Agreement and correlation of pH, bicarbonate, base excess and lactate measurements in venous and arterial blood of premature and term infants

BACKGROUND AND OBJECTIVES: Determine the extent of agreement and correlation between arterial samples and venous (obtained from a venous umbilical catheter), with respect to measurements of pH, bicarbonate, base excess and lactate, in critically ill term and premature newborns. METHODS: Arterial blood samples (0.5-1 mL) were obtained for gas analysis by radial artery puncture, and, within the limit of 5 minutes, samples were obtained from venous umbilical catheters. Bland-Altman plots were used to depict agreement between arterial and venous measurements. Limits of agreement were defined as the mean difference ± 2SD (Standard Deviation). Correlation was assessed by Pearson's method. RESULTS: A hundred and six samples (53 pairs) were taken from 53 patients for analysis of bicarbonate, pH and base excess. Lactate was analyzed in 49 pairs of samples. Differences were within the limits of agreement in 94.3% of pairs of samples for pH, and the same percentage was observed for bicarbonate. There was agreement in 96.2% of pairs for base excess, and in 91.8% for lactate. Mean differences were 0.03 units for pH, -1.2 mmol/L for bicarbonate, -0.24 mmol/L for base excess and 0.33 mmol/L for lactate. Pearson's correlation coefficients (r) were 0.87 for pH, 0.76 for bicarbonate, 0.86 for base excess and 0.95 for lactate. CONCLUSIONS: Although single venous values cannot be used as equivalent to arterial for assessing acid base status in newborns, venous blood samples can be used serially for monitoring trends over time.