'Which treatment do you believe you received?' A randomised blinding feasibility trial of spinal manual therapy

Abstract

Background: Blinding is essential for mitigating biases in clinical trials. Our main objectives were to assess the feasibility of blinding: (1) participants randomly allocated to active or placebo-control spinal manual therapy (SMT) interventions, and (2) outcome assessors. We also explored blinding by levels of SMT experience and low back pain, and factors contributing to beliefs about assigned intervention. Methods:A two-parallel-arm, single-centre, placebo-controlled, blinding feasibility trial. In April 2023, we randomised 81 adults with or without SMT experience or low back pain to either active (n = 40) or placebo-control SMT (n = 41). The primary outcome was participant blinding (beliefs about assigned intervention) using the Bang blinding index (BI) at two study visits. The Bang BI is chance-corrected and ranges from –1 (all incorrect beliefs) to 1 (all correct beliefs), with 0 indicating equal proportions of correct and incorrect beliefs. Secondary outcomes were blinding using an alternative BI, outcome assessor blinding, treatment credibility/expectancy, and factors contributing to beliefs about assigned intervention. Results: Of 85 adults screened, 81 participants were randomised and 80 (99%) completed follow-up. At study visit 1, 50% of participants in the active (Bang BI: 0.50 [95% confidence interval (CI), 0.26 to 0.74]) and 37% in the placebo-control arm (0.37 [95% CI, 0.10 to 0.63]) had a correct belief about their assigned intervention, beyond chance. At study visit 2, BIs were 0.36 (0.08 to 0.64) and 0.29 (0.01 to 0.57) for participants in the active and placebo-control arms. BIs among outcome assessors suggested adequate blinding at both study visits (Active: 0.08 [-0.05 to 0.20] and 0.03 [-0.11 to 0.16]; Placebo-control: -0.12 [-0.24 to -0.00] and -0.07 [-0.21 to 0.07]). BIs varied by participant levels of SMT experience, and low back pain. Participants and outcome assessors described different factors contributing to their beliefs. Conclusions: Adequate blinding of participants assigned to active SMT may not be feasible with the intervention protocol studied, whereas blinding of participants in the placebo-control arm may be feasible at the end of the study period. Blinding of outcome assessors seemed adequate. Further methodological work on blinding of SMT interventions is needed. Trial registration number: NCT05778396