研究表明，在乳腺癌患者中，miR-21表达异常，提示miR-21可能作为一种可能的诊断生物标记物被利用于临床应用。为了提供研究支持的临床证据，我们在本研究中调查了miR-21在乳腺癌诊断中的效用。自创建以来至2022年1月23日，我们检索了PubMed、EMBASE、Web of Science、Cochrane Library和Scopus数据库的所有相关英文文献。使用QUADAS-2评估文献质量，使用GRADE评估证据等级。使用R 4.0.1和Revman 5.3进行统计分析。使用Stata 15.1软件对结果进行验证。还根据miR-21和miR-21组合的来源进行亚组分析。共回顾了9篇文献，涉及2048名患者。所有纳入的研究均为中高质量。使用混合效应模型进行了荟萃分析。汇总敏感度、特异度、诊断比值比（DOR）、阴性似然比（NLR）和阳性似然比（PLR）分别为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)]和6.35 [95% CI (3.66, 11.16)]。miR-21的GRADE分类为A，表明强烈推荐用于乳腺癌筛查。现有证据表明，miR-21作为乳腺癌生物标志物具有足够的诊断价值。通过与其他miRNA结合，其诊断精度可以进一步提高。基于GRADE综述，强烈推荐miR-21用于乳腺癌筛查。

Studies 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.From 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.Nine 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.The 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.