Table 1 Outcome parameters and other parameters

 Behaviour change (stage of change)

Questionnaire containing 1 question per advice received to determine the current stage of change (transtheoretical model (Fig. 4) [43,44,45]).

 Behaviour change (verification)

Physical activity: the validated Dutch version of the International Physical Activity Questionnaire (IPAQ) [46].

Smoking: questions on current smoking behaviour and quantity of smoking.

Dietary and alcohol intake: questions on regular intake of all food groups and alcoholic consumptions, for which advice is received by the participant.

Body weight and waist circumference: measured at home by the participant, following detailed instructions and using the same weighing scale each time.

 Motivational regulation

Validated Treatment Self-Regulation Questionnaire (TSRQ) [47], translated into Dutch. This questionnaire has been modified to ask motivational regulation with regard to the ‘advice given’ and ‘healthy lifestyle’.

 Attitude

Questionnaire consisting of 7 Likert scale items on outcome expectations, 16 Likert scale items on the reasons for (not) undergoing testing by Health Potential®, 50 Likert scale items on the attitude towards five primary lifestyle behaviours, 1 Likert scale item on the importance of preventing disease, and 6 items on the evaluation of the Health Potential® report.

 Behaviour-specific self-efficacy

Questionnaire which was developed using the ‘Guide for constructing self-efficacy scales’ by Bandura [48]. It includes one self-efficacy question for each advice given in which the participant indicates on a scale of 0–100% how sure they are of their ability to follow the advice.

 Risk perception

Likert scale questions on the perceived probability of getting and severity [37] of each of the diseases included in the standard Health Potential® service.

 Perceived stress

Dutch translation of the validated 14-item Perceived Stress Scale (PSS) [49].

 Discussion of test results with health professionals and/or family and friends

Multiple-answer question in which the participant can indicate with whom the results were discussed.

 Test-related distress (arms 1 and 2 only)

Dutch translation [50, 51] of the validated Impact of Event Scale (IES), which has been used previously to study test-related distress after genetic testing for common disease risk [39].

 Self-reported health status

Dutch translation of the validated RAND 36-Item Short Form Health Survey, V2 (SF-36 V2) [52, 53].

 Health locus of control

Dutch translation of the validated Multidimensional Health Locus of Control (MHLC) Scale [54, 55].

 Comprehension of risk estimates (arms 1 and 2 only)

Questions based on the questions used by Kaufman et al. [56].

 Genetic knowledge (arms 1 and 2 only)

Dutch translation of the questions as used by Carere et al. [57].

 Cue to action

Two Likert scale questions on whether the participant felt the Health Potential® report and the general health check were a cue to action.

 Personal and family history of tested diseases

Questionnaire asking about personal and family history.

 General self-efficacy

Validated Dutch General Self-Efficacy Scale (DGSES) [58].

 Social modelling, social support

One Likert scale question per advice received.

 Social support

One Likert scale question per lifestyle domain (diet, alcohol, physical activity, smoking, body weight).

 Pro-active coping skills

Validated Utrechtse Pro-actieve Coping Competenties (UPCC) questionnaire [59].

 Vitality

Validated Vita-16 [60].

 Previous participation in genetic testing

Two yes/no questions.

 Temporal discounting

Validated 10-item Delaying Gratification Inventory short form, translated into Dutch (DGI-10) [61].

 Personality

Validated Dutch IPIP-50 [62].