Are arterial and venous samples clinically equivalent for the estimation of pH, serum bicarbonate and potassium concentration in critically ill patients?

Aims To assess the comparability of venous and arterial samples for pH, bicarbonate and potassium measurements in critically ill patients. Methods Simultaneous arterial and venous samples from 206 critically ill patients were analysed in duplicate. Coefficients of variation and 95% limits of agreement were calculated for arterial and venous samples. Bland-Altman plots were constructed to assess agreement between sampling sites. Results The median (range) of arterial pH, bicarbonate concentrations, potassium concentrations and glucose concentrations were 7.40 (7.017.56), 25 (941) mmol/l, 4.2 (3.16.8) mmol/l and 7.4 (3.013.5) mmol/l, respectively. Coefficients of variation for arterial and venous pH were both 0.1%, with bias (95% limits of agreement) of 0.01 (0.03 to 0.01) for arterial and -0.01 (-0.02 to 0.01) for venous samples. The bias (95% limits of agreement) between arterial and venous samples was 0.03 (-0.02 to 0.08). Coefficients of variation for arterial and venous bicarbonate results were 0.8 and 0.7%, respectively, with bias (95% limits of agreement) of 0 (-0.5 to 0.5) mmol/l for both sample types. The bias (95% limits of agreement) between venous and arterial samples was 0 (-1.3 to 1.3) mmol/l. Coefficients of variation for arterial and venous potassium samples were 0.8 and 1.1%, respectively, with bias (95% limits of agreement) of 0 (-0.1 to 0.1) for both sample types. The bias (95% limits of agreement) between venous and arterial samples was 0.1 (-0.4 to 0.6) mmol/l. Conclusions A venous blood sample, analysed on a blood gas machine, is sufficiently reliable to assess pH, bicarbonate and potassium concentrations in critically ill patients, suggesting that venous sampling alone is appropriate in the management of diabetic ketoacidosis.