Background Low back pain is often long-lasting, and implementation of low-cost interventions to improve care and minimise its burden is needed. GLA:D® Back is an evidence-based programme consisting of patient education and supervised exercises for people with low back pain, which was implemented nationwide in primary care clinics in Denmark. To assess how the intervention was received and factors influencing adherence to the program, we aimed to evaluate participants’ adherence to the intervention and identified characteristics related to the completion of GLA:D® Back. Specifically, we investigated: 1) level of attendance of participants enrolled in the programme, and 2) participant-related factors associated with low attendance. Methods Primary care clinicians delivered GLA:D® Back, a standardised 10-week programme of 2 educational and 16 supervised exercise sessions, to patients with low back pain. Attendance was defined as low, medium or high based on self-reported number of attended sessions. Additional participant-reported data included demographic characteristics, pain, prognostic risk profiles, self-efficacy, illness-beliefs, function and clinician-reported physical performance tests. Results for high, medium, low, and unknown attendance were reported descriptively. Odds ratios for low attendance compared to medium/high attendance were calculated by including all baseline factors in a mixed-model logistic regression model. Results Of 1730 participants, 52% had high, 23% medium, and 25% low levels of attendance. Level of attendance was not strongly associated with participants’ individual factors, but in combination, prediction of low attendance was fair (AUC 0.77; 95% CI 0.74–0.79). The strongest indicator of low attendance was not completing the baseline questionnaire. Conclusions Most participants of a 10-week low back pain programme attended almost all session. Non-response to the baseline questionnaire was strongly associated with low attendance, whereas individual patient characteristics were weakly related to attendance. Not completing baseline questionnaires might be an early indicator of poor adherence in programs for people with persistent low back pain. Trial registration The Health Research Ethics for Southern Denmark decided there was no need for ethical approval (S-20172000-93). The Danish data collection has obtained authorisation from the Danish Data Protection Agency as part of the University of Southern Denmark’s institutional authorisation (DPA no. 2015-57-0008 SDU no. 17/30591). The trial was registred at ClinicalTrials.gov NCT03570463.

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