Our initial data search yielded a total of 788 citations (Fig 1
). We excluded 754 articles because they did not meet our inclusion criteria. Thirty-four studies3,7,9,11–13,16,21–47 were identified that had reported mortality separately for ARDSp and ARDSexp (Table 1
). These studies were from around the globe (Table 1). Only nine studies,3,7,11–13,16,29–30,47 dealt primarily with the ARDSp and ARDSexp categories. The other references were studies involving ARDS patients that evaluated different aspects of the disease but reported mortality separately for ARDSp and ARDSexp groups. Of the studies evaluated, 4 studies16,23,30,47 were retrospective and the remaining 30 studies3,7,9,11–13,21–22,24–29,31–46 were prospective (Table 1). Twenty-two studies,3,7,9,11,13,21–22,24,26–29,31,34–35,38–40,44,46–47 had < 100 patients in each study, whereas the remaining 12 studies16,23,25,30,32–33,36–37,41–43,45 each had > 100 patients. In all, the studies involved 4,311 patients with 2,330 patients belonging to the ARDSp group and 1,981 patients belonging to the ARDSexp groups. The OR of mortality in the ARDSp group compared to the ARDSexp group was 1.11 (95% CI, 0.88 to 1.39) as determined by the random-effects model; 1.04 (95% CI, 0.92 to 1.18), as determined by the fixed-effects model; and 1.04 (95% CI, 0.92 to 1.18), as determined by the exact method, indicating that mortality is similar in the two groups.