BackgroundEffective care services for people whose work participation is at risk require low-threshold access, a comprehensive diagnostic clarification of intervention needs, a connection to the workplace and job demands, and interdisciplinary collaboration between key stakeholders at the interface of rehabilitation and occupational medicine. We have developed a comprehensive diagnostic service to clarify intervention needs for employees with health restrictions and limited work ability: this service is initiated by occupational health physicians.Methods/designOur randomized controlled trial tests the effectiveness of a comprehensive diagnostic service for clarifying intervention needs (GIBI: Comprehensive clarification of the need for intervention for people whose work participation is at risk). The comprehensive intervention comprises three elements: initial consultation, two-day diagnostics at a rehabilitation center and follow-up consultations. We will include 210 employees with health restrictions and limited work ability, who are identified by occupational health physicians. All individuals will receive an initial consultation with their occupational health physician to discuss their health, work ability and job demands. After this, half the individuals are randomly assigned to the intervention group and the other half to the waiting-list control group. Individuals in the intervention group start two-day diagnostics, carried out by a multi-professional rehabilitation team in a rehabilitation center, shortly after the initial consultation. The diagnostics will allow first recommendations for improving work participation. The implementation of these recommendations is supported by an occupational health physician in four follow-up consultations. The control group will receive the comprehensive two-day diagnostic service and subsequent follow-up consultations six months after the initial consultation. The primary outcome of the randomized controlled trial is self-rated work ability assessed using the Work Ability Score (0 to 10 points) six months after study inclusion. Secondary outcomes include a range of patient-reported outcomes regarding physical and mental health, impairment, and the physical and mental demands of jobs.DiscussionThis randomized controlled trial is designed to test the effects of a new complex intervention involving a comprehensive clarification of intervention needs in order to promote work participation and prevent the worsening of health and work disability.Trial registrationGerman Clinical Trials Register (DRKS00027577, February 01, 2022).