BackgroundThe degree to which elevated mortality associated with weekend or night-time hospital admissions reflects poorer quality of care (‘off-hours effect’) is a contentious issue. We examined if off-hours admissions for primary percutaneous coronary intervention (PPCI) were associated with higher adjusted mortality and estimated the extent to which potential differences in door-to-balloon (DTB) times—a key indicator of care quality for ST elevation myocardial infarction (STEMI) patients—could explain this association.MethodsNationwide registry-based prospective observational study using Myocardial Ischemia National Audit Project data in England. We examined how off-hours admissions and DTB times were associated with our primary outcome measure, 30-day mortality, using hierarchical logistic regression models that adjusted for STEMI patient risk factors. In-hospital mortality was assessed as a secondary outcome.ResultsFrom 76 648 records of patients undergoing PPCI between January 2007 and December 2012, we included 42 677 admissions in our analysis. Fifty-six per cent of admissions for PPCI occurred during off-hours. PPCI admissions during off-hours were associated with a higher likelihood of adjusted 30-day mortality (OR 1.13; 95% CI 1.01 to 1.25). The median DTB time was longer for off-hours admissions (45 min; IQR 30–68) than regular hours (38 min; IQR 27–58; p<0.001). After adjusting for DTB time, the difference in adjusted 30-day mortality between regular and off-hours admissions for PPCI was attenuated and no longer statistically significant (OR 1.08; CI 0.97 to 1.20).ConclusionHigher adjusted mortality associated with off-hours admissions for PPCI could be partly explained by differences in DTB times. Further investigations to understand the off-hours effect should focus on conditions likely to be sensitive to the rapid availability of services, where timeliness of care is a significant determinant of outcomes.