Literature review > Issue 1 > Review Morre et al. 

There is clearly a need for a less technically demanding, more reproducible, and less expensive serological test for Chlamydia trachomatis than the microimmunofluoresence (Micro IF) test. In experienced hands, this test has been very useful in defining the seroepidemiology of chlamydial infections but results vary even between experienced laboratories. Thus the development of ELISA serological assays using specific chlamydial peptides as antigens is a welcome development.

This study by Morre et al compared the performance of three new ELISA serological tests with the MicroIF. Several points regarding the study design and analysis are of importance. First, as mentioned above, the performance characteristics of the MicroIF test vary from laboratory to laboratory so the results presented here could differ from a second Micro IF standard in another laboratory. Assessment of such variation as compared with the ELISA results would be of interest in future studies. Second, the population studied was a very high prevalence group (29% positive by PCR) and serological test performance would likely be different in a low prevalence population. Thus, performance of the tests for screening in a lower prevalence setting cannot be inferred from these results. Third, it was disappointing that no data on C. pneumoniae serology in the study population was provided in the report since cross-reacting C. pneumoniae antibodies have been a problem in performing C. trachomatis Micro IF assays and may be a problem with the peptide-based assays as well. The Micro IF assay was said to show no cross-reactivity with C. pneumoniae and cross-reactions were not thought to be responsible for discrepant results in the study, but data on these issues would have been interesting to see.

In terms of analysis, the authors used a modified discrepant analysis to define a true positive result. The approach taken was not fully described in the article but appears to have combined both discrepant analysis and a "composite" gold standard. Thus, a test that was MicroIF positive, but negative in the comparative peptide ELISA, was subjected to discrepant analysis by repeating both the original MicroIF and then a second Micro IF. A positive in either of the two MicroIFs was defined as a true positive. This approach likely increased the calculated sensitivity of the MicroIF since the same test was used (in fact, used twice) to define positivity in the subset in which discrepant analysis was performed. If the specimen was positive in two of the three peptide assays, it was also considered positive as using a "composite positive" approach. The sensitivity of the new tests, given these methods of calculation, did not appear to be very different from the MicroIF when looking at IgG antibody. However, the lack of concordance of the tests suggests that the tests were performing quite differently in a substantial minority of specimens since there were 13 specimens positive by peptide ELISAs only and 10 positive by Micro IF only. The reasons underlying these differences are not clear. While it is difficult to know exactly what "gold standard" is most appropriate in this type of study, an alternative approach using a composite analysis in which true positives are defined as specimens in which any two of the four serological tests performed on all samples are positive would be an interesting alternative.

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