Statistical methods for primary and secondary outcomes {20a}
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REGRESSION, SECONDARY
The patient is the unit of measurement. The primary outcome, development of a new sacral HAPI, is dichotomous. The main analysis will be based on comparisons of the cumulative incidence of pressure injuries in the two groups. Relative risk (RR) of HAPI for the intervention group compared to the control group will be computed and reported along with 95% confidence intervals and We anticipate that prognostic imbalance of known (e.g. age, BMI, number of comorbidities) and unknown prognostic factors is unlikely between the groups due to central randomisation and treatment concealment. However, if we find any differences, we will conduct adjusted analyses using multiple regression models. We do not expect effect sizes to vary between hospitals, but centre-to-centre variability will be assessed, and cluster-adjusted analysis will be carried out as a form of sensitivity analysis if required. Patients’ declining to participate in the trial are given the opportunity to consent to some non-identifiable data being collected (i.e. admission type, age, sex, and PI risk score). These data will enable us to compare some characteristics of eligible and consenting patients (i.e. participants) versus patients who are eligible but not consenting (i.e. non-participants).For the secondary endpoints, “time to HAPI” will be analysed using the Kaplan-Meier curve and log rank tests. Hazard ratios along with 95% confidence intervals will be reported. Additional Cox regression models will be used to detect the influence of prognostic factors. As for the “stage of HAPI”, a chi-square test will evaluate if the intervention group differs from the control patients. All data will be entered directly into a web-based data collection tool (REDCap) and then exported into SPSS (V 24). Prior to analysis, a rigorous process of data cleaning to check outlying figures, missing, and implausible data against source data will be undertaken. Sample attrition will be managed via intention-to-treat analysis to ensure an unbiased comparison of the groups by randomisation. An expert biostatistician will lead all analyses.A parallel economic evaluation from the health system perspective using patient-level data from the trial to compare costs and effects of prophylactic silicone foam sacral dressings in addition to routine care with routine care alone will be undertaken. In the first step, a cost-effectiveness analysis will compare the costs and effects of prevention using the trial primary outcome (cumulative incidence) as the measure of effect. Data collected on costs and clinical outcomes will be used to calculate the incremental cost per sacral HAPI avoided, which is the difference in the mean costs of prevention (i.e. measured to the trial endpoint) divided by the difference in sacral HAPI incidence. In an extended step, a cost-benefit analysis will be undertaken to incorporate the benefits of prevention measured in dollars using data from the extended 28-day follow-up. Benefits will include any difference in costs of sacral HAPI treatment and HLOS between groups (documented on discharge for all participants).As already described, a parallel process evaluation is being undertaken alongside the trial [