Table 1.

Adaptation of the new letter compared to the original PHS letter.

New letter		Comparable information from PHS letter
1. First Impression Bias and Primacy Effects
	Visiting address: Het Overloon 2 6411 TE Heerlen 045-8506613 (9.00h-12.00h)	Visiting Address: Het Overloon 2 6411 TE Heerlen 045-850 66 13 (9.00h-12.00h) Aids STD Info line (for questions about sex, STD and The Pill): 0900-2042040
2. Self-Affirmation
	“Your health is very important. Not only for yourself, but also for a possible partner and family.”	“When you are (or have been) sexually active, it is important to do this test, even if you’ve done this test before or if you’re in a solid relationship.”
3. Advantages and Convenience
	“Therefore, you should take the opportunity to take a free and easy chlamydia test at home.”	“Via this letter, we invite you again to participate in the Chlamydia Screening South-Limburg.”
4. Reactance (I)
	“When you have sex, it is possible to contract chlamydia without realizing it.”	“That [ie, treat quickly] is difficult, because most people don’t know whether they are infected with chlamydia or not.”
5. Efficacy and Prompt to Action
	“Chlamydia is simple and easy to trace, and very trouble-free to treat. However, it is important to do the test as early as possible!”	“It is easy to treat, but important to be quick. Severe and frustrating health issues can be prevented by testing for chlamydia.”
6. Negative Consequences, Severity
	“If you wait too long for treatment, you can get severe and permanent health issues like infertility.”	“If chlamydia is not treated in time, men can get epididymitis and women can become infertile.”
7. Moral Norm
	“Above all, you can infect others without knowing it.”	“That [ie, treat quickly] is difficult, because most people don’t know whether they are infected with chlamydia or not.”
8. Perceived Risk, Unrealistic Optimism
	“Chlamydia does not only occur in people who have unsafe sex with many partners, but also in people with a few partners or just one partner.”	“Chlamydia is an STD (sexually transmitted disease) with a high prevalence in the Netherlands, especially in young people aged 16 to 29.”
9. Reactance (II)
	“To decrease the number of chlamydia infections, all 16-29 year olds from your region are invited to request a free chlamydia test via www.chlamydiatest.nl.”	“On www.chlamydiatest.nl The PHS South Limburg invites all men and women, aged 16-29 years, to participate in the annual free chlamydia test. On www.chlamydiatest.nl, ”
10. Self-efficacy (II) and Procedure
	“When you have requested and received the test package, you can do the test in less than five minutes at home, after which you can send it to the laboratory.”	“How does it work: You log in on with your personal log-in code from this letter. There you can create immediately your own username and password, making sure that no one else can log in. Subsequently, you can request a test package. You will receive this in a blank package at the address of your choice. You will find instructions in the test package about how to collect your test sample. Subsequently, you can send the package back to the laboratory at no cost. The test result is available within two weeks and will be online for three months via www.chlamydiatest.nl. With your username and password, you can request your test result. If you’ve forgotten your personal details, you need your personal log-in code from this letter. Therefore, you should keep this letter!”
11. Descriptive Norm and Implicit Attitudes
	“A lot of your peers have already tested for chlamydia, not finding it annoying or threatening, but more reassuring.”	N/A
12. Moral Norm (II) and Anticipated Regret
	“Also, some of them said that they would feel guilty if they didn’t do the test.”	N/A
13. Privacy
	“The entire test can be performed anonymously and at no charge. Your personal details are kept confidential. Only you, with your username and password, are able to request your test result (within two weeks!). No one, not your parents nor your general practitioner, is notified of the results.”	“All your personal details are kept confidential. Only you, with your username and password, are able to request your test result. And only you, not your parents nor your general practitioner, are notified of the results. In the folder, attached to this letter, and on www.chlamydiatest.nl, you can find more information about chlamydia and the test.”
14. Response Efficacy
	“If you are infected with chlamydia, it is very easily treated with antibiotics.”	N/A
15. Log-in code
	“Your strictly personal log-in code for www.chlamydiatest.nl is: ”	“To log in, you need a personal log-in code. Your strictly personal log-in code for www.chlamydiatest.nl is: ”
16. Prompt to Action (II)
	“Don’t delay − request your test package today!”	“Think about your health and participate in the chlamydia screening!”
17. Sender Information
	“With kind regards, Dr. Christian J.P.A. Hoebe Doctor/Epidemiologist, Infectious Diseases Project Leader, Chlamydia Screening South Limburg”	“With kind regards, Dr. Christian J.P.A. Hoebe Doctor/Epidemiologist, Infectious Diseases Project Leader, Chlamydia Screening South Limburg”
18. Footnote Information
	“For questions and information: www.chlamydiatest.nl. For questions about participation: ms H.L.G. ter Waarbeek (independent GP: 045-8506264). For questions about sex, STDs and The Pill: Aids STD info line: 0900-2042040.”	“There (ie, on www.chlamydiatest.nl can read about all the things you have to do for this test. P.S. This invitation is sent district by district across 10 municipalities to all individuals aged 16 to 29 years, so not everybody receives the invitation at the same time. If you want to know which municipalities, see www.chlamydiatest.nl. If you want to discuss questions about participation in this study with an independent general practitioner: contact ms H.L.G. ter Waarbeek (045-8506264).”