Objectives: To estimate the extent, nature and consequences of adverse events in a large National Health Service (NHS) hospital, and to evaluate the reliability of a two-stage casenote review method in identifying adverse events. Design: A two-stage structured retrospective patient casenote review. Setting: A large NHS hospital in England. Population: A random sample of 1006 hospital admissions between January and May 2004: surgery (n = 311), general medicine (n = 251), elderly (n = 184), orthopaedics (n = 131), urology (n = 61) and three other specialties (n = 68). Main outcome measures: Proportion of admissions with adverse events, the proportion of preventable adverse events, and the types and consequences of adverse events. Results: 8.7% (n = 87) of the 1006 admissions had at least one adverse event (95% CI 7.0% to 10.4%), of which 31% (n = 27) were preventable. 15% of adverse events led to impairment or disability which lasted more than 6 months and another 10% contributed to patient death. Adverse events led to a mean increased length of stay of 8 days (95% CI 6.5 to 9). The sensitivity of the screening criteria in identifying adverse events was 92% (95% CI 87% to 96%) and the specificity was 62% (95% CI 53% to 71%). Inter-rater reliability for determination of adverse events was good (k = 0.64), but for the assessment of preventability it was only moderate (k = 0.44). Conclusion: This study confirms that adverse events are common, serious and potentially preventable source of harm to patients in NHS hospitals. The accuracy and reliability of a structured two-stage casenote review in identifying adverse events in the UK was confirmed. S tudies across the world have shown that between 3% and 17% of hospital admissions result in an adverse event (defined as any unintended event caused at least partly by healthcare and which resulted in harm), and that between 28 and 75 percent of them are preventable.

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