The Performance of Immunoassays to Measure Antibodies to the Chlamydia trachomatis Antigen Pgp3 in Different Epidemiological Settings for Trachoma

Programs to eliminate trachoma as a public health problem use prevalence of the clinical sign trachomatous inflammation-follicular (TF) in 1-to 9-year-olds in endemic districts to make decisions to begin or end mass drug administration with azithromycin. Trachomatous inflammation-follicular is used as a proxy for transmission of ocular Chlamydia trachomatis infection. Long-term monitoring of previously endemic districts for recrudescence of ocular C. trachomatis infection would benefit from a simple blood test that could be integrated with other public health programs. In this study, we evaluated multiple tests to measure antibodies against the C. trachomatis antigen Pgp3-a multiplex bead assay (MBA), an ELISA, and two versions of a lateral flow assay (LFA)-in four districts of the Amhara region of Ethiopia with varying levels of TF. Seroprevalence and seroconversion rate (SCR) results were proportional to TF prevalence by district for most tests, with the notable exception of the LFA using colloidal gold as the developing reagent. Changing the test developing reagent to black latex improved agreement between serological measures and TF prevalence and in inter-rater agreement. Seroconversion rate estimates using data derived from the LFA-gold assay were inconsistent with the shape of the ageseroprevalence curve, which did not increase in older ages. These data revealed potential complications with using SCR that will need further evaluation. Data from MBA, ELISA, and LFA with the black test line showed good agreement with each other and proportionality to TF estimates, providing further data that serology has potential utility for trachoma surveillance.

Automated summary

This study evaluated multiple tests to measure antibodies against the C. trachomatis antigen Pgp3 in different districts of Ethiopia. Results showed that one of the tests initially had inconsistent results with TF prevalence, but improved after changing the developing reagent. Long-term monitoring for recrudescence of ocular C. trachomatis infection could benefit from serological testing, which showed good agreement and proportionality with TF estimates.