In March 2012, Australia's National Blood Authority published national patient blood-management guidelines for perioperative care developed by a systematic review and clinical expert opinion. This study assesses how blood transfusions and patient outcomes in cardiac surgery changed after the guidelines were published.Blood transfusions and patient outcomes in cardiac surgery were compared before and after implementation of the guidelines using an interrupted time series analysis. The evaluation included red blood cells, platelets, cryoprecipitate, fresh-frozen plasma, 30-day mortality, 30-day readmissions, and hospital and intensive care length of stay. Patient characteristics were controlled for along with hospital characteristics using fixed effects. Different responses across institutional settings were assessed with an expanded difference-in-differences model.After the guidelines were published, our model found a significant reduction in red blood cell, platelet, and fresh-frozen plasma transfusions. There was also a significant reduction in hospital length of stay but no significant impact on cryoprecipitate, 30-day mortality, 30-day readmissions, or intensive care unit length of stay. The subgroup analyses found no differences with regards to institutional settings.Following the publication of the guidelines, there was a measurable reduction in perioperative blood transfusions in cardiac surgery with an associated reduction in hospital length of stay but no detectable differences in other patient outcomes.