Diagnostic characteristics of tests for ocular Chlamydia after mass azithromycin distributions.

Trachoma is a blinding disease caused by ocular infection with Chlamydia trachomatis.1 Ocular chlamydial infection manifests clinically as conjunctival inflammation, characterized by conjunctival hyperemia and germinal lymphoid follicles on the upper tarsal conjunctiva. Repeated or persistent episodes of infection are associated with conjunctival scarring, which in turn may lead to entropion, trichiasis, and blindness.2–4 Trachoma-endemic districts are treated with mass administration of azithromycin to the entire population, regardless of each person's disease status. Given the central role of antibiotic therapy for trachoma, an ideal test for trachoma would assess chlamydial infection to provide information about the usefulness of future antibiotic treatments. The most widely used test for monitoring trachoma is conjunctival examination using the World Health Organization (WHO) simplified grading system.5 In this system, a grade of follicular trachomatous inflammation (TF) indicates the presence of five or more follicles at least 0.5 mm in diameter in the upper tarsal conjunctiva. A grade of intense trachomatous inflammation (TI) indicates obscuration of more than half the deep tarsal blood vessels. Both TF and TI are used to assess the burden of trachoma in a community. Laboratory tests for chlamydial infection are used primarily for trachoma research.6 Most commonly used is a nucleic acid amplification test (NAAT) that targets the cryptic DNA plasmid of chlamydia (Amplicor; Roche Diagnostics, Indianapolis, IN). A second-generation rRNA-based NAAT with an additional target capture step has been well characterized in the sexually transmitted disease literature. In a duplex format that detects both Chlamydia trachomatis and Neisseria gonorrhoeae, this rRNA-based test (Aptima Combo 2; Gen-Probe, San Diego, CA) has been shown to be among the most sensitive and specific of the NAATs for the detection of chlamydia.7–15 A version of this test that detects only chlamydia (Aptima CT) has proven to be an even more sensitive indicator of chlamydial infection.8,11,16 NAATs targeting chlamydial rRNA are not widely used by trachoma control programs or researchers, though several trachoma studies have reported results of conjunctival swabs using Aptima CT.17–19 Because of its superior sensitivity, the RNA test is a logical gold standard test against which to compare other tests for their ability to predict ocular chlamydia. The WHO recommends treating trachoma-endemic communities with annual mass azithromycin treatment, followed by reassessment after at least 3 years of treatment using the simplified grading system.20,21 However, the results of clinical examination are often discordant with results from laboratory tests for chlamydia, especially after mass azithromycin treatments.22 This finding may reflect differences in the kinetics of infection versus inflammation (i.e., there is an incubation period before inflammatory clinical signs develop and a period of persistent clinical signs after infection has cleared).6,23 Because the simplified grading system is used to determine whether mass antibiotic treatments should continue, its ability to predict chlamydial infection is important. We compared the diagnostic test performance (defined as the ability to correctly identify the presence of chlamydia) of the WHO simplified grading system with that of a DNA-based chlamydial test after three rounds of mass azithromycin using an RNA-based chlamydial test as a gold standard.