The OXISITE-ILD study is a prospective, multicenter, observational study designed to evaluate the agreement between finger (digital) and earlobe (auricular) pulse oximetry for measuring oxygen saturation (SpO2) during the six-minute walk test (6MWT) in patients with interstitial lung disease (ILD). In routine clinical practice, exercise SpO2 is usually measured at the finger; however, the finger reading can be unreliable in some patients, and there is currently no recommendation on the best sensor location in ILD. This study compares the two sensor locations, recorded at the same time, and evaluates whether any disagreement changes how exercise desaturation is classified and whether ambulatory oxygen is indicated.
All patients undergoing a 6MWT as part of routine ILD care are included consecutively to ensure a pragmatic, real-world representation of the ILD population. The primary objective is to measure the agreement between the two locations in the lowest SpO2 reached during the test, including the size and direction of any difference. Secondary objectives include the reclassification of patients at the clinical desaturation thresholds, the comparison between autoimmune and non-autoimmune ILD, the rate of invalid readings at each location, and the clinical, vascular and functional factors associated with disagreement.
Adults aged 18 years or older
Confirmed diagnosis of interstitial lung disease (ILD) based on clinical, radiological and/or histological criteria
Newly diagnosed or under follow-up for ILD
Physically and cognitively able to perform the 6-minute walk test (6MWT)
Inability to obtain a reliable, stable baseline pulse oximetry reading at the finger and/or the earlobe, preventing a valid paired baseline measurement and the calculation of agreement
ILD exacerbation, decompensated comorbidity, or acute respiratory infection at the time of the test
Consecutive adults with a confirmed diagnosis of interstitial lung disease (ILD), either newly diagnosed or under follow-up, attending the ILD/pulmonary fibrosis units of the participating centres and undergoing a clinically indicated 6-minute walk test. Patients with or without prior oxygen therapy are enrolled consecutively, and baseline oxygen status (none, exertional only, or at rest) is recorded. Each patient is assessed on a single occasion, consistent with the cross-sectional design.
Cohort
Cross-sectional
Dual-site pulse oximetry (finger and earlobe)
Other interventionDuring a single 6MWT, SpO2 and heart rate are recorded simultaneously at the finger (digital) and the earlobe (auricular) with two pulse oximeters, at eight timepoints (baseline, minutes 1 to 6, and a 2-minute recovery). No treatment is administered and routine clinical management is not modified; the procedure adds the simultaneous earlobe measurement for comparison with the finger.
Intraclass correlation coefficient for agreement in minimum SpO2 between sensor locations
Absolute agreement in minimum SpO2 during the six-minute walk test between the finger and earlobe sensors, quantified with the intraclass correlation coefficient (two-way mixed-effects model, absolute agreement) and its 95% confidence interval. ICC values are interpreted as below 0.50 poor, 0.50 to 0.75 moderate, 0.75 to 0.90 good, and above 0.90 excellent agreement.
Bland-Altman agreement in minimum SpO2 between sensor locations
Agreement in minimum SpO2 during the six-minute walk test between the finger and earlobe sensors, assessed with Bland-Altman analysis: mean bias (earlobe minus finger) with its 95% confidence interval and 95% limits of agreement, evaluated against a prespecified clinically acceptable difference of 3 percentage points. The direction of the bias is reported, as the finger is hypothesised to register lower values than the earlobe.
Reclassification of exertional desaturation between sensor locations at SpO2 below 88%
Proportion of participants whose classification of significant exertional desaturation changes between the finger and earlobe sensors at the threshold of minimum SpO2 below 88% (American Thoracic Society indication threshold for ambulatory oxygen). Agreement is assessed with the Cohen kappa coefficient (Landis-Koch interpretation) and the difference in proportions with the McNemar test for paired data. At this threshold the McNemar test and the raw reclassification proportions are the primary read, as kappa can be unstable when prevalence is extreme.
Reclassification of exertional desaturation between sensor locations at SpO2 below 90%
Proportion of participants whose classification of significant exertional desaturation changes between the finger and earlobe sensors at the threshold of minimum SpO2 below 90% (British Thoracic Society indication threshold for ambulatory oxygen). Agreement is assessed with the Cohen kappa coefficient (Landis-Koch interpretation) and the difference in proportions with the McNemar test for paired data.
Reclassification of exertional desaturation between sensor locations at a fall in SpO2 of 4% or more
Proportion of participants whose classification changes between the finger and earlobe sensors at the prognostic threshold of a fall in minimum SpO2 of 4% or more from baseline. Agreement is assessed with the Cohen kappa coefficient (Landis-Koch interpretation) and the difference in proportions with the McNemar test for paired data.
Agreement in minimum SpO2 by autoimmune versus non-autoimmune ILD
Comparison of the agreement between sensor locations (intraclass correlation coefficient and Bland-Altman) in autoimmune-associated ILD versus non-autoimmune ILD, with interstitial pneumonia with autoimmune features (IPAF) described separately. Where sample size permits, agreement is also described, exploratorily and without formal hypothesis testing, by major ILD subtype (for example idiopathic pulmonary fibrosis, hypersensitivity pneumonitis, or connective tissue disease-associated ILD), and by autoimmune subtype (for example systemic sclerosis or rheumatoid arthritis).
Agreement in SpO2 at additional timepoints
Agreement between sensor locations in SpO2 at baseline, maximum, per-minute intervals during the walk, and recovery, assessed with the intraclass correlation coefficient and Bland-Altman analysis.
Quantitative paired difference and Passing-Bablok regression between sensor locations
Exploratory analysis of systematic differences between the finger and earlobe in SpO2. The paired difference (earlobe minus finger) is summarised with its 95% confidence interval (paired t test, or Wilcoxon signed-rank test with a Hodges-Lehmann interval, as appropriate). Passing-Bablok regression is used to detect constant and proportional differences between the two locations, with 95% confidence intervals for the intercept and the slope.
Agreement of each sensor location with baseline arterial oxygen saturation
Exploratory comparison of the agreement of finger and of earlobe SpO2 with baseline arterial oxygen saturation, in the subgroup of participants undergoing arterial blood gas sampling.
Direct and indirect cost associated with invalid finger readings
Exploratory description of the direct and indirect costs associated with invalid or poor-quality finger readings. No prespecified cost model is applied.
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