Prognostic models for identifying risk of poor outcome in people with acute ankle sprains: the SPRAINED development and external validation study.

Background Ankle sprains are very common injuries. Although recovery can occur within weeks, around a third of patients have longer-term problems. Objectives To develop and externally validate a prognostic model for identifying people at increased risk of poor outcome after an acute ankle sprain. Design Development of a prognostic model in a clinical trial cohort dataset and external validation in a prospective cohort study. Setting Emergency departments in the UK. Participants Adults with an acute ankle sprain (within 7 days of injury). Sample Size There were 584 clinical trial participants in the development dataset and 682 recruited for the external validation study. Predictors Candidate predictor variables were chosen based on availability in the clinical data set, clinical consensus, face-validity, a systematic review of the literature, data quality and plausibility of predictiveness of the outcomes. Main Outcome Measures Models were developed to predict two composite outcomes representing poor outcome. Outcome 1 was the presence of at least one of the following symptoms at 9 months after injury: persistent pain, functional difficulty or lack of confidence. Outcome 2 included the same symptoms as outcome 1 with the addition of recurrence of injury. Rates of poor outcome in the external dataset were lower than in the development dataset, 7 vs. 20% for outcome 1 and 16 vs. 24% for outcome 2, respectively. Analysis Multiple imputation was used to handle missing data. Logistic regression models, together with multivariable fractional polynomials, were used to select variables and identify respective transformations of continuous predictors that best predicted the outcome based on a nominal alpha of 0.157, chosen to minimise over fitting. Predictive accuracy was evaluated by assessing model discrimination (c-statistic) and calibration (flexible calibration plot).