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Background: Trauma centers are increasingly advocating the replacement of arterial blood gas measurements with venous blood gas measurements for simplification of base deficit (BD) determination. These values have never been demonstrated to agree in important trauma populations, such as for patients in occult shock (OS) or the elderly. The goal of this study was to investigate the level of agreement between venous and arterial BDs from blood gases in critically ill or injured patients, specifically in OS and the elderly., Methods: This is a retrospective, consecutive, cohort study using matched pairs of venous and arterial blood gases from patients admitted to the Trauma and Neurosurgery Intensive Care Unit in a Level I trauma center in Toronto, Ontario, Canada. Agreement between near simultaneous arterial and venous BD was calculated using the Bland-Altman method. McNemar's test was used for differences in BDs in the presence or absence of OS and in elderly patients., Results: BDs for 466 arterial and venous samples from 72 patients were compared pairwise. There was no significant difference between samples (p = 0.88). Ninety-eight percent of samples were within 3.0 mmol/L of each other. No significant differences were detected between venous and arterial BD in the presence of OS or in the elderly (p = 0.72 and p = 0.25, respectively)., Conclusion: Arterial and venous BDs agree, including in the presence of OS and in the elderly. Consideration may be given to venous sampling both in the intensive care unit or in other areas of care, such as the trauma bay., Level of Evidence: Diagnostic study, level III.