Background: Prioritising patients in mass casualty incidents (MCI) can be extremely difficult. Therefore, triage systems are important in every emergency medical service. This study reviews the accuracy of primary triage systems for MCI in trauma register studies. Methods: We registered a protocol at PROSPERO ID: CRD42018115438. We searched MEDLINE, EMBASE, Central, Web of Science, Scopus, Clinical Trials, Google Scholar, and reference lists for eligible studies. We included studies that both examined a primary triage system for MCI in trauma registers and provided sensitivity and specificity for critically injured vs non-critically injured as results. We excluded studies that used paediatric, chemical, biological, radiological or nuclear MCIs populations or triage systems. Finally, we calculated intra-study relative sensitivity, specificity and diagnostic odds ratio for each triage system. Results: Triage Sieve (TS) significantly underperformed in relative diagnostic odds ratio (DOR) when compared to START and CareFlight (CF) (START vs TS: 19.85 vs 13.23 (p<0.0001)│CF vs TS: 23.72 vs 12.83 (p<0.0001)). There was no significant difference in DOR between TS and Military Sieve (MS) (p<0.710). Compared to START, MS and CF TS had significantly higher relative specificity (START vs TS: 93.6% vs 96.1% (p=0.047)│CF vs TS: 96% vs 95.3% (p=0.0006)│MS vs TS: 94% vs 88.3% (p=0.0002)) and lower relative sensitivity (START vs TS: 57.8% vs 34.8% (p<0.0001)│CF vs TS: 53.9% vs 34.7% (p<0.0001)│MS vs TS: 51.9% vs 35.2% p<0.0001)). CF had significantly better relative DOR than START (CF vs START: 23.56 vs 27.79 (p=0.043)). MS had significantly better relative sensitivity than CF and START (MS vs CF: 49.5% vs 38.7% (p<0.0001)│MS vs START: 49.4% vs 43.9% (p=0.01)). In contrast, CF had significantly better relative specificity than MS (MS vs CF: 91.3% vs 93.3% (p<0.0001)). The remaining comparisons did not yield any significant differences. Conclusion: As the included studies were at risk of bias and had heterogenic characteristics, our results should be interpreted with caution. Nonetheless, our results point towards inferior accuracy of Triage Sieve compared to START and CareFlight, and less firmly point towards superior accuracy of Military Sieve compared to START, CareFlight and Triage Sieve