Background: Rapid accurate assessment of metabolic derangements is crucial in the critically ill. We evaluated if arterial blood gas (ABG) samples transported through a pneumatic tube system (PTS) agreed with values transported by a human courier. Methods: In this prospective study of 50-paired ABG samples, the couriered reference ABG was compared with those transported by PTS. Agreement was summarised by the mean difference with 95% limits of agreement (LOA) and Lin's concordance correlation (pc). Results: The mean (±SD) time from sampling to analysis was 35.7±23.2 (courier) and 38.6±22.1 (PTS) minutes. Agreement was good between courier and PTS for pH, PaCO 2, bicarbonate, oxygen saturation and PaO 2 values (pc>0.97). Although the mean difference in PaO 2 values between PTS and courier was small (-0.9 mm Hg) and the agreement was good, individual differences were clinically significant (95% LOA -40.8 to 39.0). For PaO 2 <160 mm Hg, analysis of PTS samples yielded erroneously high PaO 2 values and vice versa for PaO 2>160 mm Hg compared to manual courier. This suggested exaggerated oxygen movement between the blood sample and air in the PTS. Conclusions: In this study, analysis of samples transported through the PTS resulted in clinically unacceptable PaO 2 values. Delay in transport and analysis of ABG samples should be avoided and samples transported manually if they cannot be assessed on-site.