Sample size requirements and intra-cluster correlations for stepped wedge cluster randomised trials in intensive care medicine: A practical guide.

Objective: To estimate key statistical parameters and provide practical guidance for planning stepped wedge cluster randomised trials in Australian and New Zealand intensive care units (ICUs). Design: Cross-sectional retrospective observational study using routinely collected ICU data. Setting: Adult public hospital ICUs contributing to the Australian and New Zealand Intensive Care Society Adult Patient Database between 2010 and 2023. Participants: All adult ICU admissions to 132 ICUs. Subgroups included unplanned admissions and admissions involving invasive mechanical ventilation or vasopressor use. Main outcome measures: In-hospital mortality during the index hospitalisation within 90 days of ICU admission. Intra-cluster correlation coefficients (ICCs) and cluster auto-correlations (CACs) were estimated using exchangeable, block-exchangeable, and discrete time decay models using a cross-sectional design. Results: Among 1,291,849 eligible ICU admissions, observed mortality ranged from 10.3% (all ICU admissions) to 23.0% (non-elective invasively ventilated patients in Mega-ROX ICUs). ICCs ranged from 0.008 to 0.022 and CACs from 0.83 to 1.00, with block-exchangeable or discrete time decay models most often providing the best fit. In a worked example, a 50-ICU stepped wedge trial with 10 steps (11 two-month periods) enrolling 45 unplanned ventilated patients per ICU per period (total ≈24,750 patients) would have 81.6% power to detect an absolute mortality reduction of 2.7%. Conclusions: Stepped wedge cluster randomised trials are feasible for evaluating ICU-wide interventions when routine data are available. The ICC and CAC estimates presented here provide Australian and New Zealand-specific parameters for future trial planning and demonstrate the potential of this design for pragmatic large-scale ICU research.