Ordinal vs dichotomous analyses of modified Rankin Scale, 5-year outcome, and cost of stroke.

OBJECTIVE: To compare how 3 common representations (ordinal vs dichotomized as 0-1/2-6 or 0-2/3-6) of the modified Rankin Scale (mRS)-a commonly used trial outcome measure-relate to long-term outcomes, and quantify trial ineligibility rates based on premorbid mRS.

METHODS: In consecutive patients with ischemic stroke in a population-based, prospective, cohort study (Oxford Vascular Study; 2002-2014), we related 3-month mRS to 1-year and 5-year disability and death (logistic regressions), and health/social care costs (generalized linear model), adjusted for age/sex, and compared goodness-of-fit values (C statistic, mean absolute error). We also calculated the proportion of patients in whom premorbid mRS score >1 or >2 would result in exclusion from trials using dichotomous analysis.

RESULTS: Among 1,607 patients, the ordinal mRS was more strongly related to 5-year mortality than both the 0-1/2-6 and 0-2/3-6 dichotomies (all p < 0.0001). Results were similar for 5-year disability, and 5-year care costs were also best captured by the ordinal model (change in mean absolute error vs age/sex: -$3,059 for ordinal, -$2,805 for 0-2/3-6, -$1,647 for 0-1/2-6). Two hundred forty-four (17.1%) 3-month survivors had premorbid mRS score >2 and 434 (30.5%) had mRS score >1; both proportions increased with female sex, socioeconomic deprivation, and age (all p < 0.0001).

CONCLUSION: The ordinal form of the 3-month mRS relates better to long-term outcomes and costs in survivors of ischemic stroke than either dichotomy. This finding favors using ordinal approaches in trials analyzing the mRS. Exclusion of patients with higher premorbid disability by use of dichotomous primary outcomes will also result in unrepresentative samples.