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Trends in Chlamydia Positivity Over Time Among Women in Melbourne Australia, 2003 to 2007

O’Rourke, K M. BN, MPH, MHlthSc*†; Fairley, Christopher K. PHD, FRACP‡§; Samaranayake, Anil MSc, MD‡§; Collignon, Peter MBBS, BSc, FASM, FRACP, FRCPA; Hocking, Jane S. MPH, PHD*

Author Information
doi: 10.1097/OLQ.0b013e3181b12765
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National surveillance data indicate that chlamydia diagnosis rates have been steadily increasing over the past decade from 47.4 per 100,000 population in 1997 to 253 per 100,000 in 2008 (58,458 notifications).1 As chlamydia is asymptomatic in over 80% of cases2 and chlamydia testing rates remain low (less than 12% of 15–24-year-old women and less than 5% of similarly aged men) in Australia,3 these surveillance data may not provide a valid measure of the burden of chlamydia in the population, nor whether this burden has changed over time. It has been shown that among the Australian population, increased testing will lead to higher population chlamydia diagnosis rates4 but surveillance data in several countries indicate that the chlamydia positivity rate amongst those tested has also been steadily increasing, particularly in Sweden,5 United States,6 Norway,7 and Denmark.8 There has been much speculation about possible reasons for these increases in chlamydia positivity, including use of more sensitive tests9 and changes in sexual behavior.10 Analyses of sentinel site data, such as sexual health or family planning clinic data, might be useful in exploring chlamydia positivity trends over time.

A recent large analysis of chlamydia testing data from family planning clinics in the United States found that after adjusting for risk factors, chlamydia positivity had increased by 5% per year.11 While this was a comprehensive analysis, the authors were required to make certain assumptions about the sensitivity and specificity of the tests used to account for changes in diagnostic testing during the study period; this may have led to biases in their final estimate of trends in chlamydia positivity over time. We present the results of an analysis of the computerized client records at a large urban sexual health clinic in Australia during the period from 2003 to 2007 when chlamydia testing methods were constant. This analysis aimed to determine whether chlamydia positivity among female clients has changed over time after adjusting for demographic, clinical, and sexual behavioral risk characteristics associated with chlamydia infection.


Study Population and Data Collection

This was a retrospective review of computerized records at the Melbourne Sexual Health Centre (MSHC), a large public sexually transmitted infections (STI) clinic in an Australian capital city in the State of Victoria. The clinic conducts over 28,000 consultations annually and provides free HIV and STI testing. MSHC policy is to offer chlamydia testing to all new clients unless they have been tested recently elsewhere or have not been sexually active. This policy was consistent throughout the study period. Clients diagnosed with chlamydia are treated in accordance with the National Management Guidelines for the Treatment of Sexually Transmitted Infections, with azithromycin 1 g taken orally as a single dose being the preferred treatment.12

Computerized records for all heterosexual women who attended MSHC for the first time between 2003 and 2007 and were tested for chlamydia, were included in the analysis. Clients attending for follow-up visits, identified from their unique identifying number, were excluded. Individual computerized records include basic demographic information such as age, sex, and postcode; behavioral information such as number of male and female sexual partners in the last 3 and 12 months, condom use, and history of sex work; and clinical information including the presence or absence of genital symptoms, and whether the client reports contact with chlamydia. The presence or absence of genital symptoms is recorded during triage when the client first arrives at the clinic and meets with a nurse before consultation with a clinician. Symptoms can be chlamydia associated such as discharge or pelvic pain, or nonchlamydia associated such as genital ulcers or genital lumps. The computerized record does not specify the type of symptoms identified at triage. The male partners reported by sex workers refer only to their nonpaying partners.

Laboratory Methods

Women are tested for chlamydia using either an endocervical swab, high vaginal swab, or first pass urine specimen. All specimens are analyzed for Chlamydia trachomatis using BD ProbeTec Strand Displacement Amplification.

National Chlamydia Surveillance Data

In order to examine whether changes observed at MSHC are consistent with changes observed in Australia, chlamydia surveillance and chlamydia testing data for females were obtained for Australia. All genital chlamydia diagnoses are required by law to be notified to the relevant State or Territory health authority. These data are compiled by National Notifiable Diseases Surveillance System.1 Chlamydia testing performed outside the settings of public hospital and STI clinics is eligible for a government rebate under the Medical Benefits Schedule. The Health Insurance Commission collects these chlamydia testing data. Unique chlamydia testing codes existed in the Medical Benefits Schedule (item 69369 and item 69370) until November 2005 when a more generic code for testing genital specimens for a number of different infections was introduced.

Antibiotic Data

We further explored whether any change in chlamydia positivity was associated with changing antibiotic utilization over time in Australia.13 The number of daily defined doses of antibiotics utilized was obtained from National Prescribing Services, Australia.14

The antibiotics included in our analysis were tetracyclines, macrolides, amoxicillin, amoxicillin with clavulanic acid, trimethoprim with sulphamethoxazole, and ciprofloxacin as these have been found to be effectively treat chlamydia infection.15

Statistical Analyses

Chlamydia positivity estimates and 95% confidence intervals were calculated using exact methods for binomial proportions for MSHC data. Logistic regression was used to assess any possible change in chlamydia positivity over time adjusting for demographic, clinical, and sexual behavioral risk factors. Contact with chlamydia and symptomatic presentation were collapsed together for pragmatic purposes: only small numbers of women (3%–4%) reported contact with chlamydia each year, and both factors can be strongly associated with chlamydia infection.16,17

For the national surveillance data, we calculated chlamydia positivity rates for each year from 1999 to 2004 by dividing the total number of notifications by the total number of tests. Trends over time in the surveillance and antibiotic utilization data were evaluated using linear regression.

All analyses were undertaken using Stata version 9.18


There were 10,498 chlamydia tests conducted among female clients presenting to the clinic for their first time over the 5 years (2003–2007). The average age of women tested was 27.7 years (median, 25.7 years; range, 12.2–80.7 years). About 45% of tests each year were conducted in women under 25 years of age (Table 1).

Characteristics of Women Attending the Melbourne Sexual Health Centre for the First Time Between 2003 and 2007 and Associations With Chlamydia Infection

The overall prevalence of chlamydia among heterosexually active female clients attending MSHC for the first time during the time frame was 5.9% (95% CI: 5.5%–6.4%). Chlamydia positivity increased each year from 4.2% in 2003 to 6.7% in 2007 (P <0.01). This increase was consistent within each age group, although the 2007 data showed a decrease in positivity for those aged over 25 years (Fig. 1).

Figure 1.:
Chlamydia positivity trends in women by age group, 2003 to 2007.

Univariate analysis found that the odds of chlamydia positivity increased on average 14% per year (OR = 1.14; 95% CI: 1.08–1.21) (Table 1). Increased chlamydia positivity was also associated with young age, an increasing number of male sexual partners in the last 3 and 12 months, symptomatic presentation or contact with infection, and inconsistent condom use during the last 3 or 12 months. History of sex work was associated with decreased chlamydia positivity.

After adjusting for age, number of sex partners, condom use, presence of symptoms and contact with infection, and history of sex work, chlamydia positivity among women attending MSHC for the first time increased on average 12% per year (OR = 1.12, 95% CI: 1.05–1.20) between 2003 and 2007. The proportion of women reporting 2 or more male sexual partners increased slightly over the study period from 25% to 30% for 3 months (P = 0.03) and from 54% to 58% for 12 months (P = 0.09), but there was negligible change in condom use or proportion reporting symptoms or contact with infection at time of testing (Fig. 2).

Figure 2.:
Self-reported sexual risk factors among women attending MSHC for the first time between 2003 and 2007.

Australia chlamydia surveillance and testing data show that between 1999 and 2004 the number of chlamydia tests conducted annually among women increased from about 88,000 in 1999 to 199,258 in 2004 and the number of annual notifications increased from 8325 in 1999 to 21,142 in 2004. Consistent with the findings at MSHC, the chlamydia positivity rate across Australia also increased during this time (P = 0.06). The overall use of antibiotics effective against chlamydia as measured by the average daily defined dose per 1000 population declined steadily between 1992 and 2001 (P <0.01) in Australia at a time when chlamydia positivity was increasing, and has been fairly stable since (Fig. 3).

Figure 3.:
Chlamydia positivity among women and antibiotic use, Australia 1999 to 2004.


We observed that during the 5-year period from 2003 to 2007, chlamydia positivity among women attending the centre increased by 12% per year after adjusting for age, behavioral risk factors, presence of symptoms, and history of contact with infection. These findings suggest that the steadily increasing chlamydia diagnoses observed in recent years in Australia are not only due to increased testing,4 but are also likely to reflect a true increase in chlamydia prevalence in Australia.

Our study has a number of limitations that need to be considered. First, they represent findings from a retrospective analysis of medical records data from a sexual health centre and may not be representative of changes in the prevalence of chlamydia elsewhere in the State. However, MSHC see clients from a range of postcodes throughout the State and not just a single geographical area around the clinic; while is it possible that the changes we have observed are specific only to clients attending the clinic, this would be unlikely. Additionally, a recent analysis of chlamydia notification and testing data showed similar chlamydia testing and notification patterns in each of the States and Territories across Australia.19

Second, we have analyzed the proportion of individuals who tested positive each year rather than following a single cohort of women over time. However, such cohort studies are expensive and logistically challenging to conduct over considerable follow-up periods. In the absence of cohort studies, carefully conducted analyzes of medical records databases provide an acceptable surrogate for investigating changes in chlamydia epidemiology over time.

Finally, we were unable to evaluate all possible factors that may have affected the prevalence of chlamydia. For example, we did not measure the clients’ sexual network characteristics such as concurrency during this time period, and past research has shown that these are important in influencing an individual’s risk of infection.20 We did measure other factors however and found that there was negligible change in sexual and clinical risk factors over time suggesting that the client profile over the 5 year period was constant.

Our study has a number of strengths. First, it involved a large number of women tested over a number of years, providing a high degree of precision. Second, the same laboratory and same diagnostic testing methods were used throughout the study period. Third, the same testing criteria were in place during the study period and the number of new women tested only increased marginally from 2110 in 2003 to 2267 in 2007. Figure 2 shows that there was little change in the risk profile of women attending the clinic during this time. We specially chose to only analyze women because the presence of symptoms in women is less strongly related to chlamydia21 and we were able to adjust for the presence of symptoms in our analysis.

There have been a number of other studies undertaken in Australia that have suggested that chlamydia positivity may be increasing among women, but none of these studies adjusted simultaneously for age, sexual risk behavior, or the presence of symptoms in their analysis. A study of 14,020 sexual health clients at a large sexual health clinic in Sydney, New South Wales found that between 1994 and 2000, the positivity among women increased from 1.8% to 3.5%22; this analysis did not model chlamydia positivity over time, adjusting simultaneously for age, sexual risk behavior, or presence of symptoms. Another analysis of laboratory data for all chlamydia tests conducted in Canberra, Australian Capital Territory examined 26,419 laboratory records and found that between December 1998 and November 2004, chlamydia positivity among women increased from 3.3% to 4.8%.23 While this analysis stratified positivity by age, it did not adjust for sexual risk behavior.

Increases in chlamydia positivity have been documented in other countries including the United States,11 Norway,7 and Sweden.9 An analysis of chlamydia testing data from family planning clinics in the United States found that after adjusting for a range of variables including age, ethnicity, sexual risk behavior, condom use and symptoms, chlamydia positivity had increased by 5% per year between 1997 and 2004.11 However, changes in diagnostic methods during the study period are a potential limitation in this study. An analysis of surveillance data in Norway found that chlamydia positivity increased from 6% in 2000 to 7.7% in 20067; while based on a large number of tests, there were no data available on sexual risk behavior. Laboratory data in Sweden showed that chlamydia positivity increased from 4.1% in 1994 to 5.4% in 1999.9 This study also did not adjust for sexual risk behavior in the analysis.

We have shown that our findings of increasing chlamydia positivity over time are consistent with national surveillance data for Australia where the chlamydia positivity among women tested increased from about 9.4% in 1999 to 10.7% in 2004 (P = 0.06). There are a number of possible explanations for rising chlamydia positivity among women in the national surveillance data in Australia. Changing sexual risk behavior may be contributing to increased chlamydia transmission in the community. Australian data show a decreasing age of first sexual intercourse among young adults.24 Other explanations include reduced control measures such as less active contact tracing and increasingly restricted access to health services or evenly possibly reduced use of antibiotics for non–chlamydia-related reasons.13 Interestingly consistent with this later hypothesis, is our finding of decreasing antibiotic utilization across Australia since 1992 (Fig. 3). It should be remembered that the antibiotic data are only available at a population level and any inference about causality is subject to the many limitations of an ecological study. A number of other studies have shown a significant reduction of about 70% in the prevalence of chlamydia among individuals who have reported that they consumed antibiotics for other reasons in the last 3 months.25,26

There is evidence to suggest that the underlying prevalence of chlamydia in Australia is increasing. These findings have important implications for the future of chlamydia control activities in Australia, particularly in terms of their cost-effectiveness. Analyses of clinic data such as these are useful for monitoring chlamydia prevalence over time.


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