Does the diagnosis of trachoma adequately identify ocular chlamydial infection in trachoma-endemic areas?

We evaluated the validity of clinically determined active trachoma as a surrogate for chlamydial eye infection in 1059 children from the Egyptian arm of the Azithromycin in the Control of Trachoma study. Participants were determined to be "clinically active" if they had >or=5 follicles or intense inflammatory infiltration on the tarsal conjunctiva. Conjunctival swabs were tested using ligase chain reaction (LCR) to detect chlamydial DNA. Of clinically active children aged 1-10 years, 31% did not have infection, as determined by LCR. Conversely, 31% of infected children were not clinically active; 78% of clinically active children aged 1-5 years were infected, versus 17% of those aged 11-15 years. The proportion of clinically active children who were infected decreased from 67% before treatment to 10% 14 months after mass azithromycin treatment. Clinically active trachoma is not always a reliable marker of infection, particularly in teenagers and after treatment.