Introduction
Chlamydia trachomatis is the world's most common bacterial sexually transmitted infection (STI), with an estimated 89 million new cases per year (1). Easily treated but largely asymptomatic, C. trachomatis infections are a challenge for primary prevention (2). If left untreated, the long term consequences of pelvic inflammatory disease (PID), ectopic pregnancy, and tubal factor infertility are detrimental (3). In Sweden, the number of C. trachomatis cases has increased for the fourth year in a row, and only 23% of chlamydia specimens in Sweden come from men (4, 5). Despite years of intensive case finding strategies, the prevalence of C. trachomatis remains high in many countries (6, 7).
Prior attempts to screen a population by means of urine samples obtained at home have proved feasible (8-10). Participation rates have, however, been too low to allow continuous screening and an accurate determination of C. trachomatis prevalence. A large problem in C. trachomatis screening is the consistently lower testing frequency among males than among females. As C. trachomatis is a sexually transmitted infection, it is important to encourage men to participate in chlamydia screening (7, 11). New strategies are needed to identify and treat those infected, to limit the spread of the disease, and to reduce complications.

The aim of this study was to increase male interest in C. trachomatis screening participation by using the internet and a home sampling strategy. Young men aged 22 years, a group with high incidence and low testing rate, were screened (5, 12).

Methods
Eligible persons
The study was conducted during February and March 2002 among all 22 year old men registered as living in Umeå, Sweden. In the population register, 1074 men who were 22 years of age had permanent addresses in Umeå. Umeå is a university city in northern Sweden where students constitute a large proportion of the population. The local medical ethics committee approved the study.

Survey
The men were sent a test package containing a cover letter, a urine specimen container, and a questionnaire regarding social and sexual behaviour. The cover letter had a six digit code written on it. The code was also written on the urine specimen container and on the questionnaire. The cover letter briefly described the study, gave the address of the study's chlamydia web site and presented information about C. trachomatis, including the possibility of being asymptomatically infected. Written instructions were also provided on how to obtain a first void urine specimen and how to store the specimen before mailing. They were informed that only one person was aware of their identity (the central research figure). The participants could then send in their coded questionnaire and coded urine specimen container in a prepaid, preaddressed biological substance envelope.
After three weeks, those who had not responded or from whom we had not received a returned, unopened letter were sent a reminder letter giving them the opportunity to request a new test package. After another two weeks, a second reminder was sent out. They were classified as no longer living at their registered address on the basis of the return of an unopened letter or an absence confirmed by relatives. One month later a follow up questionnaire was mailed to all non-respondents. They were asked why they did not participate in the screening study and their viewpoints about the project (Table 1).

Sample analysis
Upon receipt, the urine specimens were analysed for C. trachomatis DNA by means of a commercially available polymerase chain reaction (PCR) test (COBAS AMPLICOR C. trachomatis test, Roche Diagnostics, Basel, Switzerland) according to the manufacturer's instructions. For each specimen an internal control was included. A negative result was reported as negative only if the sample was negative for C. trachomatis and the internal control was positive. A sample positive for C. trachomatis was reanalysed and reported as positive if the reanalysis, using the same PCR test, was also positive.

Obtaining the results
The participants obtained the test results from a web site, which also provided information about chlamydia and gave relevant internet links for more information about other STIs. In order to develop an internet site that was attractive to young people, we engaged teenagers in the design process (http//www.vanster.nu/test/klamydia). The web site was hosted by the council of Vasterbotten municipality.
The laboratory test results of each coded urine specimen were fed into a database as a simple text file. The participants could access this database by entering their six digit personal code at the chlamydia web site. If their urine specimen was positive for C. trachomatis a message appeared on the screen instructing them to contact us for treatment. If their urine sample was negative for C. trachomatis a message appeared on the screen telling them that they were not infected with C. trachomatis.

For study participants without access to a computer, a phone number was provided so that they could call the testing centre (ie the central research figure) and obtain their results.
Treatment
When infected persons contacted us, arrangements were made for a visit to a clinic for treatment, counselling, and partner tracing. Partner tracing was performed by a specially trained social worker.

Results
Overall population characteristics
The 1074 subjects surveyed were all men with registered addresses in Umeå, whose twenty second birthday fell during the year in which the study was performed.

Study response rates
After three postal contacts 39% (396/1016) of the men responded, and 362 urine specimen were obtained for C. trachomatis analysis (Figure 1). Six men contacted us requesting new urine specimen containers. Three men requested new return envelopes. Four men requested new test packages after receiving our reminders. Four men actively refused participation in the study. All 362 urine specimens arrived intact without apparent damage during transport.

Characteristics of respondents
Of the study group, 62% were students, 76% were living alone, and 50% were in a steady relationship. The median age of first intercourse was 17.5 years and the median number of lifetime sexual partners was 3.5. Thirty males responded with only a questionnaire (Figure 1). Sixty seven percent (20/30) of these men had never had sexual intercourse and the rest declined to submit urine samples for other reasons, the most common of which was that their partner had recently been tested for C. trachomatis.

Characteristics of non-respondents
Follow up questionnaires were sent to 640 of the initial non-responding men. One hundred and ten men (17%) replied to these questionnaires. The most common reasons for not participating were that they thought it unnecessary, because they believed that they were not infected (50%) or because they had a steady relationship with their partner (55%) (Table1). Sixty nine per cent (76/110) of the men who answered the follow up questionnaire had two or more reasons for not participating.

Test results
Of the 362 urine specimens tested, four were positive for C. trachomatis, giving a prevalence of 1.1% in the study group.
Obtaining the test results from the Internet
The web site had 1834 hits during the study period. Multiple hits from the same internet protocol (IP) address during a 30 minute interval, were considered to originate from a single visitor. The number of visits was therefore calculated to be 634. Test results were obtained at all hours of the day.

Of the four infected men, three obtained their test results by typing their codes into the web site and also contacted us voluntarily for treatment. The fourth man was contacted since he had not approached us. It then became evident that he did not understand the language of the covering letter and therefore did not understand how to obtain the test result.
Twenty three codes not remotely similar to our study's codes were entered during seven different periods. In other words, one or more persons attempted to crack our codes seven times.
Three men obtained their test results by telephone, since they did not have internet access.
Discussion
In this study we evaluated a new C. trachomatis screening method based on a home sampling strategy and using the internet as a convenient facility for the participants to obtain the test results.

The internet proved to be an accessible tool in screening. The results were easily inserted into the database by the investigators and conveniently retrieved by the participants. Only three of the men in the study did not have internet access and had to obtain their test results by telephone. All of the other men were able to obtain their results from the internet at any time of the day, and infected men contacted us independently for treatment. Only one man had to be contacted, because language difficulties prevented him from understanding the instructions.
The Web site had 634 visits, exceeding the number of urine samples tested. This could mean that people not participating in the study also visited the web site. We think that this could indicate an importance and a value of presenting essential information on the web site.

Strengths and limitations
The strength of our study was that the combination of internet and home sampling strategy gave a male answer response of 38.5%. To our knowledge, this is the highest ever participation rate yet published for a C. trachomatis population based screening using home obtained urine samples. Two recently published population based screening studies of 21 to 23 year olds, using postal urine specimens as test material, had a 26.8% and 0.4% answer rate (9, 10).
Among the respondents we detected four C. trachomatis positive men. The C. trachomatis prevalence among the 22 year old men in our study group was thus 1.1%. In this age group and in this area the percentage of positive C. trachomatis tests during customary care, which involves visits to healthcare centres, STI clinics, and youth clinics, is usually around 10% (12). Most men tested during customary care are partners of an infected person and thus obliged to be tested according to Swedish law. These males comprise a high risk group with a high percentage of positive C. trachomatis test results. The low prevalence rate of our study indicates our strategy reached participants outside the high risk groups, which we consider to be important for efficient disease control. Furthermore, the C. trachomatis male prevalence in this study is lower than those previously detected in population based studies, which range from 2.5-5.9% (8,9). The low prevalence of C. trachomatis in the survey could be due to a selection bias, since participants could not be anonymous. The lower male prevalence could also be due to the fact that treatment and partner tracing of C. trachomatis infected persons is mandatory under Swedish law. The partner tracing of the four infected men found six female partners who were at risk of infection with C. trachomatis.

The limitation of our study was that participation was too low to allow an accurate estimation of the population chlamydia prevalence. Our analysis of the non-respondents showed some obvious reasons for not participating: they had never had sexual intercourse (14.5%) or they or their partner had recently been tested for C. trachomatis (36.4%). Since it is difficult to encourage this group to participate in screening, we need to focus on the men who responded that did not care (25.5%) or that they did not consider themselves to be at risk (50%). It is also generally more difficult to involve men than women and we find it vital to include men for successful C. trachomatis screening. And yet, even in the late twentieth century, a proposed national screening programme for C. trachomatis in the United Kingdom suggested that only women should be tested (13). Restricting male participation in screening to that of traceable contacts makes successful eradication of chlamydia unlikely (14).
A further limitation of our study was that, under Swedish law, the participants could not be anonymous.

Implementation
In countries where C. trachomatis treatment is not mandatory according to law, participants may be anonymous which could further increase the participation rate. The cover letter and the internet site for such a study should also present information in several languages.
Considering the large proportion of men who do not believe themselves to be at risk, the web site could be further developed to include information about risk factors and possible symptoms of C. trachomatis infection.
The internet could also be used in C. trachomatis screening as a facility where testing kits can be ordered anonymously. The 'worried-well', or the young person who assesses that he is at risk could then have the opportunity to order a coded sampling kit. The C. trachomatis test results could then be obtained on the web site. This method could be used for continuous testing of C. trachomatis and could also allow young people to learn more about chlamydia and other sexually transmitted infections.

Conclusions
The internet proved to be an accessible tool in C. trachomatis screening. The internet C. trachomatis screening strategy achieved the highest male participation rate yet published, and also reached young men outside the high risk groups. Methods of improvements and future implementations of the internet as a tool in C. trachomatis screening are suggested.

Acknowledgements

This study was supported by grants from Folkhälsoinstitutet Sweden and the Virology Department, Umeå University Hospital, Sweden.