Trachoma Control: A Potential Field Test for Ocular C. Trachomatis Infection

Abstract

Purpose: There has been an increasing demand for an inexpensive field test for ocular C. trachomatis (CT) detection in resource-limited settings. The purpose of our study was to determine the sensitivity, specificity, and field utility of the Cephied GeneXpert (CGX) CT/NG RUO test done in rural Tanzania compared to the Roche Amplicor test done in a standard laboratory in the US.

Methods: 144 children ages 0 to 9 in a trachoma-endemic community in Kongwa Tanzania were surveyed to assess clinical trachoma and have two ocular swabs taken from the same eye. Both swabs were stored dry, one shipped to JHU and stored frozen, the other stored frozen in Kongwa, Tanzania. With one freeze-thaw cycle at both sites, the specimens at Johns Hopkins were reconstituted and tested using Amplicor PCR test, where positivity was defined >0.8; the specimens at Kongwa were reconstituted and tested using CGX, where positivity was defined as a field value of 1. The sensitivity and specificity of the new test against the commercial standard was compared, as well as the field utility of CGX.

Results: Of the samples tested in the field, 127/144 (88%) yielded results. Of the 12% of samples that could not be processed, half were due to insufficient volume (likely the result of drying out during reconstitution in the field) and the other half were electricity issues and clogged machine. The prevalence of follicular trachoma was 44% and of infection according to the Amplicor PCR was 29%. The sensitivity of CGX was 100% and the specificity was 95% when compared to Amplicor PCR as the “gold-standard”. CGX identified 5 more positives than Amplicor, and in four of those, the children had either trachoma or another test for infection that was positive.

Conclusions: The sensitivity and specificity of the GeneXpert CT/NG RUO assay, as done in the field in rural Tanzania, was extremely good compared to Amplicor testing done at a reference laboratory in the US. This is among the first PCR based test to show these results in a field setting. Pending a cost analysis and evaluation of pooling methods, it is a promising new testing method for evaluating trachoma control in the field in resource limited environment.

Keywords: 736 trachoma • 467 clinical laboratory testing • 468 clinical research methodology