Risk of bias assessment in a meta-analysis investigating the efficacy and safety of intrathecal diamorphine

We read with great interest the article by Grape et al. [1]. The authors conducted a meta-analysis that included meta-regression and trial sequential analysis, providing a comprehensive synthesis of 12 prospective, randomised controlled trials comparing intrathecal diamorphine with controls across various surgical procedures [1]. We identified a concern that may compromise the quality of this meta-analysis. The authors stated that the methodological quality of the included trials was assessed using the Cochrane Collaboration's Risk of Bias tool 2 (RoB2) [1]. The risk of bias assessment they describe encompassed seven domains: random sequence generation; allocation concealment; blinding of participants and personnel; blinding of outcome assessment; incomplete outcome data; selective reporting; and other biases. The approach employed, along with the reference cited, was actually consistent with the Cochrane Collaboration's Risk of Bias tool 1 (RoB1) [1, 2]. It is important to note that RoB2 resembles but differs from RoB1. Specifically, RoB2 evaluates six domains: the randomisation process; deviations from intended interventions; missing outcome data; measurement of the outcome; selection of the reported results; and overall bias [3]. The article by Sterne et al. discusses this in detail [3]. Additionally, two common figures depicting the risk of bias for the included studies can be generated using the RoB2 assessment spreadsheet (ROB2_IRPG_beta_v9) [3], or through the use of a web application called robvis® [4]. We suggest that the authors review their risk of bias assessment approach to align with the latest standards and improve the rigour of their meta-analysis.