Preventable Adverse Drug Reactions – Meta-Analysis Figure 5. Percentage of inpatients with preventable adverse drug reactions, during hospitalisation. *not provided directly in the study, interpreted from other presented data. doi:10.1371/journal.pone.0033236.g005 Sensitivity analyses The preventability of ADRs among outpatients and inpatients and the percentage of outpatients with PADRs were higher in studies with shorter study periods. When outliers were removed from the main analysis, the preventability of ADRs among outpatients was 45% (95% CI: 40–59%), lower than in the main analysis. The preventability of ADRs among inpatients was 54% (95% CI: 51–56%), higher than in the main analysis, when outliers were removed. However, all sensitivity analyses’ estimates, both according to study period and after removal of outliers, were within the confidence intervals of the overall estimates of the main analyses. Discussion We found that 2% of adult outpatients being hospitalised or visiting emergency care experience PADRs. Approximately half of all ADRs among outpatients were preventable. As no studies in primary care were identified, the percentage of patients with PADRs and the preventability of ADRs remain unknown among outpatients without an admission or emergency visit. Among inpatients, close to half of ADRs present during hospitalisation were preventable, but the percentage of inpatients with PADRs could not be estimated precisely. Strengths and limitations of the review Our study is the first to estimate in a meta-analysis the percentage of patients with PADRs and the preventability of ADRs, among both outpatients and inpatients. Our search strategy was comprehensive, but laborious with a large number of citations. We included published studies in English, as research is to be shared internationally, but this may have lead to overlooking some relevant studies. Authors were not contacted for unpublished data which may have lead to excluding studies that would have fulfilled the inclusion criteria if more data had been available. Further, although an excluded full-text article could include several exclusion criteria, only one exclusion criterion per article was recorded in the order of notifying the criterion. As it was not interpretable in most original studies whether ADRs were incident or prevalent, we used the percentage of patients with PADRs as an outcome measure. To prevent heterogeneous estimates and avoid bias, we required a standard- ised definition for ADRs, inclusive data sources and an original preventability assessment. Yet, the included studies were hetero- geneous, perhaps due to varying study designs, settings and criteria for preventability. However, our overall estimates did not differ substantially when two sensitivity analyses were performed. We found ADRs among adult outpatients more preventable than in earlier reviews, in which the median and pooled preventability of ADRs among patients being hospitalised has been 31% and 29%, respectively [4,10]. The difference may arise from inconsistent definitions for ADRs. For providing consistent estimates, we required WHO’s [16] or a similar [17] definition for ADRs. Studies included in previous reviews on outpatients accepted various definitions for ADRs [4,10], some of which found ADRs less preventable [47-50]. The criteria for prevent- ability may also have influenced differing preventability estimates. While it was not an inclusion criterion, most studies in our meta- analysis used the criteria introduced by Hallas et al [39] or Schumock et al [40] to establish preventability. Some studies in Figure 6. Preventability of adverse drug reactions among inpatients, during hospitalisation. *not provided directly in the study, interpreted from other presented data. doi:10.1371/journal.pone.0033236.g006 PLoS ONE | www.plosone.org 7 March 2012 | Volume 7 | Issue 3 | e33236