Bias in the measurement of the outcome is associated with effect sizes in randomized clinical trials on exercise therapy for chronic low back pain: a meta-epidemiological study

OBJECTIVE: To explore the relationships between the risk of bias and treatment effect estimates for exercise therapy interventions on pain intensity and physical functioning outcomes in randomised controlled trials (RCTs) involving patients with chronic low back pain.

STUDY DESIGN AND SETTING: Cross-sectional meta-epidemiological study.

STUDY SELECTION: The 230 RCTs (31,674 participants) in the 2021 'Exercise therapy for chronic low back pain' Cochrane Review were included.

DATA EXTRACTION: Study design characteristics, sample size, prospective trial registration, flowchart information, interventions and comparisons were extracted. Independent pairs of reviewers assessed the risk of bias using the Cochrane Risk of Bias 2 (RoB 2) tool.

RESULTS: The meta-regression included 220 (pain intensity) and 203 (physical functioning) effect sizes. Unadjusted and adjusted meta-regression models showed no significant associations between the bias domains and pain intensity effect sizes. Only domain 'bias in the measurement of the outcome' was significantly associated with physical functioning (standardised mean difference: -0.40, 95% confidence interval: -0.77 to -0.02) when adjusted for flowchart reported (yes/no), prospective trial registration, sample size, and comparator type.

CONCLUSIONS: The risk of bias in the measurement of the outcome could lead to slight overestimates of the effect size for physical functioning. Clinicians should consider this when they read and assess RCT results in this field. We encourage meta-researchers to replicate our findings using a consistent approach for evaluating the risk of bias (i.e. the RoB 2 tool) in other musculoskeletal conditions and interventions to investigate their generalisability.