Accuracy of Transcutaneous CO2 Values Compared With Arterial and Capillary Blood Gases

BACKGROUND: Transcutaneous monitors are utilized to monitor a patient's respiratory status. Some patients have similar values when comparing transcutaneous carbon dioxide (P-tcCO2) values with blood gas analysis, whereas others show extreme variability. A retrospective review of data was performed to determine how accurately P-tcCO2 correlated with CO2 values obtained by arterial blood gas (ABG) or capillary blood gas. METHODS: To determine whether P-tcCO2 values correlated with ABG or capillary blood gas values, subjects' records were retrospectively reviewed. Data collected included the P-tcCO2 value at the time of blood gas procurement and the ABG or capillary blood gas P-CO2 value. Agreement of pairs of methods (ABG vs P-tcCO2 and capillary blood gas vs P-tcCO2) was assessed with the Bland-Altman approach with limits of agreement estimated with a mixed model to account for serial measurements per subject. RESULTS: A total of 912 pairs of ABG/P-tcCO2 values on 54 subjects and 307 pairs of capillary blood gas/P-tcCO2 values on 34 subjects were analyzed. The P e% range for ABG was 24-106 mm Hg, and P-tcCO2 values were 27-133 min Hg. The P-CO2 range for capillary blood gas was 29-108 mm Hg, and P-tcCO2 values were 30-103 mm Hg. For ABG/P-tcCO2 comparisons, the Pearson correlation coefficient was 0.82, 95% CI was 0.80-0.84, and P was <.001. For capillary blood gas/P-tcCO2 comparisons, the Pearson correlation coefficient was 0.77, 95% CI was 0.72-0.81, and P was <.001. For ABC/P-tcCO2, the estimated difference +/- SD was -6.79 +/- 7.62 mm Hg, and limits of agreement were -22.03 to 8.45. For capillary blood gas/P-tcCO2 the estimated difference +/- SD was -1.61 +/- 7.64 mm Hg, and limits of agreement were -16.88 to 13.66. The repeatability , coefficient was about 30 mm Hg. CONCLUSIONS: Based on these data, capillary blood gas comparisons showed less variation and a slightly lower correlation with P-tcCO2 than did ABC, comparisons. After accounting for serial measurements per patient, due to the wide limits of agreement and poor repeatability, the utility of relying on P-tcCO2 readings for this purpose is questionable.