Randomised controlled trials (RCTs) and non-RCTs, controlled and uncontrolled before-after studies, and time-series analyses that focused on planned collaboration directly between primary care physicians and specialists, working as individuals or clinical teams, and using any method of interactive communication, were eligible for inclusion in the review. Studies had to be conducted in outpatient and community primary care settings, and in countries where health care systems were generalisable to the United States context. Professionals specialising in treating patients with diabetes, psychiatric conditions, or cancer (including palliative care) were the collaborative groups of interest. Studies of collaborative arrangements where less than 75% of professionals belonged to these clinical groups were excluded.
Outcomes of interest were selected from the included studies by two independent and blinded reviewers. The selected patient outcome measures were haemoglobin A1c in studies of diabetes, the Centre for Epidemiologic Studies Depression Scale, a version of the Symptom Check List, or the Hamilton Depression Scale in studies of psychiatric conditions.
Most included studies related to psychiatric patients; there were no studies of cancer. Approximately half of the studies were conducted in the United States or the United Kingdom. Included interventions comprised face-to-face meetings, paper or electronic letters or notes, telephone discussions and video-conferencing, with some studies combining various methods.
Two reviewers independently selected studies for inclusion in the review. Disagreements were resolved by discussion, and by reference to other reviewers.