CHLAMYDIA TRACHOMATIS is the most common bacterial pathogen transmitted by sexual intercourse. The prevalence of the infection ranges from 2% to 40%, depending on the population screened. 1–4 In the Western world, genital chlamydial infection accounts for the majority of preventable pelvic inflammatory disease and infertility. 4 As the infection is usually asymptomatic, screening is the best strategy to prevent its spread. Although universal screening has been shown to be cost-effective when the prevalence of chlamydial infection exceeds 3.9%, resources to test all sexually active women presenting to a family planning clinic are often inadequate. 5 Without a specific local screening protocol, many practitioners assume that testing for C trachomatis would be necessary only if the patient has symptoms or signs of genital tract infection. Consequently, all too often, asymptomatic women at the highest risk for chlamydial infection do not receive testing at a sufficient level. 6
The strategy of selective screening based on risk factors offers an alternative to testing all sexually active women. In a population with low C trachomatis prevalence, the selection of patients for screening is particularly important. Some of the previous studies have suggested that demographic and behavioral characteristics are successful in identifying women at high risk of the infection, whereas others have found no reliable criteria for screening in a low-prevalence population. 2,3,7–11 So far, most research attempting to find risk factors that could be used as selective screening criteria have been conducted in racially/ethnically or socioeconomically diverse populations, with the identification of multiple sex partners in the past year, black race, and age younger than 25 years as the most important predictors of chlamydial infection. 2,3,7–15 As the risk of chlamydial infection and other sexually transmitted diseases (STDs) depends on cultural factors associated with ethnicity and socioeconomic status, the risk markers might be different in low-prevalence populations with relatively homogenous background and access to preventive health care.
We undertook a cross-sectional study in two Finnish family planning clinics to find screening criteria for a population with low prevalence of genital chlamydial infection. In Finland, examination and treatment for C trachomatis infection are performed in various settings for free, and public health education campaigns have informed people about C trachomatis infection since the 1980s.
Sexually active women attending family planning clinics at the municipal health center in the City of Porvoo and at the University Student Health Clinic, University of Helsinki, Helsinki, from January 1995 through January 1996 were asked to participate in this study. All women who had not received antibiotics during the previous month and who were able to give a sample of urine that had been in the bladder a minimum of 2 hours were eligible. After giving a written informed consent, participants were asked to anonymously complete a questionnaire about demographic data and history of urogenital symptoms and genital tract infections, as well as sexual behavior. Women using condoms as their primary contraceptive method were asked to report the consistency of use by choosing one of two categories: “always” or “sometimes.” Each participant sealed her answered questionnaire in an envelope, which was marked by a nurse practitioner with a unique study number assigned to each subject. The study was approved by local committees on human research in the Porvoo Health Center and at the University of Helsinki.
Subjects were instructed to collect 25 ml of first-void urine. A clinician then performed a standard gynecologic examination and obtained an endocervical swab specimen for chlamydia antigen detection. Urine specimens were centrifuged and the pellets were frozen at −20 °C. The endocervical specimens were kept at +4 °C until transported with urine specimens, consent forms, and envelopes containing answered questionnaires to the chlamydia laboratory of the Department of Virology, Haartman Institute, University of Helsinki, within 5 days of collection. Urine specimens were processed and tested by polymerase chain reaction (PCR) for C trachomatis DNA (Amplicor; Roche Diagnostic Systems, Nutley, NJ), as described previously. 16 Cervical swabs were tested for C trachomatis antigen by enzyme immunoassay (EIA, MikroTrak II; Syva and Behring Diagnostic Products, Cupertino, CA), and positive samples were confirmed by direct immunofluorescence (DFA, MicroTrak; Syva, Palo Alto, CA), according to the manufacturers’ instructions. A comparison of antigen detection method and PCR was also made, and the data have been reported separately. 16 Each week a list of infected subjects was sent to the participating clinics. The infected subjects were contacted and treated with a single 1-g dose of azithromycin or a 100-mg dose of doxycycline twice a day for 1 week.
Associations between C trachomatis infection and potential explanatory variables were analyzed by t tests and chi-square tests. Statistical significance for all associations was set at P < 0.05. Odds ratios associated with 2 × 2 tables were calculated as reported by Rothman. 17 Logistic regression analysis with backward selection was used to identify variables that were predictive of chlamydial infection from among those that had any effect in models with single explanatory variables. All statistical analyses were calculated with SPLUS 4 Statistical Software (MathSoft; Data Analysis Products Division, 1997, Seattle, WA).
We enrolled 1198 subjects (80%) from the estimated total of 1500 women attending these clinics during the study period. The mean age of the participants was 29.8 years (range, 15–56 years). The majority of women were using oral contraceptive pills and had had one or two sex partners in the past year (Table 1). Five hundred six women (42%) were living in union (defined as married or cohabiting) with their partner. A total of 42 women had a confirmed positive C trachomatis test for at least one sample, yielding an overall prevalence of 3.5%. While one in five women reported a previously diagnosed genital chlamydial infection, the history of STDs was not associated with current C trachomatis–positivity. The mean (± SD) age of the C trachomatis–positive women was significantly lower than that of the uninfected women (27 ± 5 years versus 30 ± 7 years;P < 0.01). The prevalence of chlamydial infection decreased with increasing age, from 4.1% to 5.8% among women aged 15 to 30 years and to 1.8% or less among those older than 31 years (Figure 1). Of the 42 C trachomatis infections, 12 (28%) were found among the 277 (23%) who were 25 years of age or younger, whereas 23 (55%) were detected among the 398 (33%) between 26 and 30 years of age.
There was a relationship between the prevalence of C trachomatis and the use of different contraceptive methods. None of the 132 women who reported always using condoms had chlamydial infection. In comparison, 30 (4.4%) of the 679 women using oral contraceptives and 5 (2.2%) of the 223 women using intrauterine devices (IUDs) were infected (P < 0.05) (Table 2). The highest C trachomatis prevalence (6.1%) was among the 48 women who reported inconsistent condom use. C trachomatis infection was significantly associated also with marital status. The 632 women not living in union were five times more likely to be infected than the 506 women who were either married or cohabiting. Casual sexual relationships were reported by 654 participants (55%). They were twice as likely as women without such relationships to have chlamydial infection. The vast majority of women (96%) intended to use condoms for casual sex, either as their primary contraceptive method or in addition to another method. Although not significantly different, subjects who reported an intention to use condoms every time with casual sex partners were less likely to have chlamydial infection than subjects who reported occasional use of condoms (4% versus 6%;P = 0.1).
A significantly higher percentage of women with chlamydial infection than uninfected women reported having had three or more sex partners in the past year (43% versus 20%;P < 0.001). Indeed, women with three or more partners in the past year were four times more likely than those with one or two partners to have chlamydial infection (Table 2). In our sample, adolescent women were less likely than older women to have had multiple partners; only 13% of women aged 15 to 20 years reported three or more partners in the past year, compared to 19% of women aged 21 to 25 years (P = 0.20) and 23% of women aged 26 to 35 years (P < 0.05). Similarly, women with only one or two partners during their lifetime were significantly less likely than those with three or more partners to be infected with C trachomatis.
All variables that were associated with C trachomatis infection were included in logistic regression analysis. By stepwise backward elimination, three independent variables were left in the final model. Probability of having chlamydial infection tended to decrease with increasing age, so that an increase of 5 years cut the probability in half (adjusted OR, 0.5; 95% CI, 0.3–1.0). With adjustment for other variables associated with chlamydial infection, women with three or more sex partners during their lifetime were nine times more likely (adjusted OR, 8.5; 95% CI, 2.0–35.0) than those with one or two partners to have chlamydial infection. The majority of women (98%) had male partners; nine women (0.8%) reported having female partners, and six (0.5%) reported having both male and female partners. Unexpectedly, bisexual women were more likely (adjusted OR, 22; 95% CI, 3.4–140.0) than women with male partners to be infected with C trachomatis.
In our population with low C trachomatis prevalence, genital chlamydial infection was most prevalent among women between 26 and 30 years of age. Previously, most reports have recommended selective screening of sexually active women 25 years of age or younger, with the highest C trachomatis prevalence reported among women 15 to 20 years of age. 2,3,7–10,12–15,18–22 However, if only women aged 25 years or younger had been screened in our sample, only 28% of the infections would have been detected among 23% of women tested. In comparison, with the extension of the screening criteria to include all women up to 30 years of age, 83% of the infections would have been detected by testing 56% of women.
The number of sex partners was positively associated with genital chlamydial infection. In line with the findings of previous studies, multiple sex partners in the past year increased the risk of having C trachomatis infection. 7,8,10,12 Having multiple partners is usually associated with younger age and with not living in union. In our sample, women aged 26 to 30 years were more likely than younger women to report three or more partners in the past year, which may explain their higher prevalence of C trachomatis. In concordance with the findings of a study by Handsfield and colleagues, consistent condom use appeared to protect women from chlamydial infection effectively: none of the condom users were infected with C trachomatis. 15
We were surprised to find that bisexual women had a higher probability of having chlamydial infection than women with male partners, in a model adjusted for age and the number of sex partners. Factors connected to bisexual lifestyle may make a subject more prone to genital chlamydial infection. As the number of bisexual women in our study was small, more research is needed to determine the role of sexual orientation in chlamydial infection. However, a notable proportion of women attending family planning clinics may be unwilling to disclose sensitive data on their sexual orientation or number of sex partners. Thus, information about sexual behavior or lifestyle is probably too unreliable to be used for selective screening.
Nineteen percent of women in our sample reported a history of genital chlamydial infection, compared to 4% to 9% of women in other populations. 8,12 This finding is similar to a situation where a remarkable part of the target population has been screened for C trachomatis before. Family planning clinics will encounter this situation more often as awareness of chlamydial infection increases and as easily performed and reliable testing becomes more available. Repeated screening may change the selection of characteristics associated with the infection. Also, the positive predictive value of previously determined risk markers decreases if the prevalence of the infection decreases, thus calling for reevaluation of the screening criteria.
Our results emphasize the importance of determining screening criteria individually for each clinical setting, based on the local epidemiology and the characteristics of the population. Although screening of women aged 25 years or younger might be sufficient in most settings, extension of the screening criteria to include all women up to 30 years of age may be a better strategy in some populations.
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