What is the impact of implementing mandatory minimum nurse-patient ratios in general acute care hospitals?

Achieving safe staffing levels in nursing has been a challenge for health systems globally, and several policies have been adopted to promote provision of safe nursing care to patients in hospitals. Among these, mandatory minimum ratios have been implemented in some jurisdictions. The first were the states of Victoria, Australia, and California, USA, in the late 1990s, and Queensland, Australia in 2016. More recently, there has been a growing interest in establishing minimum nurse-to-patient ratio policies in England, with unions and professional bodies lobbying and advocating for mandates. Mandatory minimum staffing ratios have sparked and continue to spark debate, with opponents claiming that they constitute a 'one size fit all' approach when staffing should be dynamic and dependent on patients' needs, whereas enthusiasts highlight that mandates offer a safety net that is otherwise absent. After 30 years of research on the topic, what do we know about mandatory minimum nurse-patient ratios? In this umbrella review, we analysed 14 systematic reviews and discussion papers to find out.