External validation, extension and recalibration of Braunwaldʹs simple risk index in a community-based cohort of patients with both STEMI and NSTEMI

Background Using the simple risk index (SRI) that is based on age, heart rate and systolic blood pressure, we sought to evaluate the ability to predict outcome in AMI patients in a community-based population. Methods and results We identified and evaluated 3684 consecutive patients with an admission diagnosis of possible AMI, who attended between 1st September and 30th November 1995. Two thousand one hundred fifty three patients had confirmed evidence of WHO definition AMI, of whom 1656 survived to hospital discharge. We evaluated the ability of the SRI to predict mortality over 30 days using the score generated by the equation (heart rate × [age / 10]2) / systolic blood pressure. The SRI was a strong (c-statistic = 0.77 CI 0.74–0.79) predictor of 30-day mortality in both STEMI and all consecutive cases of WHO definition AMI. However, the model showed poor calibration when used on a community-based population with 30-day mortality being underestimated across all risk quintiles. Consequently we sought to recalibrate the quantitative aspects of the model for the total AMI population in the following way (Risk Index; 30-day mortality) (≤ 29.2; 9.2%), (29.3–37.8; 23.9%), (37.9–47.3; 34.6%), (47.4–61.5; 40.3%), (≥ 61.6; 65.5%). Conclusion We have externally validated the SRI in an unselected cohort of consecutive WHO definition AMI patients. However, the original model consistently underestimated the likelihood of death at 30 days regardless of the calculated risk score. We have therefore suggested corrections to the risk estimates, to allow its application in an unselected community cohort.