Preventable Adverse Drug Reactions – Meta-Analysis Ethics Statement According to Swedish regulations on medical research on humans, approval by an ethical review board is not required in review studies and meta-analyses that use aggregated patient data from previous studies. As no individual patient data was used or stored for the current study, informed consents from the participants of the original studies were not required. Results A total of 5770 citations were found from electronic database searches and additional 59 records were identified from reference lists (Figure 2). After removal of duplicate records, the inclusion and exclusion criteria were applied on 4220 unique citations. After title and abstract review, the inclusion and exclusion criteria were applied on 399 articles’ full texts. Most full-text articles were excluded due to not assessing or reporting the percentage of patients with PADRs or the preventability of ADRs (n = 290). Excluded articles commonly focused on potential drug-related problems or other types of adverse events without a category for ADRs or their preventability. Many also lacked a denominator for calculating the percentage of patients or preventability. After applying all inclusion criteria, we finally included 22 articles in the review. Of these, 14 studies reported PADRs among outpatients exclusively, six among inpatients, and two separately for outpatients and inpatients (Tables 1,2,3). Table 1. Characteristics of included studies on preventable adverse drug reactions among outpatients being hospitalisedw. Study (Country) Study period Study design Pearson 1994 [23] 6 months, (United States){ 1992–1993 Courtman 1995 [24] (Canada) Dartnell 1999 [25] (Australia) Chan 2001 [27] (Australia) Olivier 2002 [28] (France) Dormann 2003 [29] (Germany) Dormann 2004 [30] (Germany){ Alexopoulou 2008 [33] (Greece) Fransceschi 2008 [34] (Italy) Hopf 2008 [35] (United Kingdom) Ruiz 2008 [36] (Spain)* Van Der Hooft 2008 [37] (Netherlands) Pirmohamed 2004 6 months, [31] (United Kingdom) 2000–2002 20 weeks, 1992–1993 30 days, 1994 8 weeks, 1998 Weeks, 1998 13 months, 1998–1999 12 months+6 months for related readmission, no year 6 months, 2005 14 months, 2004–2005 2 weeks, 2006 21 months, 2001–2003, for each patient 60 days 12 months, 2003 Prospective, observational Prospective, observational Prospective, observational Cross-sectional, observational Prospective, observational Prospective, observational Prospective, observational Prospective, observational Prospective observational Prospective, observational Prospective, observational Prospective, observational Retrospective, observational Population characteristics No age limitation Age.65 years, median 78 years (range 65–108), 41% male Median age 58 years (range 15–91), 38% male Mean age 82 years (range 75–94), 45% male Age.15 years Mean age 54 years (range 17–97), 85% male Mean age 57 years (range 18–97), high proportion of those between 55–70 years Median age 66 years (range 46–79), 48% male Mean age 65 years (range 15–100), 2/3 participants.65 years, 50% male Age $65 years People without ADR: mean age 62 years, 35% male Unknown Mean age 38 years, 48.5% male Characteristics of patients with ADR Mean age unknown, 50% male Unknown Unknown Unknown Definition for ADR{ Similar to WHO [16] WHO [16] Criteria for preventability Schumock [40] Modified Hallas [39] WHO [16] Own criteria WHO [16] Modified Hallas [39] Mean age 58 years, 54% male Unknown WHO [16] WHO [16] Imbs [57] and own criteria# Schumock [40] Unknown WHO [16] Schumock [40] Median age 76 years (range 65–83), 41% male Mean age 71 years (range 69–73), 47% male Mean age 77 years (range 60–93), 41% male Mean age 67years (range 19–91), 43% male Aged.65 years represent 65%, 58% male Unknown Edwards and Aronson [17] Hallas [39] WHO [16] Modified Hallas [39] Edwards and Aronson [17] Hallas [39] integrated with Gurwitz [58] WHO [16] Hallas [39] WHO [16] Schumock [40] Similar to WHO [16] Hallas [39] ADR = adverse drug reaction; WHO = World Health Organization. wADRs are present at admission and may or may not be the main reason for hospitalisation. {Data from study used for analyses for both outpatients and inpatients. *ADR reason for re-hospitalisation. {Studies using WHO’s definition may have referenced another publication for the definition. #Compared the two methods for assessing preventability of which the ‘‘own criteria’’ was chosen to be included in the meta-analysis. doi:10.1371/journal.pone.0033236.t001 PLoS ONE | www.plosone.org 4 March 2012 | Volume 7 | Issue 3 | e33236