Epidemiology of medical error

Newspaper and television stories of catastrophic injuries occurring at the hands of physicians spotlight the problem of medical error but provide little insight into its nature or magnitude. 1Physicians, patients, and policymakers may underestimate the magnitude of risk and the extent of harm.We review the epidemiology of medical error, concentrating primarily on the prevalence and consequences of error, which types are most common, which physicians make errors, and the risk factors that increase the likelihood of injury from error. PREVALENCE AND CONSEQUENCES IN HOSPITALS Benchmark studiesThe Harvard study of medical practice is the benchmark for estimating the extent of medical injuries occurring in hospitals.Brennan et al reviewed the medical records of 30,121 patients admitted to 51 acute care hospitals in New York State in 1984. 2 They reported that adverse events-injuries caused by medical management that prolonged admission or produced disability at the time of discharge-occurred in 3.7% of admissions.A subsequent analysis of the same data found that 69% of injuries were caused by errors. 3In a study of the quality of Australian health care, a population-based study modeled on the Harvard study, investigators reviewed the medical records of 14,179 patients admitted to 28 hospitals in New South Wales and South Australia in 1995. 4 An adverse event occurred in 16.6% of admissions, resulting in permanent disability in 13.7% of patients and death in 4.9%; 51% of adverse events were considered to have been preventable.The number of preventable adverse events is important because both preventable and potentially adverse events (or "near misses") imply medical error.In contrast, nonpreventable adverse events suggest that anticipated and unavoidable complications were present.In the Australian study, the higher rate of adverse events was attributed in part to methodologic differences between the 2 studies, but a real difference in the rate of injuries to patients in the 2 populations could not be excluded.No study rivals the scope of the Harvard and the Australian studies except for a recent replication of the Harvard study in Colorado and Utah. 5 Even so, the results probably represent an estimate of the lower boundary of the prevalence of medical injury and error.The Harvard investigators defined adverse events stringently, using disability and injury as prerequisites.This underestimates the error rate because many errors do not produce injury; they are caught in time, the patient is resilient, or luck is good.