Question
How well do three nucleic acid amplification assays (NAATs) perform for the detection of C. trachomatis in urine specimens from students attending school-based health clinics?
Design
The performances of three NAATs (ligase chain reaction, transcription-mediated amplification, and strand displacement amplification) were compared for the detection of C. trachomatis in urine samples from symptomatic and asymptomatic males and females attending high school health clinics using any two positive NAATs as a true-positive result.
Participants
Five hundred six urine specimens from asymptomatic and symptomatic males and females attending various school-based health clinics for routine screening for STD were tested. The age range was 12 to 20 years.
Description of Tests and Diagnostic Standard
Subjects, who were asked not to urinate for at least 1 h prior to providing a specimen, collected 20 mL of the initial urine stream. Urine specimens were transported, stored, and tested according to the manufacturers‚ instructions for each assay. The specimens were each tested by three NAAT, including 1) a transcription-mediated amplification assay, the APTIMA Combo 2 (AC2 assay, Gen-Probe, Inc., San Diego, CA), which detects both C. trachomatis and N. gonorrhoeae and targets C. trachomatis rRNA; 2) a ligase chain reaction assay (LCx Probe, Abbott Laboratories, Abbott Park, IL), which targets a sequence within the cryptic plasmid DNA of C. trachomatis; and 3) a strand displacement amplification assay (BD ProbeTec, Becton Dickinson Diagnostic Systems, Sparks, MD), which also targets a sequence within the cryptic plasmid DNA of C. trachomatis. A patient was considered infected with C. trachomatis if his or her urine specimen was positive by at least two of the three NAATs.
Samples that were positive only by the AC2 were retested using a transcription-mediated amplification assay (ACT, APTIMA C. trachomatis, Gen-Probe) that targeted the 16S rRNA of C. trachomatis. Samples that were positive only by the LCx were retested using a polymerase chain reaction assay (PCR, Roche Diagnostic Systems, Indianapolis, IN).
Main Outcome Measures
The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of each assay were evaluated by comparing the assay‚s results with the infected patient status.
Main Results
The prevalence of C. trachomatis was 14.8% (75 infected individuals of 506). Sixty-nine specimens were positive and 422 were negative by all three assays. Four and five samples were positive only by LCx and AC2, respectively. Three and three samples were negative only by LCx and ProbeTec, respectively. The performance of each NAAT for the detection of C. trachomatis in urine specimens as determined by any two of three NAAT giving a positive result is shown in the table. There were no statistical differences in the performance characteristics between any of the 3 assays. Four of the 5 samples positive by AC2 only were confirmed positive by a TMA-based assay that amplified a different target sequence than the AC2. Of the 4 specimens positive only by LCx, the 2 that were available for further testing were both positive by PCR.
Performance of three NAAT for the detection of C. trachomatis in 506 urine specimens from young men and women attending high school health clinics.
|
NAAT
|
Assay performance (%)
|
|
Sensitivity
|
Specificity
|
PPV
|
NPV
|
|
LCx
|
96.0
|
99.1
|
94.7
|
99.3
|
|
ProbeTec
|
96.0
|
100
|
100
|
99.3
|
|
AC2
|
100
|
98.8
|
93.8
|
100
|
Authors‚ Conclusions
All three assays demonstrated high performance. NAATs present a great improvement in C. trachomatis detection with urine specimens and can be recommended for C. trachomatis screening programs where noninvasive and convenient self-collection is preferred.
Source of funding: Kits were donated by Becton Dickinson and GenProbe.
For correspondence: Charlotte A. Gaydos, Johns Hopkins University, 720 Rutland Ave., Ross 1159, Baltimore, MD 21205. E-mail address:
cgaydos@jhmi.edu.
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