An adverse event is “an unintended injury or complication resulting in death, disability or prolonged hospital stay caused by healthcare management.”
Studies conducted in North America, Britain, Europe, Australia, and New Zealand have shown that the percentage of adverse events occurring in hospitals is between 3 and 17%, the large variation probably due to differences in observational method, but other factors may also play a role. For the sake of simplicity, I have summarized the study results to one figure – 10% (or one in every ten hospital admissions). The references are found below.
Medical error is clearly a major problem in healthcare; in the USA, an estimated one million people are injured by errors during hospital treatment each year and 120,000 people die as a result of those injuries, according to a study by Dr Lucien Leape of the Harvard School of Public Health.6 Medical error contributes to more deaths in the USA than motor vehicle accidents, falls, drowning and plane crashes combined – see Figure 1 above. The risk of adverse events in developing countries may be even higher than those in which the above studies were conducted.
Most medical errors are related to system problems, not individual negligence or misconduct, and are preventable (see our other summary statistic on the preventability of adverse events). Medical errors can be reduced significantly if healthcare delivery systems are improved.
Baker GR, Norton PG, Flintoft V, Blais R., Brown A, Cox J, et al. The Canadian adverse events study: the incidence of adverse events among hospital patients in Canada. CMAJ 2004;170:1678–86.
Brennan TA, Leape LL, Laird NM, Hebert L, Localio AR, Lawthers AG et al. Incidence of adverse events and negligence in hospitalized patients. Results of the Harvard Medical Practice Study I. _N Engl J Med_1991;324(6):370–7.
Davis P, Lay-Yee R, Briant R, Ali W, Scott A, Schug S. Adverse events in New Zealand public hospitals I: occurrence and impact. NZ Med J 2002;115(1167):U271.
Davis P, Lay-Yee R, Briant R, Schug S, Scott A, Johnson S, et al. Adverse events in New Zealand public hospitals: principal findings from a national survey. Wellington: NZ Ministry of Health; 2001. Available: https://www.health.govt.nz/publication/adverse-events-new-zealand-public-hospitals-principal-findings-national-survey (accessed 17 Sep 2020).
Davis P, Lay-Yee R, Briant R, Ali W, Scott A, Schug S. Adverse events in New Zealand public hospitals II: preventability and clinical context. NZ Med J 2003;116(1183):U624.
Leape LL, Brennan TA, Laird N, Lawthers AG, Localio AR, Barnes BA, et al. The nature of adverse events in hospitalized patients. Results of the Harvard Medical Practice Study II. N Engl J Med 1991;324(6):377–84.
National Study on Hospitalisation-Related Adverse Events (ENEAS). >Report. February 2006. Ministry of Health and Consumer Affairs. Technical Secretary. Publications Center. Paseo del Prado, 18-28014 Madrid. Official Publication Identification No. (NIPO) 351-06-009-2, 2005.
Thomas EJ, Studdert DM, Burstin HR, Orav EJ, Zeena T, Williams EJ, et al. Incidence and types of adverse events and negligent care in Utah and Colorado. Med Care 2000;38(3):261–71.
Schioler T, Lipczak H, Pedersen BL, Mogensen TS, Bech KB, Stockmarr A, et al. Danish Adverse Event Study [Incidence of adverse events in hospitals. A retrospective study of medical records]. Ugeskr Laeger 2001;163(39):53708.
Soop M, Fryksmark U, Köster M, Haglund B. The incidence of adverse events in Swedish hospitals: a retrospective medical record review study. Int J Qual Health Care 2009;21(4):285–91.
Vincent C, Neale G, Woloshynowych M. Adverse events in British hospitals: preliminary retrospective record review [published erratum in BMJ_ 2001;322:1395]. BMJ 2001;322(7285):517–9.
Wilson RM, Runciman WB, Gibberd RW, Harrison BT, Newby L, Hamilton JD. The Quality in Australian Health Care Study. Med J Aust 1995;163(9):458–76.
Zegers M, Bruijne MC de, Wagner C, Hoonhout LHF, Waaijman R, Smits M, et al. Adverse events and potentially preventable deaths in Dutch hospitals: results of a retrospective patient record review study. Quality & Safety in Health Care 2009;18(4):297–302.