In a large study of Medicare enrollees, Gurwitz and colleagues (2003) found 5 ADEs per 100 patient-years and 1.4 preventable ADEs per 100 patient-years. The study took place in a large New England multispecialty ambulatory practice providing health care for more than 30,000 persons aged 65 and over. In total, 1,523 ADEs were identified, 421 of which were adjudged preventable (28 percent). ADEs were identified using multiple methods: reporting from health care providers, review of hospital discharge summaries, review of emergency department notes, computer-generated signals, free-text review of electronic clinical notes, and review of administrative incident reports of medication errors. Generalizing these results to the population of all Medicare enrollees, the authors estimated that 530,000 preventable ADEs occur among the 38 million enrollees (Gurwitz et al., 2003).
Another study, which contacted patients directly, found a much higher rate of ADEs but a lower proportion adjudged preventable. In a study (Gandhi et al., 2003) carried out in four primary care practices in Boston, of the 661 patients who had received at least one prescription during a 4-week period and who responded to a survey, 181 ADEs were identified (27 per 100 patients). Many more ADEs were identified by surveying the patients than by reviewing charts: of the 181 ADEs, 166 (92 percent) were identified by surveying patients, 50 (28 percent) by reviewing charts, and 35 (19 percent) by both means. Of the 181 ADEs identified, 20 were considered preventable (11 percent).
In a study on ADEs in ambulatory care (Gandhi et al., 2003), of the 20 preventable ADEs identified, 9 were due to the selection of an inappropriate dose, 2 to wrong dose, and 2 to wrong frequency of dose. It was considered that CPOE, including checking of dosages, interactions with other drugs, and allergies to the drug, could have prevented 7 of the 20 preventable ADEs. In a study of ADEs among elderly patients in the ambulatory setting (Gurwitz et al., 2003), of the 421 preventable ADEs identified, 246 were found in the prescribing stage. Among these prescribing errors, 46 percent involved wrong drug/wrong therapeutic choice and 41 percent wrong dose. (See Table 3-10 for rates of preventable ADEs in ambulatory care.)
In total, the committee estimates that at least 1.5 million preventable ADEs occur each year in the United States: