Agreement between arterial and capillary pH, pCO2 and lactate in patients in the emergency department

Study objective The primary objective of the study was a quantitative analysis to assess the mean difference and 95% confidence interval of the difference between capillary and arterial blood gas analyses for pH, pCO2 and lactate. Secondary objective was to measure the sensitivity and specificity of capillary samples to detect altered pH, hypercarbia and lactic acidosis. Methods Adults admitted to the ED for whom the treating physician deemed necessary an arterial blood gas analysis (BGA) were screened for inclusion. Simultaneous arterial and capillary samples were drawn for BGA. Agreement between the two methods for pH, pCO2 and lactate were studied with Bland-Altman bias plot analysis. Sensitivity, specificity, positive and negative predictive value as well as AUC were calculated for the ability of capillary samples to detect pH values outside normal ranges, hypercarbia and hyperlactatemia. Results 197 paired analyses were included in the study. Mean difference for pH, between arterial and capillary BGA was 0.0095, 95% limits of agreement were -0.048 to 0.067. For pCO2, mean difference was -0.3 mmHg, 95% limits of agreement were -8.5 to 7.9 mmHg. Lactate mean difference was -0.93 mmol/L, 95% limits of agreement were -2.7 to 0.8 mmol/L. At a threshold of 7.34 for capillary pH had 98% sensitivity and 97% specificity to detect acidemia; at 45.9 mmHg capillary pCO2 had 89% sensitivity and 96% specificity to detect hypercarbia. Finally at a threshold of 3.5 mmol/L capillary lactate had 66% sensitivity to detect lactic acidosis. Conclusion Capillary measures of pH, pCO2 and lactate cannot replace arterial measurements although there is high concordance between the two methods for pH and pCO2 and moderate concordance for lactate. Capillary blood gas analysis had good accuracy when used as a screening tool to detect altered pH and hypercarbia but insufficient sensitivity and specificity when screening for lactic acidosis.