The Norwegian Sickness Absence Clinic (NSAC) is a publicly funded specialist outpatient health service, which is uniquely available for the work force. The overall aim of the NSAC is prevention of sickness absence, promote return to work (RTW) among those on sickness absence and prevent long term disability benefit dependency. In addition to being a health service, the NSAC has a focus on work and functional recovery, including also non-health related factors. Patients can be referred by general practitioners for mental health problems and musculoskeletal problems. The NSAC has a lower threshold for severity than specialist health services generally, and in particular for mental health problems. The efficacy of this service is unknown.

The NSAC Efficacy Study is a randomized controlled multicentre trial which aims to assess the effect of the NSAC service. "Helse i Arbeid" is the Norwegian name for NSAC, and the Norwegian abbreviation is "HiA". The Norwegian study name is HIANOR.

The NSAC Efficacy Study involves five different NSACs across northern Norway, and will recruit 2500 patients, randomized to in equal proportions to three treatment arms:

1. NSAC - rapid: treatment at the NSAC at- or within 4 weeks
2. NSAC - ordinary: treatment at the NSAC after 10-14 weeks
3. NSAC - active control: monodisciplinary examination at the NSAC close to diagnosis-specific deadline for examination as suggested by guidelines (8-26 weeks, the majority at the end of this interval) The overall aim is to assess the effect of the NSAC service, with the hypothesis that the NSAC service is superior to what resembles treatment as usual (TAU) for outcomes such as return to work or improved health (waiting list control). Many of the diagnoses or problems for which patients are referred to the NSACs naturally improve regardless of health interventions, and - as of date - no research has been conducted to assess the efficacy of the service.

The Norwegian Sickness Absence Clinic (NSAC) is a publicly funded specialist outpatient health service, which is uniquely available for the work force. The overall aim of the NSAC is prevention of sickness absence, promote return to work (RTW) among those on sickness absence and prevent long term disability benefit dependency. In addition to being a health service, the NSAC has a focus on work and functional recovery, including also non-health related factors. Patients can be referred by general practitioners for mental health problems and musculoskeletal problems. The NSAC has a lower threshold for severity than specialist health services generally, and in particular for mental health problems. The efficacy of this service is unknown.

The NSAC Efficacy Study is part of a broader project, the Norwegian Sickness Absence Clinic Study (NSACS). The NSACS currently has two sources of funding: Northern Norway Regional Health Authority and The Norwegian Labour and Welfare Administration and involves two RCTs: NSAC Efficacy and NSAC Nudge and fifteen work packages concerning health economics, scalability, implementation, profiling of patient groups, and non-RCT related research questions to improve understanding of the patient group and their challenges. To do this the project will use patient survey data, clinician-reported procedures, opinions and outcomes, linked to registry data for work benefits and health service use. The randomized controlled trial NSAC Efficacy is the subject of this trial registration.

The NSAC Efficacy Study is a naturalistic randomized controlled multicentre trial, carried out in northern Norway and involving five NSACs. The study invites 2500 patients, randomized to either of three treatment arms:

1. NSAC rapid: treatment at the NSAC at- or within 4 weeks
2. NSAC ordinary: treatment at the NSAC after 10-14 weeks
3. NSAC - active control: monodisciplinary examination at the NSAC close to diagnosis-specific deadline for examination as suggested by guidelines (8-26 weeks, the majority at the end of this interval)

The NSACs are staffed by teams of medical doctors specializing in physical medicine and rehabilitation, psychologists, physiotherapists, and employment counsellors with experience from case management in the Norwegian Labor and Welfare Administration (NAV).

All patients are asked to complete an electronic survey about their mental health and musculoskeletal pain, work conditions, motivation for work and barriers for return to work (RTW). The control group only completes the survey on health, and only on either mental health or musculoskeletal pain, according to referral diagnosis.

The survey tool manages the randomization algorithm.

The NSAC is a relatively new clinical service, available for the labour force, and publicly funded with the intention to reduce sickness absence and prevent retirement from the labour force and transitions to disability benefits. The NSAC clinic welcomes patients with the most common diagnoses for sickness absence and is supposed to have a low threshold.

There is no single alternative clinical service for similar patients outside the labour force, thus no single treatment as usual (TAU) alternative. Service availability will depend on diagnosis and severity:

• The majority of patients eligible for treatment in the NSAC referred with mental disorders would not be eligible for treatment in specialist psychiatric services as conditions in most cases would be too mild. The most common treatment alternative would be treatment at the general practitioners.
• For patients referred for musculoskeletal problems, some may be eligible for consultations and treatment at physical medicine and rehabilitation outpatient clinics. For those not eligible, the general practitioner is the most common alternative, and other options such as private physiotherapists. Eligibility criteria may vary between catchment areas.
• All alternatives to the NSAC would be without employment counsellors, little cross disciplinary assessment, and little to no focus on work and functional rehabilitation.

The active control group aims to be comparable to TAU, and differs from the NSAC in the following respects:

1. Patients receive a monodisciplinary examination from either a doctor, physiotherapist of psychologist at the NSAC. The focus of the examination is on health-related factors. The patients will not receive further follow-up at the clinic beyond the first examination. Upon indication, the patient is referred to other treatment or examination outside of the NSAC.
2. Employment counsellors are not involved in patient consultations or in discussions about patients.
3. Patients in the active control group will not be posed questions concerning work, motivation for work or barriers for return to work. Patients will during registration in Tivian be classified as having predominantly either musculoskeletal- or common mental disorders, and this will dictate the type of questions posed: musculoskeletal patients will be asked questions on musculoskeletal factors but not psychological factors, while patients will common mental disorders will be asked questions about psychological factors but not musculoskeletal issues. For patients being examined by a psychologist, if issues concerning musculoskeletal health arise, the patient is asked to discuss these with his/her GP. If issues concerning mental health arise which requires competence beyond what the physiotherapist possesses, the physiotherapist may confer with the medical doctor at NSAC.
4. The clinician is not to take initiative to discussing work-related factors with the patient, and as far as possible avoid these becoming central topics of the consultation. If the patient on his/her own initiative brings up such topics, the clinician is not restricted from addressing them.
5. Employment counsellors are not part of patient discussions prior to examination.
6. Meetings or discussions about patients are kept within profession; i.e. a physiotherapist is allowed to discuss his/her patient with other physiotherapists if need be.

The data necessary to answer the research questions are gathered from national registries via personal identifier, registry data on population level, questionnaires filled out by the patient (data on health and working conditions) and questionnaires filled out by the clinician (data on number of treatments, diagnosis, professions involved, and types of treatment provided).

The patient questionnaire covers 9 themes, split in two sections. Section 1 covers health, and includes musculoskeletal problems, mental health, and other health related issues (such as alcohol consumption, medication, and physical activity). Section 2 includes working conditions (including inter alia questions on social support, work/family conflict and bullying), barriers for RTW, labour market affiliation, other personal aspects (such as demography and motivation for work), questionnaires for health economic evaluation (such as the EQ-5D) and expectations for treatment.

By and large the questionnaire consists of test batteries which have been validated for specific topics. To reduce the number of questions posed to each patient, the baseline questionnaire will employ index questions which have proven psychometric properties in terms of factor loading or similar, meaning that if a problem on a specific topic such as neck pain is not indicated, the patient will not be presented with the Neck Disability Index questionnaire. The patient questionnaires will be filled out electronically at before first treatment and at 6- and 12 months after first treatment. In addition, before treatments, patients will be given shorter versions of the same questionnaire to be filled out at the clinic. These shorter questionnaires will consist of questions to which the patient at baseline indicated a high score, and thus includes more relevant information to the clinician.

In the NSAC Efficacy Study, receiving treatment at NSACs presupposes consent to participate in research, as the effect of the treatment is as of date unknown. Hence, receiving treatment at the NSAC is not necessarily superior to other available health services. Other available health services that may be utilized by patients serving in the control groups include mono-disciplinary treatment by physiotherapists, psychologists, general practitioners, gyms, electronically delivered self-help tools etc. All patients referred to NSACs will be offered treatment, but patients that decline to participate in research are provided an examination in line with randomization arm #3.

Inclusion Criteria:

* The patient must be considered eligible for treatment at the NSAC by the admission team at the NSAC.

Exclusion Criteria:

• Patients considered too healthy for treatment at the NSAC, i.e. not within target group
• Patients considered too sick for treatment at the NSAC, i.e. not within target group
• Patients that do not have a diagnosis that is relevant for treatment at the NSAC, i.e. not within target group
• Patients otherwise considered not within target group for the measure
• Patients which were included in the NSAC Nudge trial (clinicaltrials identifier: NCT05006976) may not be recruited within the first year of follow-up of that trial
• Patients which are secondary referred from other departments/clinics within the hospital
• Non-Norwegian or non-English speaking patients
• Patients which could not be reached by the research coordinator at the outpatient clinic for information about the study

There are no plans to share individual patient data with other researchers. At the stage of analyses, associated researchers will be provided with an anonymized data set including all relevant data to answer the research questions.