Background The admission time of patients with ST-segment elevation myocardial infarction (STEMI) may affect the quality of care they receive. This study aimed to explore the pattern and magnitude of variation in quality of care for patients with STEMI in both the process and outcome domains.

Methods We performed a retrospective study based on STEMI data from China. We estimated the adjusted ORs of six process indicators and one outcome indicator of STEMI care quality by fitting multilevel multivariable regression models across 42 4hour time periods per week.

Results The study cohort comprised 98 628 patients with STEMI. Care quality varied by time of arrival to the emergency department. We identified three main patterns of variation, which were consistent across days of the week. In the first pattern, which applied to electrocardiographic examination within 10 min of arrival and to aspirin or clopidogrel use within 10 min of arrival, quality was lowest for arrivals between 08:00 and 12:00, rose through the day and peaked for arrivals between 24:00 and 04:00. Percutaneous transluminal coronary intervention treatment within 90 min showed the same pattern but with maximal performance for those arriving 20:00–24:00. In the third pattern, applying to lipid function evaluation within 24 hours and beta blocker use within 24 hours, quality was best for arrivals between 04:00–08:00 and 16:00–19:00 and worst for arrivals between 24:00–04:00 and 12:00–16:00.

Conclusions The quality of care for STEMI shows three patterns of diurnal variation. Detecting the times at which quality is relatively low may lead to quality improvement in healthcare. Quality improvement should focus on reducing the weekend effect and off-hour effect and the diurnal temporal variation.