, reactive syphilis serologic results, and confirmed ocular/otic abnormalities at examination, a diagnostic cerebrospinal fluid (CSF) examination is not necessary, because up to 40% and 90% of patients, respectively, would have no CSF abnormalities. Based on the results of 2 studies, repeated CSF examinations are not necessary for HIV-uninfected patients or HIV-infected patients on antiretroviral therapy who exhibit appropriate serologic and clinical responses after treatment for neurosyphilis. Finally, several important gaps were identified and should be a priority for future research.

Agreement and error of titration assays

Titration assays can be used to define positivity either in terms of a change over time, i.e. seroconversion, or relative to a fixed threshold. The operating characteristics of these definitions depend on the precision of the assay. We present methods for estimating the distribution of errors, at the level of a single replicate, from the distribution of within-pair agreement. When the maximum replicate-level error is one dilution, a simple probability argument is used, with estimation by method of moments. For the more general case, a discretized Gaussian model is used, with maximumum likelihood estimation. These models fit well to eight published datasets. The discretized Gaussian model also allows the potential performance of alternative dilution factors to be assessed. For influenza hemagglutination-inhibition, the approach is compared to a previous Markov chain Monte Carlo data augmentation model. These methods allow the estimation of the underlying error distribution from observed between-replicate differences under repeatability conditions. The results can be used to guide the choice of the fold change necessary to infer seroconversion. Finer dilution factors, e.g. 1.5 rather than 2, could facilitate a better balance between the sensitivity and specificity of titration assays.

Post-treatment serological changes in some patients with early syphilis exhibit a parabolic trend

Early syphilis accounts for a large proportion of patients with syphilis. Non- Treponema pallidum tests are commonly used to assess treatment effectiveness by analyzing the serological titer before treatment and six months after treatment. However, serological changes during the first three months after completion of treatment have not been completely understood. This prompted us to investigate whether serum titers of patients exhibit a continuous decrease post-treatment and to assess the trend of change in serological titer during this period. One hundred and seventy-three eligible patients with early syphilis were included in the analysis. Pre-treatment serological titers and those at three and six months post-treatment were compared and analyzed. Serological recovery was defined as a 4-fold or greater decrease in titer from pre-treatment level. Forty patients (23.1%) were found to have an increased serum titer at three months after treatment. Among the 40 patients, 13 patients had primary syphilis, 5 patients had secondary syphilis, and 22 patients had early latent syphilis. The proportion of patients with primary syphilis was higher, and their initial titers were significantly lower. No significant differences were observed with respect to age, gender, or initial treatment. The assessment results of 17 patients (9.8% of the total patients) change. Serological changes in some patients exhibit a parabolic pattern that may affect the clinician’s assessment of patient recovery. Therefore, more frequent assessment of serological titer might be required within the first six months post-treatment.

Challenges of Rapid Plasma Reagin Interpretation in Syphilis Screening in Uganda: Variability in Nontreponemal Results Between Different Laboratories

BACKGROUND

Syphilis is a cause of morbidity and mortality and is of particular concern in pregnancy in low-income countries because of the risks associated with maternal-fetal transmission. Ugandan national guidelines recommend a nontreponemal rapid plasma reagin (RPR) followed by treponemal testing for diagnosis of syphilis. The RPR test confirms a reactive specific treponemal test, or confirms serological "cure" with a 4-fold dilutional decrease; RPR is beset with technical and biological limitations, making accurate diagnosis and appropriate treatment problematic. The aim of this analysis was to compare performance of RPR testing in different laboratories.

METHODS

Stored, freeze-thawed sera from 215 participants were additionally tested for RPR and dilutional titer in 2 different reference laboratories. Discrepant results were tested at a third reference laboratory which served as a tie-breaker. Equivalence in RPR titer was defined as within 2-fold or less. All patients with reactive rapid tests were treated as per Ugandan National Guidelines.

RESULTS

Of 215 sera, 97 (45.1%) were RPR reactive in clinic laboratory A, 81 (37.7%) and 65 (30.2%) were RPR reactive in laboratories B and C, respectively. All reported positive in laboratory C were positive in laboratory B. Discrepant results were tested in laboratory D. χ Test was highly significant (P = <0.001) for difference between each dyad of laboratories (A and B, A and C, and B and C) RPR results. There were significant differences between RPR titers by paired t test and Wilcox rank test (P = <0.001); with up to a 3-fold difference between laboratories. Two one-sided test approach demonstrated nonequivalence. Agreement between laboratories B-D, and C-D: 48 (98.0%) of 49 and 34 (69.4%) of 49, respectively (P = <0.001). Laboratories B and D showed no significant difference and had equivalent RPR titers. Laboratories C and D had different titers (P = <0.001) and were not equivalent.

CONCLUSIONS

We found significant interlaboratory discrepant RPR results. A 3-fold difference in results is likely to be clinically significant and could result in undertreatment or overtreatment. These data demonstrate a key limitation of the RPR test and underline the urgent need for a more reproducible quantitative test than the current RPR for diagnosing and determining cure of syphilis.

Repeat Syphilis Is More Likely to Be Asymptomatic in HIV-Infected Individuals: A Retrospective Cohort Analysis With Important Implications for Screening

There is conflicting evidence as to whether repeat syphilis is more likely to present asymptomatically than initial syphilis. If it is, then this would motivate more frequent and long-term syphilis screening in persons with a history of multiple episodes of syphilis. We conducted detailed folder reviews of all individuals with 4 or more diagnoses of syphilis between 2000 and 2017 at the Institute of Tropical Medicine, Antwerp, and assessed if there was a difference in the proportion presenting with symptomatic (primary and secondary) vs asymptomatic (latent) syphilis in initial vs repeat syphilis. Forty-five clients with 4 or more episodes of syphilis were included in the study. All were HIV-infected. Repeat episodes of syphilis were less likely to be symptomatic than initial episodes (35/160 [21.9%] vs 28/45 [62.2%]; P < .001). Frequent screening in those with HIV infection may be the only way to diagnose repeat episodes of syphilis. Care providers can use this information to motivate persons with multiple episodes of syphilis to be screened every 3 to 6 months.