Literature review > Issue 8 > Review on Shrier et al. 

Chlamydia trachomatis (CT) infection causes substantial morbidity which could be diminished by detection and treatment prior to the occurrence of permanent damage to the female reproductive tract. Because the majority of women with CT infection do not recognize associated symptoms, detection based upon highly sensitive laboratory methods is necessary for control of this infection. Previous studies have shown that testing urine and endocervical specimens from asymptomatic women using nucleic acid amplification tests (NAAT) for CT provides superior sensitivity to cell culture methods. Early studies of NAAT generally used single specimens tested with multiple methods, a “specimen standard”. Because CT may be detected from urine, endocervical, vaginal and urethral sites independently in a single patient, investigators have recognized the need for a broader consensus definition for a true positive test. By performing each detection method on specimens collected from a variety of anatomic sites in a single individual, investigators can derive an “infected patient standard”. Because previous studies have shown high specificity for each of the methods of laboratory testing for CT currently available, consensus definitions of true positive would be expected to increase the total numbers of infected patients identified and provide data which might improve the sensitivity of screening tests.

Shrier et al. examined the sensitivity of nucleic acid amplification tests and cell culture using nine collected specimens from each patient. The study population consisted of asymptomatic women aged 16-25 presenting for routine gynecological care. Patients were excluded if they had never been sexually active, if they were pregnant, if they had been diagnosed with chlamydial infection within the preceding six weeks, if they had taken antibiotics within the preceding 21 days, or if they were known to be a sexual contact of a partner with an STD. After exclusions, 139 patients constituted the study population. Of these, 126 patients had a complete set of laboratory specimens obtained, and were further analyzed.

Subjects self-collected vaginal swabs for polymerase chain reaction testing (PCR; Cobas Amplicor, Roche Diagnostic Systems, Branchburg, NJ). Each participant also collected the first 30 ml of voided urine for PCR and ligase chain reaction testing (LCR; LCx assay, Abbott Laboratories, North Chicago, IL), with the timing of the urine collection during the visit left to the subject. A clinician performed a pelvic examination and collected 1) a distal urethral swab for CT PCR, 2) a distal urethral swab for CT cell culture, and 3) a vaginal swab for CT PCR. Following wiping of the cervix, endocervical specimens were obtained for 4) Neisseria gonorrhoeae culture, 5) CT LCR, 6) CT PCR, 7) CT cell culture.

If only one specimen from a subject was positive by NAAT, that specimen was retested by another laboratory using nested PCR to target conserved portions of the major outer membrane gene (ompA). The specimen was considered positive in the presence of a positive ompA PCR result. Eight patients with positive urine LCR and one patient with a positive vaginal PCR were found to be negative by nested PCR. These were considered as false positives in the subsequent analysis.

A consensus true-positive was defined as either 1) a positive cell culture result, 2) a positive PCR and positive LCR from the same specimen, 3) 2 positive NAAT from different specimens, or 4) a single positive NAAT confirmed by nested PCR.

CT was diagnosed in 27 of 126 (21.4%) subjects. Four subjects had positive test results on all 9 specimens. Fifteen subjects had 2-8 positive results. Eight patients had a single positive NAAT result confirmed by nested PCR.

As expected for a study in which four specimen sources and three test types were used to derive an “infected patient standard”, the sensitivity of a single test for any anatomical site was lower than reported for studies using multiple tests on single specimens or testing against cell culture. The authors investigated combination testing strategies using their results, but found a test sensitivity of >80% only for the combination of urine LCR plus endocervical LCR. Unfortunately, the specificity of the urine LCR was found to be only 90.9%, whereas all other tests had specificities >99%. This produced a positive predictive value of only 71.9%.

The authors' conclusion that a single specimen may have limited sensitivity in CT detection is appropriate. In 2003, the LCR test utilized in this study was withdrawn by the manufacturer due to quality control problems. This does not negate the conclusions, which reflect the fact that CT infection can be detectable at a variety of anatomic sites, and that detection is compromised by sampling error. Other studies using the infected patient standard for multiple tests of CT infection have corroborated the insensitivity of single specimens [1,2].

Although NAAT provides a substantial improvement over cell culture in the detection of CT infection in asymptomatic women, further large-scale studies are necessary to define a practical algorithm for screening asymptomatic populations. Studies of NAAT which combine patient-collected specimens from different sites may ultimately enhance single-test sensitivity to acceptable levels. Whether the manufacturers of commercial laboratory tests for CT will be enthusiastic in sponsoring trials aimed at decreasing the number of tests used to identify infected patients is uncertain.

References

1. Moncada J, Schachter J, Hook EW 3rd, Derrero D, Gaydos C, Quinn TC, Willis D., Weissfeld A, Martin DH. The effect of urine testing in evaluations of the sensitivity of the Gen-Probe Aptima Combo 2 assay on endocervical swabs for Chlamydia trachomatis and Neiserria gonorrhoeae: the infected patient standard reduces sensitivity of single site evaluation. Sex Transm Dis 2004; 31 (5): 273-7.

2. Marrazzo JM, Johnson RE, Green TA, Stamm WE, Schachter J, Bolan G, Hook EW 3rd, Jones, RB, Martin DH, St. Louis ME, Black CM. Impact of patient characteristics on performance of nucleic acid amplification tests and DNA probe for detection of Chlamydia trachomatis in women with genital infections. J Clin Microbiol 2005; 43(2): 577-84.

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