BACKGROUND General health checks are seen as one way to mitigate the rising prevalence of chronic diseases. However, general health checks have failed to show an impact on population health and may also widen health inequalities. During recent years, stepwise approaches have been advanced as an alternative. In 2013, we set up the TOF project in a partnership with the general practitioners' organisation (PLO) and ten municipalities of the Region of Southern Denmark to develop a stepwise model

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... sisting of a joint intervention and a targeted intervention aimed at patients at high-risk or with health risk behaviour. A novel feature was the use of a personal digital mailbox for recruiting participants. A personal digital mailbox is a secure digital mailbox provided by the Danish public authorities. Apart from being both safe and secure, it is a low-cost, quick and easy way to reach Danish residents (95% coverage). OBJECTIVE To analyse the association between the rates of acceptance of two digital invitations sent to a personal digital mailbox and socio-demographic determinants, medical treatment, and health care usage in a stepwise, primary care model for the prevention of chronic diseases. METHODS A cross-sectional analysis of the rates of acceptance of two digital invitations sent to randomly selected residents born between 1957 and 1986 and residing in two Danish municipalities. The outcome is acceptance of the two digital invitations. Exposure variables were retrieved from Statistics Denmark as well as EPR information from participating GPs. Statistical associations were determined by Poisson regression. Data-driven chi-squared automatic interaction detection (CHAID) method was used to generate a decision tree analysis predicting acceptance of the digital invitations. RESULTS 8,814 patients received an invitation in their digital mailbox from 47 GPs. A total of 40.2% (N=3,545) accepted the first digital invitation, and 30.3 % (N=2,661) accepted both digital invitations. The rates of acceptance of both digital invitations were higher among women, older patients, patients of higher SES, and patients not diagnosed with or in treatment of diabetes mellitus, COPD, and/or CVD. Patients who had seen their GP within the past two years were also more likely to accept the digital invitations. Registering for a preventive consultation or having a partner who accepted the first digital invitation showed no association with acceptance of either invitation. CONCLUSIONS This is the first paper to report on the rates of acceptance of digital invitations to participate in a stepwise model for prevention of chronic diseases. More studies of digital invitations are needed to determine if the acceptance rates seen in the present study should be expected from future studies as well. Similarly, more research is needed to determine whether a multimodal recruitment approach including digital invitations to personal digital mailboxes will reach harder-to-reach sub-populations more effectively than digital invitations only. CLINICALTRIAL Registered at Clinical Trial Gov (Unique Protocol ID: TOFpilot2016). Registered on 29 April 2016.