The tertiary outcome measure is the cost-utility of the collaborative care intervention, compared to CAU. The cost-utility is evaluated by relating the difference in direct medical costs per patient receiving collaborative care or CAU to the difference in terms of Quality Adjusted Life Years (QALY) gained, which yields a cost per QALY estimate. We will also estimate the cost per QALY including the productivity costs. The costs will be assessed with the TiC-P, a measure commonly applied in economic evaluations of treatment in mental health care [
74,
75]. Quality of life will be assessed with the EuroQol (EQ-5D) [
76] and the Short Form-36 (SF-36) [
77], both of which are validated instruments for the measurement of general health-related quality of life. The EQ-5D descriptive system consists of five dimensions: mobility, self-care, usual activities, pain/discomfort and anxiety/depression. Each has three levels: no problems, some problems and extreme problems, thus defining a total of 243 (3
5) distinct health states. A study that was recently carried out in the Netherlands evaluated the EQ-5D in a national setting, resulting in the 'Dutch EQ-5D tariff'. The resulting tariff is used to calculate utilities for EQ-5D health states for the cost-utility analyses of health care programmes and treatments [
78,
79]. Additionally, presenteeism will be assessed with the presenteeism scale of the WHO Health and Work Performance Questionnaire Short Form (HPQ Short Form) [
80].
Calculating the total direct medical costs with the TiC-P, the total number of medical contacts (among which outpatient visits, length of stay in hospital, use of medication) will be multiplied by unit costs of the corresponding health care services. Reference unit prices for health care services will be applied and adjusted to the year of the study according to the consumer price index [
81].
The second section of the TiC-P includes a short form of the Health and Labour questionnaire (HLQ) for collecting data on productivity losses, [
82] the SF-HLQ which consists of three modules that measure productivity losses: absence from work, reduced efficiency at work and difficulties with job performance [
83]. The number of days of absence from work and the actual costs of working hours missed due to health-related problems are calculated on the basis of the average value added per worker according to age and gender per day and per hour, respectively. If respondents indicate that they were absent from work during the entire recall period, data will be collected from the time when the period of long-term absence started. This additional information will be used to calculate the production losses according to the friction cost method [
84,
85]. The friction cost method takes into account the economic circumstances that limit the losses of productivity to society, which are related to the fact that a formerly unemployed person may replace a person who has become disabled [
84].
Since the collaborative care intervention used in this study is a new intervention, a unit price per session is not known yet. To determine a reference price, a detailed cost-price study will be performed. Therefore, we will perform measurements of time for face-to-face contacts as well as indirect time per contact (e.g. consultations of other specialists) for a total of 20 sessions. Furthermore, we will estimate overhead costs based on the information of the financial department of the hospital. This will result in an estimate of the actual costs per contact. The unit cost estimate per contact will be used as a reference price per contact for the collaborative care intervention.