Response - and participation rates increased significantly after letter-, email- and SMS reminders were sent in the Ct-screening among 16 to 29 year old people in the Netherlands. The reminder invitation letter may not only resulted in more test package requests, but may have also resulted in reaching more people at higher risk of a chlamydia infection (sub-Saharan Africans and Surinamese/Antilleans, persons with a casual partner or ≥ 2 partners in the past 6 months, and Ct-positives who did not participate in the re-screening after 6 months). Being at higher risk and fear for the test result may have played a role in being reluctant to request a package at the initial invitation. The reminder letter could have raised more awareness on the importance of being tested for chlamydia. In a previous study  it was shown that Ct-positives who did not participate in the re-screening after 6 months are at highest risk for a chlamydia infection in the second screening round, compared to Ct-negatives and Ct-positives who participated in the re-screening. This underlines the importance of reminders to reach this particular high risk group.
In contrast, people with a long-term steady relationship also responded more often after a reminder. Possibly, this group did not respond at first due to a perceived low risk. However, a reminder letter addressing the relevance of screening could have made them decide differently.
Compared to heterosexual men, MSM more often applied for a test package directly after the invitation, which can be related to familiarity with Internet-based interventions targeted to MSM in the Netherlands . Men in general were more likely to react after reminders than women, an effect that was also seen in PILOT CT . Although differences were small, this might be explained by the fact that participation in population-based Ct-screening programs in general is higher in women than in men [8, 9, 12, 16, 17]. In contrast, in France, women's reaction to reminders (phone call and letters) was little higher than men's, but also in this screening program women participated more often then men . Additionally, in the UK, young women (16-24 yrs) with the highest Ct-prevalence participated only after repeated reminders (by postcard, letter or phone call). In this study, again, more women participated than men [8, 18]. Concluding, reaching men with reminders is encouraging, but still they are less likely to engage in Ct-screenings.
Participation rates increased from 10% to 14% and 7% to 10% in the first respectively second screening round after email/SMS reminders were sent. Still, every 1 in 5 packages was never returned, even after several reminders. Reasons for non-participation (no sample return after package request) were described previously [2, 19]. The most important reasons were lack of time, loss of package, forgetfulness and being Ct-tested and -treated elsewhere.
Furthermore, we observed that the respondents' willingness to provide an email address or cell phone number for communication during the screening procedure was very high. Therefore, nearly all respondents were reachable by modern communication media (99% by email; 72% by cell phone). In contrast to package request, sample return after email/SMS reminders was not significantly related to (sexual) risk factors of Ct-positivity. However, 23% of all Ct-positives returned their sample after receiving an email/SMS reminder which illustrates the usefulness of automated reminders to encourage sample return and, subsequently, treating more people for their infection.
Although these findings are encouraging, the magnitude of the effect of email/SMS reminders remains uncertain due to not knowing what proportion of people would have returned their sample eventually without the necessity of receiving a reminder. Assuming that the natural trend of returning a sample would be gradual decline (trend without peaks, Figures 2 and 3), the peaks may represent the additional effect of the reminders, as they immediately follow after the reminders were sent.
We also showed that providing a cell-phone number was slightly associated with risk factors of Ct-positivity (ORs up to 1.7). Menon-Johansson et al. reported similar characteristics of people being both at higher risk of STI and more often making use of mobile phones (young aged, migrants, and lower SES) .These findings illustrate the value of using cell phones and text message (SMS) for communication to high-risk demographic groups.
To our knowledge CSI is the first systematic, population-based Ct-screening program using a combination of reminder letters, emails and SMS to enhance response- and participation rates. Generally, the only systematic  and other opportunistic Ct-screening programs implemented reminders for similar and other purposes than in CSI. Reminders were used to confirm package receipt [8, 12, 18], encourage sample return [8, 11, 12, 17, 18], checking test results [9, 10] or reminding going for treatment . Although reminders were reported in those programs, comparison was impeded due to differences in program design and reminder implementation and the lack of detailed evaluations. Some screening programs reported using modern technologies like email and SMS as reminders. Emails were used to remind on checking test results and getting treatment [9, 10], but SMS reminding was reported only once in a small study . More frequently SMS was used for partner- and test result notification [16, 17, 20–22]. The Internet-based set up of our screening program and the detailed automated monitoring of each logistic step in the screening process made it possible to thoroughly evaluate the effects and determinants of the various reminders. Another great advantage of CSI was the availability of sociodemographic data for all invitees from population registers. One limitation of our evaluation was that due to the program design (implementation), no control group of persons who did not receive reminders was embedded, which makes it more difficult to relate the additional effect to the reminders. On the other hand, the high peaks in response right after the reminders as shown in the graphs are likely showing the contributions of the reminders.