A positive IUS with both NAATs was identified in 40 of the 47 diagnosed patients, whereas discrepancies or a negative result were detected in 5 and 2 patients, respectively. Among the remaining 163 negative patients, one was identified as false-positive using only Amplicor PCR and one, using only nPCR (Table 1). Regarding the FVU, a positive result with both NAATs was identified in 41 of the 47 diagnosed patients, whereas discrepancies or a negative result were detected in 2 and 4 patients, respectively. Among the 163 negative patients, one was identified as false-positive using the Amplicor PCR and 4 different patients were identified as false-positive using nPCR (Table 1). Regarding the PSS samples, a positive result with both NAATs was obtained from 34 of the 47 diagnosed patients, whereas discrepancies or a negative result were obtained from 7 and 6 patients, respectively. Among the remaining 163 negative patients, one was identified as false-positive using only Amplicor PCR and one, using only nPCR (Table 1). The combined use of IUS and PSS with nPCR identified all 47 diagnosed patients (Table 2).
Our study showed that all 3 sampling techniques were <100% sensitive using a single NAAT assay at a time. In the literature, IUS is considered the standard sample for diagnosis of chlamydial urethritis.1 Nevertheless, as the procedure is uncomfortable and requires a visit to healthcare settings, noninvasive sampling techniques such as FVU have been alternatively proposed.1,2,13–18 In general, diagnostic sensitivity of IUS sampling has been considered superior in comparison with the FVU.10 The current study indicated that both techniques resulted in equal sensitivity when using the Amplicor PCR, whereas the IUS was slightly more sensitive when using the nPCR. Nevertheless, as these differences were only marginal, both methods were considered as equally suitable for diagnosis of chlamydial urethritis.
The combination of testing both IUS and PSS samples with nPCR yielded 100% sensitivity, increased from what was seen using 1 of the 2 sampling procedure at a time. The combined use of the 2 NAAT assays in any single sample at a time, also increased sensitivity, especially regarding PSS, and this was achieved without substantial loss of specificity. The suggestion, however, of incorporating a second NAAT in every day clinical practice cannot be easily considered because of additional cost and workload issues. The combination of 2 sampling methods with 1 NAAT is also a questionable approach and should be considered only when a significant increase in sensitivity occurs.
The 2 NAAT assays tested have different DNA targets. The Amplicor PCR is a commercially available, FDA approved, one-step method, targeting the conserved cryptic plasmid that exists in 4 to 10 copies in the genome of C. trachomatis, thus resulting in high sensitivity, and can be considered in every day clinical practice. The nPCR is an in-house established assay, targeting the single copy omp1 gene and its high sensitivity comes as a result of the nested procedure. In that respect, nPCR can be kept as a backup and confirmatory technique. As plasmid free strains19 or novel variants with deletions in the cryptic plasmid target sequence exist,20 a laboratory should be able to implement such alternative NAATs.
In conclusion, this study demonstrated that PSS, a noninvasive sampling method resulted in lower sensitivity when compared with traditional procedures and only marginally increased overall the diagnostic yield when used in combination with FVU or IUS for the molecular diagnosis of C. trachomatis urethritis. In that respect, its use should be further evaluated after optimization and in large epidemiologic studies.
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