event rates in various care settings, or estimates of the economic impact of drug-related morbidity and mortality, it is clear that medication safety represents a serious cause of concern for both health care providers and patients. Data from a variety of settings demonstrate that medication errors are common, although the frequency reported depends on the identification technique used and the definition of error employed.

A 1999 study in 36 hospitals and skilled nursing facilities found a 10 percent medication administration error rate (excluding wrong-time errors) (Barker et al., 2002). In observational studies of hospital outpatient pharmacies, prescription dispensing error rates of 0.2 to 10 percent have been found (Flynn et al., 2003). And in a national observational study of the accuracy of prescription dispensing in community pharmacies, the error rate was 1.7 percent—equivalent to about 50 million errors during the filling of 3 billion prescriptions each year in the United States (Flynn et al., 2003).

The mortality projections documented in To Err Is Human were derived from adverse event data collected in a New York State study (Brennan et al., 1991; Leape et al., 1991) and a Colorado/Utah study (Thomas et al., 2000). In these two studies, medication-related adverse events were found to be the most common type of adverse event—representing 19 percent of all such events. In a variety of studies, moreover, researchers have found even higher rates of inpatient adverse drug events than were observed in the New York State and Colorado/Utah studies (Classen et al., 1991; Bates et al., 1995b) using less restrictive definitions of adverse drug events and more rigorous detection methods. More recently, major studies have shown that many adverse drug events occur in the period after discharge from the hospital (Forster et al., 2003), in nursing homes (Gurwitz et al., 2000, 2005), and in ambulatory care settings (Gandhi et al., 2003; Gurwitz et al., 2003).

In a major recent study, moreover, researchers found high levels of errors of omission in the U.S. health care system across a wide range of measures. The chance of receiving high-quality care was only about 55 percent (McGlynn et al., 2003).

Nearly 10 years ago, researchers estimated that the annual cost of drug-related illness and death in the ambulatory care setting in the United States was approximately $76.6 billion (Johnson and Bootman, 1997). Using the same approach, this cost was estimated to be $177.4 billion in 2000 (Ernst and Grizzle, 2001).

MEASURES TO IMPROVE MEDICATION SAFETY

Efforts to improve medication safety are made at all levels of the health care system: by helping the patient avoid medication errors; by organizing



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