Low pO₂ Contributes to Potential Error in Oxygen Saturation Calculations Using a Point-of-Care Assay

Objectives: The present study addressed the accuracy of calculated oxygen saturation (sO₂) using point-of-care (POC) testing compared with measured values on a blood gas analyzer. Methods: In total, 3,323 sO₂ values were measured in 1,180 patients using a CO-oximeter (ABL 800 Flex; Radiometer, Copenhagen, Denmark). Measured parameters were then used to calculate an expected sO₂ for the POC method (Abbott i-STAT; Abbott POC, Princeton, NJ). Cases in which calculated sO₂ differed from measured sO₂ by 10% or more were analyzed. Results: Of the 3,323 comparisons performed, 260 (8%) showed discrepancies (± ≥10%) between measured and calculated sO₂ values. Ninety-four of discrepant measurements (245 of 260) occurred when pO₂ was less than 50 mm Hg. pH and bicarbonate distributions shifted to lower values in discrepant vs nondiscrepant cases. Conclusions: Our results suggest that the likelihood of discrepant sO₂ is 27% among patients with pO₂ less than 50 mm Hg. Direct measurement of sO₂ by CO-oximetry is strongly suggested in this clinical scenario.