Supplemental Testing Is Still Required in Australia for Samples Positive for <i>Neisseria gonorrhoeae</i> by Nucleic Acid Detection Tests (2006)

The introduction and widespread application of nucleic acid detection tests (NADT) for Neisseria gonorrhoeae (i.e., gonococci [GC]) in Australia has enhanced diagnostic capacity and public health control of gonococcal disease. However, the well-recognized problems with the use and interpretation of GC NADT in the difficult and widely varying conditions in Australia led to a formal request for a specially convened expert reference group (ERG) to develop guidelines relevant to these different situations (9). We therefore noted with some concern conclusions by Lowe et al. (4) suggesting that supplemental testing for diagnosis of gonorrhea using GC NADT is not required in Australia when initial testing is by the Gen-Probe APTIMA Combo 2 (ACT) assay. This assertion contradicts the recommendations contained in the national consensus guidelines that were based on a review of the published literature and the extensive experience obtained over many years in the diverse test conditions experienced here. The ERG guidelines are consistent with another recent review of GC NADT testing in Australia (11) and the current National Pathology Accreditation Advisory Council of Australia (NPAAC) guidelines (7). The latter supersedes advice contained in the earlier NPAAC (6) guidelines on which Lowe et al. (4) relied.

There were a number of important factors that were subject to formal consideration by the Australian ERG seemingly ignored by Lowe et al. (4). First, this ERG (9) pointed to problems with GC NADT sensitivity that may result from the capacity of Neisseria gonorrhoeae to repeatedly recombine/alter its genome through acquisition or loss of genetic sequences. These alterations result in gain or loss of important characteristics not only for NADT-based diagnosis but also for culture-based analyses (3, 8). These subpopulations may vary quite substantially as a proportion of the total gonococcal population in different patient subgroups and over time and place (3, 5). The test numbers mentioned by Lowe et al. (4) suggest that their study was limited to a sample obtained over approximately 1 month only, which is insufficient to properly assess any GC NADT.

Second, the Australian guidelines (9) also emphasized that cross-reactions with non-gonococcal Neisseria strains remains a significant limitation of GC NADT and that specificity may also vary depending on the patient population. Lowe et al. (4) mention that the AC2 “does not cross-react with other Neisseria spp.” To our knowledge, this has not been extensively investigated in Australia or elsewhere. Hence, the high clinical specificity of the AC2 observed by Lowe et al. (4) may not necessarily reflect the performance of the AC2 in other patient populations, either within or outside Australia. Additionally, the numbers of assays positive for GC were very low (n = 39), not all four GC NADT were used in all instances introducing a bias into the “resolved” data, and the population (not specifically defined) was seemingly a low-prevalence group, further compromising the assessment.

Third, experience in Australia has shown us that commercial GC NADT satisfying existing FDA standards and the earlier NPAAC guidelines (6) but using limited prerelease evaluation have had to be subsequently withdrawn from use in Australia (9). Some “in-house” assays (5) were similarly withdrawn after more extensive and complete “in-use” appraisals revealed substantial deficiencies in performance. The relatively small number of samples examined by Lowe et al. (4) would not reliably detect these problems.

Also in an Australian context, Lowe et al. relied on a cppB-based supplemental assay as their final “confirmatory” test (4), despite the fact that the use of this assay as both a screening and supplemental assay has been associated with false-negative and false-positive results both in Australia and elsewhere (1, 3, 9, 10).

It is important that GC NADT is subjected to rigorous, extensive, and continuing evaluation before being endorsed as being suitable for use as a single definitive diagnostic test. It is our view that Lowe et al. (4) failed to satisfy the principles and details required for proper appraisal of this GC NADT, particularly in an Australian context. Data such as those presented may be relevant to some situations and do contribute to the wider appraisal that we recommend. However, an incomplete evaluation performed at one geographic location at one time point on small numbers of positive cases is essentially meaningless if it is to be used, as implied by Lowe et al., as a basis for universal test validation for a particular GC NADT.

In the opinion of both the Australian ERG (9) and other national bodies (7), a modification of the Centers for Disease Prevention and Control approach (2) that includes supplemental testing provides a reasonable and pragmatic basis for reliable reporting of results of GC NADT in Australian conditions. It was also the considered opinion of the ERG (9) that the available local data on AC2 assays were to date incomplete and that total reliance on manufacturers' data to remedy this deficiency may be misplaced. The additional information supplied by Lowe et al. (4) does not alter these views.