Table 1. Screening rules implemented in the model.

Screening design	Rule as implemented in the model	Comments	Reference
1 Time of invitation	Each individual is assigned a number which indicates the week of the year that they are invited for CSI screening.
2 Eligibility	Individuals are eligible to participate if they are in the age group 16–29 years and meet the criteria for inclusion (e.g. have been sexually active, and for the national level model, pass a risk-score threshold to exclude those persons with negligible risk levels).	In the model, individuals who turn 16 during the year are invited for the first time in the following year; this might be slightly different in reality, where they might still be invited the same year they turn 16, depending on the exact timing of invitations per geographic area.
3 Risk score selection	The risk score is explained in table 2. It excludes about 20% of the lower-risk sexually active population in the age-range of 16–29.	The risk score is similar to the risk score-based selection applied in one of the CSI regions. As the model population was not stratified by level of education or ethnicity, the risk score threshold for inclusion used in the model was lower than the > = 6 threshold used in the CSI to exclude a similar fraction of 20–30% of the population as was excluded by risk score in the CSI screening.	[26] [13]
4 Acceptance of first invitation	The chance that an individual accepts his/her first screening invitation depends solely on their gender, and the number of years since start of the screening program Initial participation decreases for both genders over time.	Factors for decrease of initial participation over time are 2008: 1.0 2009: 0.82 2010: 0.675 2011: 0.438 2012 and further: 0.37 times participation in 2008.
5 Repeated acceptance	Repeated participation depends solely on an individual's previous decisions on participation. The chance to participate per screening round is detailed in figure 1.
6 Treatment uptake	14% of participants ignore positive test results, and do not seek treatment. In the model there is no correlation between these 14% and the 9% of the population that do not participate in baseline healthcare. 86% of those tested positive get treatment.	In the CSI 91% sought treatment after being informed positive, 94% of those 91% actually took the treatment.	[39], page 46
7 Treatment of current partners	80% of current partners of individuals treated for Chlamydia are notified and treated at the same time as the index case. The 20% of the current partners that are not treated includes the 14% that would ignore their own positive test results (point 6), and individuals who have been tested or treated themselves recently (a personal value for each individual, drawn from an exponential distribution with a median of 68 days).	“recent testing/treatment fatigue” determines whether individuals are willing to participate in testing and/or treatment as part of symptomatic and asymptomatic regular healthcare, as well as part of all forms of partner notification (both regular healthcare and CSI), for a number of days after their latest Chlamydia testing and/or treatment. Participation in the CSI program in the model is not affected by this fatigue, as the participation data upon which the participation trees are based already implicitly contains this information (on a population level)	[40]
8 treatment of ex-partners	50% of ex-partners for which the partnership ended less than one year ago are notified and treated. The 50% of the recent ex-partners that are not treated include the 14% that would ignore their own positive test results, as well as those that have been treated recently. As a model simplification, treatment of ex-partners happens immediately upon treating the positively screened individual.
9 Retesting of those tested positive	In the case of a positive test result, participants are invited for an additional test 6 months after the initial invitation. The procedure is identical to the above procedure, except that the delay between invitation and treatment is shorter by 17 days. The effect is that people are re-treated (if positive) 166 days after their first test.	In the CSI these participants immediately get a test-kit sent to their home, which shortens the delay between an invitation to be retested and actual treatment taking place by 17 days.