Background: The risk of selection bias in registries and its consequences are relatively unexplored. We sought to assess selection bias in a recent registry about acute coronary syndrome and to explore the way of conducting and reporting patient registries of acute coronary syndrome.
Methods and results: We analyzed data from patients of a national acute coronary syndrome registry undergoing an audit about the comprehensiveness of the recruitment/inclusion. Patients initially included by hospital investigators (n=3265) were compared to eligible nonincluded (missed) patients (n=1439). We assessed, for 25 exposure variables, the deviation of the in-hospital mortality relative risks calculated in the initial sample from the actual relative risks. Missed patients were of higher risk and received less recommended therapies than the included patients. In-hospital mortality was almost 3 times higher in the missed population (9.34% [95% CI, 7.84 to 10.85] versus 3.9% [95% CI, 2.89 to 4.92]). Initial relative risks diverged from the actual relative risks more than expected by chance (P<0.05) in 21 variables, being higher than 10% in 17 variables. This deviation persisted on a smaller degree on multivariable analysis. Additionally, we reviewed a sample of 129 patient registries focused on acute coronary syndrome published in thirteen journals, collecting information on good registry performance items. Only in 38 (29.4%) and 48 (37.2%) registries was any audit of recruitment/inclusion and data abstraction, respectively, mentioned. Only 4 (3.1%) authors acknowledged potential selection bias because of incomplete recruitment.
Conclusions: Irregular inclusion can introduce substantial systematic bias in registries. This problem has not been explicitly addressed in a substantial number of them.