Evaluation of test results of microbiology laboratories of North India for standard tests for syphilis under an external quality assurance scheme

Diagnostic performance of two rapid tests for syphilis screening in people living with HIV in Cali, Colombia

INTRODUCTION

There is insufficient evidence supporting the use of rapid diagnostic tests (RDTs) for syphilis in people living with HIV (PLWH). We evaluated the diagnostic performance of two commercially available RDTs (Bioline and Determine) in PLWH in Cali, Colombia.

METHODS

A cross-sectional field validation study on consecutive adults with confirmed HIV diagnosis attending three outpatient clinics. Both RDTs were performed on capillary blood (CB), obtained by finger prick, and sera, by venipuncture. A combination of treponemal enzyme linked immunosorbent assay (ELISA) and Treponema pallidum haemagglutination assay (TPHA) on serum samples was the reference standard. Rapid plasma reagin (RPR) and clinical criteria were added to define active syphilis. Sensitivity and specificity, predictive values and likelihood ratios (LR) of RDTs were estimated with their corresponding 95% confidence interval (95% CI). Stratified analyses by sample type, patient characteristics, non-treponemal titers, operator and re-training were performed.

RESULTS

244 PLWH were enrolled, of whom 112 (46%) had positive treponemal reference tests and 26/234 (11.1%) had active syphilis. The sensitivities of Bioline on CB and sera were similar (96.4% vs 94.6%, p = 0.6). In contrast, Determine had a lower sensitivity on CB than sera (87.5% vs 99.1%, p<0.001). Sensitivities were lower in PLWH not receiving ART (Bioline 87.1% and Determine 64.5%, p<0.001) and for one of the operators (Bioline 85% and Determine 60%, p<0.001). Specificities of the RDTs were > 95% in most analyses. Predictive values were 90% or higher. For active syphilis, the RDTs showed a similar performance pattern but with decreased specificities.

CONCLUSION

The studied RDTs have an excellent performance in PLWH to screen for syphilis and potentially for active syphilis, yet Determine performs better on sera than CB. Patient characteristics and potential difficulties operators may face in acquiring enough blood volume from finger pricks should be considered for the implementation and the interpretation of RDTs.

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.

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?

BACKGROUND

Repeat syphilis is playing an increasing role in syphilis transmission in several populations. The assessment of repeat syphilis and response to treatment depends on accurately measuring intraindividual changes in non-treponemal tests. For a 0- to 6-month delta rapid plasma reagin (RPR) to be determined by routine individual RPR testing, samples are tested 6 months apart with differences in reagent batches, environmental conditions, and observers all leading to measurement errors. We hypothesized that conducting paired RPR testing (simultaneous testing of acute and convalescent samples) would enable a more accurate determination of delta RPR compared with individual testing.

METHODS

A total of 120 study participants with a new diagnosis of syphilis were followed up at 0, 3, 6, 9, 12, 18, and 24 months, with RPR testing performed via individual testing at each study visit and at any suspected repeat syphilis. Rapid plasma reagin paired testing was performed on samples from 0 and 6 months and at any suspected repeat syphilis.

RESULTS

The quantitative agreement ±1 dilution among paired and individual testing was 97.2%. There was no difference in the proportion with serofast status at 6 months: 21 (19.4%) and 19 (17.6%) according to paired and individual testing, respectively (P = 0.726). There was no statistically significant difference between 0- and 6-month delta RPR as determined by paired and individual testing in predicting seroresponse at 12 months (86.1% and 91.6% agreement with 12-month serofast/nonserofast classification, respectively; P = 0.262).

CONCLUSIONS

In our setting, individual testing performed equally well compared with paired testing. Follow-up of syphilis will remain onerous for the patient and the health care provider until new tests that can more accurately assess the response to therapy and repeat syphilis/treatment failure are developed.