Blinding assessment of manual therapy interventions of the back in Swiss graduate students: a blinding feasibility randomized controlled trial

Study design Single center, two-parallel group, blinding feasibility randomized controlled trial. Background Clinical trials of manual therapy interventions for back pain face methodological challenges regarding blinding of assigned interventions. We assessed blinding of participants and outcome assessors in a methodological randomized trial of two manual therapy interventions of the back and explored factors influencing perceptions about intervention assignment. Methods In a doctoral-level Introduction to Epidemiology course on 7–8 November 2022, 24 healthy students were randomly allocated (1:1) to active or control manual therapy interventions. The active manual therapy group (n = 11) received soft tissue mobilization of the lumbar musculature. The control group (n = 13) performed deep breathing exercises while receiving light touch over the thoracic region. The primary outcome was blinding feasibility of study participants immediately after intervention, derived from a 5-point scale and measured by the Bang blinding index (Bang BI), with values ranging from − 1 (opposite guessing) to 1 (complete unblinding), with 0 indicating random guessing (50% correct). Secondary outcomes included blinding feasibility of outcome assessors and factors influencing perceptions about intervention assignment in participants and outcome assessors, explored via thematic analysis. Results 24 participants were analyzed following an intention-to-treat approach. 55% of participants in the active manual therapy group correctly identified their group allocation beyond chance immediately after intervention (Bang BI: 0.55 [95% confidence interval (CI), 0.25 to 0.84]), and 8% did so in the control manual therapy group (0.08 [95% CI, -0.37 to 0.53]). Bang BIs in outcome assessors suggested adequate blinding, with indices of 0.09 (-0.12 to 0.30) and − 0.10 (-0.29 to 0.08) for perceived allocation of active and control manual therapy participants, respectively. Blinding estimates were limited due to imprecision and suboptimal generalizability to real-world clinical settings. Participants and outcome assessors reported varying factors related to perceived intervention arm allocation. Conclusions Our findings suggest that blinding of participants and outcome assessors immediately after a one-time manual therapy intervention is feasible in randomized controlled trial settings. Careful thinking and consideration of blinding in manual therapy intervention trials is warranted and needed. Trial registration: Retrospectively registered at ClinicalTrials.gov ― currently under Protocol Registration and Results System (PRS) review.