Rationale: Landmark studies of long-term oxygen therapy (LTOT) in patients with chronic obstructive pulmonary disease (COPD) used arterial oxygen pressure (PaO2) to define severe hypoxemia; however, oxygen saturation as measured by pulse oximetry (SpO2) is commonly used instead. The Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines recommend evaluation with arterial blood gas (ABG) analysis if SpO2 is ⩽92%. This recommendation has not been evaluated in stable outpatients with COPD undergoing testing for LTOT. Objectives: To evaluate the performance of SpO2 compared with ABG analysis of PaO2 and arterial oxygen saturation (SaO2) to detect severe resting hypoxemia in patients with COPD. Methods: Retrospective analysis of paired SpO2 and ABG values from stable outpatients with COPD who underwent LTOT assessment in a single center. We calculated false negatives (FNs) as an SpO2 >88% or >89% in the presence of pulmonary hypertension with a PaO2 ⩽55 mm Hg or ⩽59 mm Hg in the presence of pulmonary hypertension. Test performance was assessed using receiver operating characteristic (ROC) analysis, intraclass correlation coefficient (ICC), test bias, precision, and accuracy root-mean-square (Arms). An adjusted multivariate analysis was used to evaluate factors affecting SpO2 bias. Results: Of 518 patients, the prevalence of severe resting hypoxemia was 74 (14.3%), with 52 missed by SpO2 (FN, 10%), including 13 (2.5%) with an SpO2 > 92% (occult hypoxemia). FNs and occult hypoxemia in Black patients were 9% and 1.5%, respectively, and were 13% and 5%, respectively, among active smokers. The correlation between SpO2 and SaO2 was acceptable (ICC = 0.78; 95% confidence interval, 0.74-0.81); and the bias of SpO2 was 0.45%, with a precision of 2.6 (-4.65 to +5.55%) and Arms of 2.59. These measurements were similar in Black patients, but in active smokers, correlation was lower and bias showed greater overestimation of SpO2. ROC analysis suggests that the optimal SpO2 cutoff to warrant LTOT evaluation by ABG analysis is ⩽94%. Conclusions: SpO2 as the only measure of oxygenation carries a high FN rate in detecting severe resting hypoxemia in patients with COPD undergoing evaluation for LTOT. Reflex measurement of PaO2 by ABG analysis should be used as recommended by GOLD, ideally at a cutoff higher than an SpO2 ⩽92%, especially in active smokers.
Keywords: COPD; hypoxemia; oxygen; pulse oximetry.