Background
Chlamydia trachomatis is a highly prevalent sexually transmitted disease.1-3 Although there is no systematic screening program in Norway, testing rates among young women are high, as are the positivity rates.4
By using data from a laboratory that covers all chlamydial diagnostics in a defined geographical area (a Norwegian county), we have previously shown that at the age of 25, one in six women had been diagnosed with C. trachomatis at least once.4 Chlamydial infections have potential long-term consequences such as pelvic inflammatory disease, tubal factor infertility, and ectopic pregnancy.5
Women with positive test results are often anxious about their future fertility.6,7 A recent review has however shown that the previously assumed high complication rates of C. trachomatis are now being increasingly questioned.8
We have previously used a registry-linkage approach and applied a retrospective cohort design to investigate ectopic pregnancy rates and birth rates by test result among women tested for C. trachomatis in a routine clinical setting.9 In the present study we have applied a similar design to assess the rate of pelvic inflammatory disease following a diagnosis of C. trachomatis.
Methods
The study was carried out in Sør-Trøndelag County, central Norway. Of the county’s 270,000 inhabitants approximately 150,000 are living in the major city, Trondheim.
Data Sources
In Norway, all citizens are given a unique 11-digit social security number at birth or at immigration. In the current study, we used two databases, both containing the personal identifier and data on residency.
In Sør-Trøndelag County, a single laboratory is responsible for all C. trachomatis diagnostics.4 The C. trachomatis database contains information on all tests (date, diagnostic method, and test outcome) in the county from November 1990 to December 2005 with person as the unit of analysis.
Information on all hospitalisations with a diagnosis of pelvic inflammatory disease was obtained from discharge registries 1990–2005 of the two hospitals in the county (Orkdal Hospital and Trondheim University Hospital). Patients with pelvic inflammatory disease were identified through computerised hospital in- and outpatient registries by using the International Classification of Diseases (ICD) 9th Revision code 614 during 1990–98 and the ICD 10th Revision codes N70 over the years 1999–2005.
Laboratory Methods
Several methods were used for C. trachomatis detection throughout the study period.4 Briefly, the IDEIA™ Chlamydia Test, Celltech Diagnostics/Novo BioLabs/DAKO was replaced by PACE 2™, GenProbe in 1992, whereas Amplicor from Roche Molecular Systems became the routine detection method in 1999.
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| Figure 1. Inclusions and exclusions of women to the study. |
Study Population
During November 1, 1990, to December 31, 2005, 101,649 women (all ages included) were tested for C. trachomatis, among whom 68,807 resided in Sør-Trøndelag County (Figure 1). We excluded 1,706 women without a valid personal identifier. In order to obtain a study population with a nearly complete testing history, we limited the population to women who were 20 years old or younger when computerised registration of test results started in 1990. Thus, 42,108 women born before 1970 or after 1984 were excluded from the study population (Figure 1). We finally excluded 46 women who were registered with a hospitalisation for pelvic inflammatory disease prior to their first registered C. trachomatis test. Thus, our study population consisted of 24,947 women who had not experienced a hospitalisation for pelvic inflammatory disease prior to their first C. trachomatis test.
Statistical Analysis
Confidence intervals (95%) for proportions were calculated using the Wilson method.10 All other analyses were carried out using SPSS for Windows, version 15.0 (SPSS, Chicago, IL).
For estimation of hazard ratio for pelvic inflammatory disease by C. trachomatis status we used Cox regression analysis with months from the first C. trachomatis test as the time variable. C. trachomatis status was entered as a time-dependent covariate. C. trachomatis status changed at the date of a positive test. Analyses were adjusted for age at first test. Women were followed up until the first date of hospitalisation with a diagnosis of pelvic inflammatory disease or censored on December 31, 2005.
Ethics
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