Sexually Transmitted Diseases:
Letters to the Editor: Author's Response
Bakken, Inger J. PhD
Department of Epidemiology; SINTEF Health Research; Trondheim, Norway
To the Editor:
The interest that Andersen et al1 have shown in our study on Chlamydia trachomatis (CT) as a risk factor for ectopic pregnancy2 is highly appreciated.
Andersen et al have several comments to our paper that will be addressed successively in the following. First, Andersen et al included only women with a registered pregnancy to the analyses, arguing that it is important to include only women at risk for an ectopic pregnancy. When we conducted our study we did not have access to data on intrauterine pregnancies. We were, therefore, not able to repeat Andersen's study design but suggested in our discussion that including only women with registered pregnancies to the analyses might possibly have led to underestimation of ectopic pregnancy risk. It would have been interesting to see analyses from the Andersen research group that included data from all women irrespective of fertility history. Such analyses would settle the discussion on whether the hazard ratio would increase or decrease, at least in their data. Second, we certainly had access to data from a single laboratory only. However, this laboratory is the only facility for CT diagnostics in the study area, as stated clearly in our paper. Third, our concern regarding the observation period in the study by Andersen et al is that the period when data on CT tests were collected is relatively short and restricted to the first few years of the total observation period.
Andersen et al also offer other explanations to the differences between the results from the two studies. Indeed, our Norwegian population is opportunistically screened whereas the Danish is more sporadically tested. Information on all tests in a population frequently tested gives better access to the history of prior chlamydial infections. Also, the results for the oldest age group in our study are similar to the results in the study by Andersen et al. As discussed in our report, we believe the main reason for the lack of association for the oldest women is the incomplete testing history in this age group. Finally, not being able to adjust for potential confounding factors is a general weakness of studies based on registry data. However, as pointed out in our paper, sexual behavior and contraceptive use are not likely to be independent risk factors or true confounders. Such characteristics are more likely to reflect an increased risk for sexually transmitted infections.
Andersen et al suggest that the conclusion by Low et al3 that the incidence of severe complications associated with chlamydial infection is lower than expected might apply to all 3 Scandinavian studies. However, these studies include women tested for CT, and although no treatment data are available, it seems unlikely that a large proportion of women with CT were left untreated. The relatively low complication rates also among women with positive tests might rather indicate that screening for chlamydial infection benefits female reproductive health.
1. Andersen B, Østergaard L, Thomsen RW, et al. Chlamydia trachomatis and risk of ectopic pregnancy. Sex Transm Dis 2007; 34:59.
2. Bakken IJ, Nordbo SA, Skjeldestad FE. Chlamydia trachomatis infections increase the risk for ectopic pregnancy: A population-based nested case-control study. Sex Transm Dis 2007; 34: In press.
3. Low N, Egger M, Sterne JA, et al. Incidence of severe reproductive tract complications associated with diagnosed genital chlamydial infection: The Uppsala Women's Cohort Study. Sex Transm Infect 2006; 82:212–218.