Literature review > Issue_1 > Review Akduman et al. 

Nucleic acid amplification assays for diagnosis of sexually transmitted diseases (STDs) have been shown to be extremely sensitive and specific. Many of these assays have been cleared by the Food and Drug Administration (FDA) for routine clinical diagnosis of Chlamydia trachomatis and Neisseria gonorrhoeae. Due to their exquisite sensitivity, it has been shown that they can be used with noninvasive specimens such as urine or self-administered swabs without a loss in either sensitivity or specificity. By utilizing noninvasive specimens, surveillance and clinical diagnosis of common STDs can be performed in routine clinic settings as well as non-clinic settings, enabling public health officials to screen for these STDs in community-based settings.

A new nucleic acids amplified assay is the Becton Dickinson Strand Displacement Assay, referred to as the "BD ProbeTec-SDA" assay. This new diagnostic test has been cleared by the FDA for diagnosis of Chlamydia trachomatis and Neisseria gonorrhoeae in routine clinical specimens and urine specimens. The BD ProbeTec-SDA system is a semi-automated system that uses strand displacement amplification for the simultaneous detection of C. trachomatis and N. gonorrhoeae. Products are detected in solution with fluorescent detector probes. Positive and negative controls for specimen processing are also included in the kit along with amplification control to monitor for assay inhibition. Aktuman and colleagues evaluated the diagnostic performance of this assay for the detection of N. gonorrhoeae infection by comparing the results of the assay with those obtained by culture, and then by retesting all specimens with positive results and 280 specimens with negative results. A total of 3,544 urine specimens were collected during a three-month period from patients attending the Denver Metro Health STD Clinic. The investigators further examined technician proficiency by repeating all the positive tests and a subsample of the negative specimens twice by two experienced technicians according to the manufacturer's instructions.

Of the 3,544 patients, 153 were positive for N. gonorrhoeae by one or both of these tests. One hundred twenty-nine were BD ProbeTec-SDA assay positive and culture positive. One specimen was BD ProbeTec-SDA assay negative but culture positive, and 23 were BD Probe Tec-SDA assay positive but culture negative. Thus, the overall discrepancy rate was 24 of 3,544 specimens (or 0.7%). These results are consistent with previous studies with other nucleic acid amplified assays where the more sensitive molecular assay has more positives than are detected by culture. While this was to be expected, the unusual finding in this study was that only seven of the 23 discrepant test results could be confirmed by repeat testing on the same assay. Furthermore, 50% of the specimens with discrepant positive results had MOTA readings in the range of 2,000 to 10,000, which the manufacturer defines as in the low positive range. The rates of discrepancy did not appear to significantly differ between the two technicians. A further disturbing finding in the study was that of the 280 specimens initially BD ProbeTec SDA assay and culture negative, eight (2.9%) were positive for N. gonorrhoeae by one of the two repeat tests.

In summary, while the test is relatively sensitive and specific and has a very high negative predictive value, its positive predictive value was only 84.9%. Furthermore, many of the culture-negative BD ProbeTec SDA assay-positive specimens were not positive upon repeating testing in the same assay. I support the authors' conclusion that due to the very high sensitivity of these new assays and potential for contamination, the test should be performed under stringent quality control conditions and laboratory technicians should be aware of the potential for sample-to-sample in environmental contamination. The authors list several areas for concern. Problems may originate during sample processing especially when transferring specimens from priming wells to the amplification wells. In order to avoid contamination, Becton Dickinson has recommended the use of a hand drill for removal of caps. Due to the heating in the heat block, liquid is frequently present around the tops of the tubes and it is difficult to remove the caps that are placed closely in the block without cross-contamination. The company now recommends that the block be tapped on the counter to shake down fluid before removal of the caps. In addition, the work area and automatic pipetter need to be cleaned frequently with 20% sodium hypochlorite-detergent solution. Environmental samples need to be taken often before and after routine cleaning to observe for further contamination. Finally, it is recommended that all specimens with MOTA values in the range of 2,000 to 10,000 need to be retested for validation purposes.

Since the positive predictive value was low with this assay and this is often a function of prevalence, it is recommended that culture confirmation or repeat testing by another assay may be needed for samples with positive results in populations with relatively low prevalence for gonorrhea. Even with a relatively high prevalence of N. gonorrhoeae (4.3% in this survey), the authors recommend repeating the assay in duplicate for all specimens that are initially positive. These conclusions are compatible with the recent guidelines published by the CDC for laboratory testing for STDs. While these assays are exquisitely sensitive and specific and allow for the use of noninvasive specimens like urine, laboratory procedures must be implemented to avoid contamination and to monitor for contamination and inhibition. Repeat testing and, on occasion, confirmation by culture, or by another assay, or with a competitive probe provides the cautious approach to diagnosis of these pathogens particularly in low prevalence populations.

References:

1. Centers for Disease Control and Prevention. Screening tests to detect Chlamydia trachomatis and Neisseria gonorrhoeae Infection-2002. MMWR 2002;51(No. RR-15):17-18.

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