Adverse Events

Last Updated: 07-29-2025

Every year, millions of Medicare patients experience adverse events and temporary harm events as a result of medical care or in a health care setting. These events can be the result of errors, substandard care, known side effects, or unexpected complications that may not have been preventable. Although most harm events resolve quickly, some have long-term and serious implications for patient health. Despite nationwide efforts to improve patient safety, reducing patient harm remains a challenge for the Department of Health and Human Services (HHS) and our Nation's health care providers. OIG is committed to helping HHS agencies determine the impact of harm events in Federal programs and supports new and existing efforts with recommendations to improve patient safety.

Initially prompted by a mandate in the Tax Relief and Health Care Act of 2006, OIG has produced a body of work related to patient safety and adverse events. This work includes publishing, in 2010, the first national incidence rate of adverse events among Medicare patients—27 percent of patients experienced some type of harm. To date, OIG has published 18 reports addressing topics ranging from the use of hospital incident reporting systems to labor and delivery in Indian Health Service hospitals. HHS agencies have implemented numerous OIG recommendations to broaden their patient safety efforts, including enhancing surveillance of adverse events and refining payment incentives to reduce the incidence of adverse events.

Patient Harm Is Common Among Medicare Patients

In 2022, OIG released an updated national incident rate of harm finding that a quarter of Medicare patients (25 percent) experienced adverse events and temporary harm events during their hospital stays in October 2018. Many of these events (43 percent) could have been prevented through better care, and nearly a quarter of patients who experienced such events required care that resulted in additional costs to the Medicare program. Although changes to the methodology and to the Medicare population preclude a direct comparison to our earlier work, these findings show that patient harm remains a significant challenge in hospitals across the Nation.

Earlier reviews also raised concerns about the prevalence of harm in post-acute care facilities and in Indian Health Service hospitals, with harm rates ranging between 13 and 43 percent. In each setting, OIG found that harm events were often preventable and were costly to the Medicare program. In light of these findings and in response to our recommendations, HHS agencies took additional steps to further promote and incentivize patient safety.

OIG recommended that HHS agencies reassess and update their efforts related to the National Strategy for Quality Improvement in Health Care, optimize national patient harm surveillance tools, and expand efforts to disseminate information about best practices and national clinical guidelines. OIG also recommended that HHS agencies take additional steps to align payment more effectively with quality and to provide guidance to surveyors that assess hospital compliance to track harm events.

In response to those recommendations, HHS took steps to advance patient safety by launching the National Action Alliance for Patient and Workforce Safety and modernizing the Quality and Safety Review System to track more types of harm events informed by OIG’s work. Other efforts include new guidance for surveyors to assess hospital quality improvement efforts, new payment models that include patient safety metrics, and more rapid promotion of patient safety best practices.

Hospitals Are Not Capturing Half of Patient Harm Events

In a followup study to our national incidence report, OIG found that hospitals did not capture half of patient harm events that occurred among hospitalized Medicare patients. Of the patient harm events that hospitals did capture, few were investigated, and even fewer led to hospitals making improvements for patient safety. Further, in a companion memo, we found that few of the captured events were required to be reported externally per CMS and State requirements and that hospitals failed to report some of those events. These results were similar to OIG’s findings from more than a decade ago on hospital incident reporting systems and reporting of events to State reporting systems. Although hospitals may have improved in their ability to capture harm events, they continue to have limited information needed to make care safer.

To address the lack of hospital awareness and response to patient harm, we recommended that HHS agencies work with Federal partners and other organizations to align harm event definitions and create a taxonomy of patient harm to drive a more comprehensive capture rate of harm events. We also recommended that CMS ensure that surveyors prioritize a Medicare requirement intended to hold hospitals accountable for patient harm. Additionally, we recommended that CMS instruct Quality Improvement Organizations to use information about harm events to assist hospitals in identifying weaknesses in their incident reporting or other surveillance systems.

Adverse Event Toolkits

OIG developed two adverse event toolkits to assist hospitals and other researchers working to identify patient harm events through medical record reviews. The toolkits serve as technical resources to assist the health care community, government agencies, and researchers in identifying and measuring patient harm events in hospitals. Hospitals around the country reported to OIG that they are using these toolkits to improve identification of patient harm events.