Severity of illness and risk of mortality from all patient refined-diagnosis related groups: Two scales of different concepts or two sides of the same coin?

All patient refined-diagnosis related groups (APR-DRGs) includes severity of illness (SOI) and risk of mortality (ROM) subclasses. For predictions, both subscales are used together or interchangeably. We aimed to compare SOI and ROM by evaluating the reliability and agreement between both. We performed a retrospective observational study using mainland Portuguese public hospitalisations of adult patients from 2011 to 2016. Reliability (quadratic weighted kappa) and agreement (proportion of agreement) between SOI and ROM were analysed overall and by APR-DRG. While overall reliability and agreement between SOI and ROM were high (weighted kappa: 0.717, 95% CI 0.717-0.718; proportion of agreement: 69.0%, 95% CI 69.0-69.0) there was high heterogeneity across APR-DRGs, ranging from 0.016 to 0.846 on reliability and from 23.1% to 94.8% on agreement. Most of APR-DRGs (263 out of 284) showed a higher proportion of episodes with ROM level above the SOI level than the opposite. In conclusion, SOI and Risk of Mortality measures must be clearly distinguished and are 'two scales of different concepts' rather than 'two sides of the same coin'. However, this is more evident for some APR-DRGs than for others.