Background and purposeStudies have revealed that miR-21 is abnormally expressed in breast cancer patients, suggesting that miR-21 could be exploited as a possible diagnostic biomarker for clinical applications. In order to provide clinical evidence that is supported by research, we investigate the diagnostic utility of miR-21 in breast cancer in this study.MethodsFrom their inception to 23 January 2022, the PubMed, EMBASE, Web of Science, Cochrane Library, and Scopus databases were searched for all pertinent English literature. QUADAS-2 for literature quality assessment, GRADE for evidence grading. Statistical analyses were performed using the R 4.0.1 and Revman 5.3. The results were validated using Stata 15.1 software. Subgroup analysis was also performed according to the source of miR-21 and miR-21 combinations.ResultsNine publications with 2048 patients were reviewed for inclusion. All of the included studies are of moderate-high quality. Meta-analysis was performed using a mixed-effects model. The pooled sensitivity, specificity, diagnostic odds ratio (DOR), negative likelihood ratio (NLR) and positive likelihood ratio (PLR) were 0.91 [95% CI (0.86, 0.95)], 0.85 [95% CI (0.77, 0.91)], 56.62 [95% CI (21.00, 184.83)], 0.11 [95% CI (0.05, 0.18)] and 6.35 [95% CI (3.66, 11.16)], respectively. The GRADE classification for miR-21 was A, indicating a strong recommendation for breast cancer screening.ConclusionsThe available evidence suggests that miR-21 has sufficient diagnostic value as a biomarker for breast cancer. Its diagnostic precision can be further improved by combining it with other miRNAs. Based on the GRADE review, miR-21 is strongly recommended for breast cancer screening.