BackgroundTo which extent trauma- induced disturbances in ionized calcium (iCa2+) levels have a linear relationship with adverse outcomes remains controversial. The goal of this study was to determine the association between the distribution and accompanying characteristics of transfusion-independent iCa2+ levels versus outcome in a large cohort of major trauma patients upon arrival at the emergency department.MethodsA retrospective observational analysis of the TraumaRegister DGU®(2015–2019) was performed. Adult major trauma patients with direct admission to a European trauma centre were selected as the study cohort. Mortality at 6 h and 24 h, in-hospital mortality, coagulopathy, and need for transfusion were considered as relevant outcome parameters. The distribution of iCa2+ levels upon arrival at the emergency department was calculated in relation to these outcome parameters. Multivariable logistic regression analysis was performed to determine independent associations.ResultsIn the TraumaRegister DGU®30 183 adult major trauma patients were found eligible for inclusion. iCa2+ disturbances affected 16.4% of patients, with hypocalcemia (< 1.10 mmol/l) being more frequent (13.2%) compared to hypercalcemia (≥ 1.30 mmol/l, 3.2%).Patients with hypo- and hypercalcemia were both more likely (P< .001) to have severe injury, shock, acidosis, coagulopathy, transfusion requirement, and haemorrhage as cause of death. Moreover, both groups had significant lower survival rates. All these findings were most distinct in hypercalcemic patients. When adjusting for potential confounders, mortality at 6 h was independently associated with iCa2+ < 0.90 mmol/L (OR 2.69, 95% CI 1.67–4.34;P< .001), iCa2+ 1.30–1.39 mmol/L (OR 1.56, 95% CI 1.04–2.32,P= 0.030), and iCa2+  ≥ 1.40 mmol/L (OR 2.87, 95% CI 1.57–5.26;P< .001). Moreover, an independent relationship was determined for iCa2+ 1.00–1.09 mmol/L with mortality at 24 h (OR 1.25, 95% CI 1.05–1.48;P= .0011), and with in-hospital mortality (OR 1.29, 95% CI 1.13–1.47;P< .001). Both hypocalcemia < 1.10 mmol/L and hypercalcemia ≥ 1.30 mmol/L had an independent association with coagulopathy and transfusion.ConclusionsTransfusion-independent iCa2+ levels in major trauma patients upon arrival at the emergency department have a parabolic relationship with coagulopathy, need for transfusion, and mortality. Further research is needed to confirm whether iCa2+ levels change dynamically and are more a reflection of severity of injury and accompanying physiological derangements, rather than an individual parameter that needs to be corrected as such.Graphical AbstractSupplementary InformationThe online version contains supplementary material available at 10.1186/s13054-023-04541-3.