Higher mortality for weekend admissions: quality of care or selection bias?

We read the article by SUISSA et al. [1] with great interest. The authors retrospectively looked at all hospitalisations for chronic obstructive pulmonary disease (COPD) exacerbation or pneumonia during the period 1990–2007 from the healthcare databases of Quebec, Canada, and found higher mortality for weekend, but not Friday, admissions, relative to Monday–Thursday admissions. Similar weekend effects were observed for multiple medical conditions across different settings [2–4]. The increased mortality for weekend acute admissions is often attributed to lower quality of care during the weekend even though it could be equally plausible that sicker people came into hospital at weekends [2].