Making the incomparable comparable: Calibrating patient prioritisation tools for independent surgical procedures onto a common scale

Long waiting times for elective surgery remain a persistent challenge in public health systems. Patient prioritisation tools (PPTs) aim to support equitable decision-making by scoring patients based on clinical and non-clinical factors. However, these tools are often developed independently for specific procedures, resulting in non-comparable scores that hinder consistent prioritisation and risk violating the principles of horizontal equity (equal treatment for equal need) and vertical equity (greater need receives higher priority). This study introduces and demonstrates a calibration method to align multiple procedure-specific PPTs onto a unified severity scale. As a proof-of-concept, ten independently developed PPTs from a general surgery unit at a single Australian public hospital were calibrated using an interactive binary search algorithm to identify clinically equivalent patient states across tools, in collaboration with clinicians from the same unit. Each PPT was then aligned to a common reference procedure, and min-max normalisation was applied to standardise scores onto a shared scale. Internal consistency was assessed by comparing calibration outputs against severity rankings from the same clinical team. The calibrated scores demonstrated strong agreement with the team's severity judgements (Kendall's {tau} = -0.734, p < 0.001), indicating that the calibration method preserved the clinicians' intuitive severity rankings. The calibrated scores also preserved the assigned urgency ordering across a cohort of 845 patients. When embedded in an existing dynamic priority formula in simulation, they redistributed waiting time between urgency categories without measurably altering severity concordance. This indicates that the operational effect of the comparable scores depends on the prioritisation formula in which they are embedded, rather than on the calibration itself. These results suggest that the proposed calibration method offers a practical and potentially scalable approach for aligning PPTs across procedures, supporting valid comparisons and enabling more consistent, transparent, and equitable prioritisation. By establishing a common severity scale, this approach addresses a real-world challenge in surgical waiting list management: enabling procedures of inherently different clinical impact to be prioritised fairly and proportionately when competing for shared resources.