C-148. Nucleic Acid Amplification Tests to Determine the Prevalence of Chlamydia trachomatis Ocular Infection in Trachoma

J. Schachter, J. Moncada, T. M. Lietman;
Univ. of California, San Francisco, CA.

Background: Trachoma, a chronic keratoconjunctivitis caused by Chlamydia trachomatis (CT) is the world’s leading cause of preventable blindness. Nucleic acid amplification tests (NAATs) have been used to detect conjunctival CT infection in trachoma. An accurate estimate of prevalence may be needed to monitor control programs. We evaluated 3 different NAATs. Methods: Conjunctival swabs from Ethiopian children in a trachoma endemic area were collected in M4 medium (Remel, Lenexa, KS), frozen at -80º C, and sent to UCSF for NAAT testing by APTIMA Combo2, APTIMA Chlamydia trachomatis (AC2 and ACT, Gen-Probe, San Diego, CA) and COBAS (PCR, Roche Molecular Systems, Branchburg, NJ). Specimen aliquots were tested according to manufacturers’ instructions. A selection of APTIMA positive samples was further tested by serial tenfold dilutions. Results: We tested 180 specimens, and found 78 negative, 56 positive by all 3 NAATs, 2 positive by AC2 and ACT but negative by PCR, and 44 positive by ACT only. We retested 24 of the positive samples of which 7 were initially positive by all 3 NAATs. At a 1:1000 dilution, 7/7 were ACT positive, and 6/7 were AC2 positive. Two specimens were positive by AC2 and ACT, but negative by PCR. Upon dilution, both were positive in ACT at 1:1000, and in AC2 at <1:10. Of the 15 positive ACT only specimens, none were positive at >1:10, and some were negative on a repeat test, but positive on further testing. Discussion: The superior analytic sensitivity of ACT seems to translate into a superior clinical sensitivity with these trachoma samples. We found that specimens with all 3 NAATs positive had a large microbial load, as shown by 1:1000 dilution results. With somewhat less target, PCR became negative, and the AC2, while still qualitatively positive, did not test positive at 1:1000 as the ACT did. When ACT was the only positive test, the highly diluted specimens did not yield positive results, and some failed to repeat at >1:10. These type of results are seen when the microbial load is near the detection limit of the test (Schachter et al, JCM, 2006). Our results suggest that ACT gives a more accurate measurement of CT prevalence than either AC2 or PCR.