Database search and risk of bias assessment
A total of 2881 articles (PubMed 2276, Cochrane Library 134, and Ichushi web 471) were identified in the initial search, and 41 articles were potentially eligible after the first screening of the titles and abstracts. Next, the full text was reviewed, and 20 articles were excluded. Eventually, 21 observational studies were selected for this study (Fig. 1). The summary of the risk of bias using the QUADAS-2 in the included studies was shown in Fig. 2. For patient selection, three or two studies were evaluated as having a high risk of bias or high concern for applicability, respectively, because of the possibility of inappropriate exclusion or mismatched definition. In the patient flow and timing assessment, three studies were assessed to have a high risk of bias because of inappropriate omission or uncertainly of evaluation timing of reference standard.
Included studies
The characteristics of the included studies are shown in Table 2. Among these 21 studies, five6,13,18,30,36 and 164,7,14,19,31,32,33,34,35,37,38,39,40,41,42,43 were prospective and retrospective studies, respectively. The category of pneumonia was eight7,19,31,33,36,38,39,42, seven13,18,32,34,35,41,43, and six4,6,14,30,37,40 for HCAP, NHCAP, and NHAP, respectively. Twelve4,7,13,14,19,30,31,36,39,41,42,43 studies for PSI, 124,7,13,18,19,32,33,34,35,38,41,43 for A-DROP, seven7,13,18,19,34,41,43 for I-ROAD, and 144,6,7,13,14,19,30,31,36,37,39,40,41,42 for CURB-65 were included in this study.
PSI
Twelve4,7,13,14,19,30,31,36,39,41,42,43 studies were included in the meta-analysis for the PSI score. Using a cut-off value of ≥ IV (moderate; n = 12), the pooled sensitivity, specificity, PLR, NLR, and DOR for mortality were calculated as 0.97 (0.94–0.98), 0.15 (0.10–0.21), 1.14 (1.08–1.20), 0.22 (0.12–0.38), and 5.09 (2.95–8.78), respectively (Table 3). Using a cut-off value of V (severe; n = 11), the pooled sensitivity, specificity, PLR, NLR, and DOR for mortality were 0.69 (0.60–0.77), 0.66 (0.60–0.72), 2.03 (1.82–2.27), 0.47 (0.38–0.58), and 4.32 (3.35–5.59), respectively. The forest plots and estimated sensitivities and specificities from each study are shown in Fig. 3a,b.
A-DROP
Twelve4,7,13,18,19,32,33,34,35,38,41,43 studies were included in the meta-analysis for the A-DROP score. Using a cut-off value of ≥ III (severe; n = 11), the pooled sensitivity, specificity, PLR, NLR, and DOR for mortality were 0.70 (0.62–0.76), 0.54 (0.45–0.62), 1.50 (1.33–1.70), 0.56 (0.48–0.66), and 2.66 (2.09–3.40), respectively (Table 3). The forest plots using these cut-offs and estimated sensitivities and specificities from each study are shown in Fig. 3c,d. In one study7, forest plots weren’t described in Figure because the data necessary to create it in both a cut-off value of ≥ I and ≥ III was insufficient.
I-ROAD
Seven7,13,18,19,34,41,43 studies were included in the meta-analysis for the I-ROAD score. Using a cut-off value of ≥ moderate (n = 5), the pooled sensitivity, specificity, PLR, NLR, and DOR for mortality were 0.92 (0.69–0.98), 0.44 (0.30–0.59), 1.66 (1.39–1.98), 0.18 (0.05–0.61), and 9.32 (2.86–30.3) respectively (Table 3). Using a cut-off value of severe (n = 7), the pooled sensitivity, specificity, PLR, NLR, and DOR for mortality were 0.67 (0.54–0.77), 0.63 (0.50–0.74), 1.78 (1.44–2.21), 0.53 (0.42–0.68), and 3.34 (2.35–4.75), respectively. The forest plots using these cut-offs and estimated sensitivities and specificities from each study are shown in Fig. 3e,f.
CURB-65
Fourteen4,6,7,13,14,19,30,31,36,37,39,40,41,42 studies were included in the meta-analysis for the CURB-65. Using a cut-off value of ≥ II (moderate; n = 13), the pooled sensitivity, specificity, PLR, NLR, and DOR for mortality were 0.91 (0.84–0.95), 0.28 (0.20–0.37), 1.26 (1.17–1.36), 0.33 (0.23–0.46), and 3.86 (2.74–5.44), respectively (Table 3). Using a cut-off value of ≥ III (severe; n = 14), the pooled sensitivity, specificity, PLR, NLR, and DOR for mortality were 0.63 (0.52–0.73), 0.63 (0.53–0.71), 1.70 (1.52–1.90), 0.58 (0.49–0.70), and 2.91 (2.34–3.62), respectively. The forest plots using these cut-offs and estimated sensitivities and specificities from each study are shown in Fig. 3g,h.
Comparisons of overall AUC among PSI, A-DROP, I-ROAD, and CURB-65
The overall AUC values were pooled from the AUC (95% CI) values reported in the included studies (Fig. 4). The overall AUCs were 0.70 (0.68–0.72), 0.70 (0.63–0.76), 0.68 (0.64–0.73), and 0.67 (0.63–0.71) for PSI, A-DROP, I-ROAD, and CURB-65 scores, respectively. No significant differences were observed (p = 0.66, I2 = 0%).
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