BACKGROUND: Transcutaneous measurements of CO₂ and O₂ (P_tcCO2, P_tcO2) are noninvasive and allow for continuous monitoring in adults with exacerbation of COPD, but substantial accuracy issues may exist. We investigated agreement between results of arterial blood gas analysis and trans-cutaneous measurements of CO₂ and O₂ in patients with COPD. METHODS: Adult subjects were monitored after acute admission to a respiratory intermediate care unit or ICU due to exacerbation of COPD and with ongoing noninvasive ventilation or immediately following extubation. Monitored variables were continuous transcutaneous measurement and simultaneous routine arterial blood gas analysis. Agreement between measurements was assessed by calculating bias with 95% limits of agreement for single-point estimates of P_tcCO2 versus P_aCO2 and versus P_aO2, and for changes in transcutaneous measurements between 2 time points (DP_tcCO2 and DP_tcO2). We considered limits of agreement within ± 7.5 mm Hg to be acceptable. RESULTS: A total of 57 transcutaneous measurements were made in 20 subjects for comparison with concurrent arterial blood gas analysis at 36 time points. The bias (limits of agreement) for P_tcCO2 and P_tcO2 was 2.5 mm Hg (–10.6 to 15.6 mm Hg) and 11.2 mm Hg (–28.2 to 50.6 mm Hg), respectively. The bias for DP_tcCO2 and DP_tcO2 was 2.3 mm Hg (–3.8 to 8.3 mm Hg) and –5.3 mm Hg (–37.5 to 27 mm Hg), respectively. CONCLUSIONS: P_tcCO2 and P_tcO2 did not accurately reflect results from arterial blood gas analyses in this study of mostly hypercapnic subjects. Agreement between changes in CO₂ during the monitoring pe-riod was acceptable, however, and transcutaneous monitoring may be used for continuous monitoring of P_CO2 in conjunction with arterial blood gas analysis for reference.