Screening for chlamydial infections and the risk of ectopic pregnancy in a county in Sweden: ecological analysis

British Medical Journal, June 13, 1998 by Matthias Egger, Nicola Low, George Davey Smith, Bo Lindblom, Bjorn Herrmann

Introduction

In industrialised countries Chlamydia trachomatis is the predominant infectious agent causing pelvic inflammatory disease[1-3] and, as a result of damage to the fallopian tubes, accounts for up to half of all ectopic pregnancies.[4-5] The substantial financial costs of genital chlamydial infections result from hospital treatment for pelvic inflammatory disease, ectopic pregnancy, and infertility, which may include in vitro fertilisation.[6] Programmes to reduce the incidence of genital chlamydial infection have not been widely implemented except in Sweden.

Policies to prevent chlamydial infection were gradually introduced in the 1980s in Sweden.[7] These included the establishment of a national diagnostic service[8]; testing of women in antenatal, family planning, and abortion clinics[8-9]; a statutory requirement since 1988 that doctors trace and treat the sexual contacts of patients with chlamydial infections[10]; and the establishment of youth clinics which provide health education, condoms, and testing and treatment for chlamydial infection.[8]

Declining rates of chlamydial infection and associated pelvic inflammatory disease in Sweden during the 1980s have been attributed to these policies.[8 9 11] The effect of these policies on the incidence of sequelae such as ectopic pregnancy and infertility are less clear. Trends in the occurrence of ectopic pregnancy are thought to lag behind those of chlamydial infection by several years,[4 5] but this association has not been examined in a population based study. Uppsala county, north of Stockholm, consists of six municipalities with a total population of 269 000 in 1990. Screening for chlamydial infection among the female population has been extensive[8] and care for ectopic pregnancies is provided in one hospital only. Uppsala county thus provides an exceptional setting for an ecological study of the association between genital chlamydial infection and ectopic pregnancy.

Material and methods

Ascertainment of infections and pregnancies

The Uppsala chlamydia database has been described in detail elsewhere.[8] Briefly, over 99% of examinations for infection with C trachomatis are performed in a single laboratory. A total of 110 834 cervical samples were collected from women of all ages between 1985 and 1995. Overall, 23% of samples came from 20 private practices, 16% from one sexually transmitted disease clinic, 15% from three family planning clinics, 14% from six youth clinics, 14% from five gynaecology clinics, 10% from 34 primary healthcare clinics, 7% from 14 antenatal clinics, and 1% from other sites. Altogether 89% (99 133/110 834) of samples were cultured in McCoy cells treated with cycloheximide, according to the methods of Ripa and Mardh[12]; 10% (10 855/110 834) were analysed by enzyme immunoassay (EIA, Abbott Diagnostics, Chicago, or Syva, Palo Alto); and 0.8% (846/110 834) were analysed by direct immunofluorescence (Syva, Palo Alto). Results of 568 examinations (0.5% of all samples) could not be interpreted.

The list of discharge diagnoses from University Hospital in Uppsala was used to determine the number of times ectopic pregnancy was the primary diagnosis for women aged 15 to 39. This list includes all episodes diagnosed in the county. Population numbers, births, and the number of induced abortions were obtained for women at each age from the county statistical office or the epidemiological centre of the National Board of Health and Welfare. The total number of pregnancies reported was calculated as the sum of live births, induced abortions, and ectopic pregnancies.

Statistical analysis

Rates of genital chlamydial infection per 100 examinations and rates of ectopic pregnancy per 1000 reported pregnancies between 1985 and 1995 were calculated for women aged 20-24, 25-29, 30-34, and 35-39 years. The association between the two rates was examined using linear regression analysis. [chi-square] tests were used to examine trends by age.

The same rates were calculated for women at each age and for each calendar year for chlamydial infection among women aged 15-39 and for ectopic pregnancy among those aged 20-39. Poisson regression models were defined to examine the strength of the association between the rates of chlamydial infection and the risk of ectopic pregnancy at each age. Models were calculated to examine the association between the rate of ectopic pregnancy and the rate of chlamydial infection in the same year and for the rates of chlamydial infection occurring 1 to 5 years earlier. For example, in the model examining the importance of the rate of chlamydial infection 5 years earlier, the rate among 15 year olds in 1985 was used to predict the risk of ectopic pregnancy among 20 year olds in 1990. This meant that some rates were missing in earlier years. The rate 1 year earlier was available from 1986 onwards and the rate five years earlier from 1990 onwards. Analyses were adjusted for age within each age group.