In patients with fibrotic idiopathic interstitial pneumonia (f-IIP), the diffusing capacity for carbon monoxide (DLCO) has been used to predict abnormal gas exchange in the lung. However, abnormal values for arterial blood gases during exercise are likely to be the most sensitive manifestations of lung disease. The aim of this study was to compare DLCO, resting PaO2, P(A-a)O2 at cardiopulmonary exercise testing peak, and oxygen desaturation during a 6-min walk test (6MWT). Results were obtained in 121 patients with idiopathic pulmonary fibrosis (IPF, ) and fibrotic nonspecific interstitial pneumonias (NSIP, ). All but 3 patients (97.5%) had low DLCO values (35？mmHg) and 100 (83%) demonstrated significant oxygen desaturation during 6MWT (>4%). Interestingly 27 patients had low DLCO and normal P(A-a)O2, peak and/or no desaturation during the 6MWT. The 3 patients with normal DLCO also had normal PaO2, normal P(A-a)O2, peak, and normal oxygen saturation during 6MWT. Our results demonstrate that in fibrotic IIP, DLCO better defines impairment of pulmonary gas exchange than resting PaO2, exercise P(A-a)O2, peak, or 6MWT SpO2. 1. Introduction According to the ATS/ERS statement, fibrotic interstitial idiopathic pneumonia (f-IIP) includes idiopathic pulmonary fibrosis (IPF) and fibrotic nonspecific interstitial pneumonia (f-NSIP) [1–4]. Although pathological abnormalities are quite different between these two diseases [5], alteration of gas exchange is a major abnormality which is thought to reflect the severity of fibrotic process [6]. Given the simplicity of pulmonary function testing, many investigators have examined the potential for simple resting physiologic measurements to stratify disease severity. The classic physiologic findings in the fibrotic IIP include a reduction in lung volumes (vital capacity; total lung capacity), a reduction in carbon monoxide diffusing capacity (DLCO), and hypoxemia that worsens with exercise [2]. Evaluation of gas exchange impairment can be performed in clinical practice by simple tests like DLCO, resting PaO2, and P(A-a)O2, measurement of SpO2 during a 6-min walk test (6MWT) or PaO2 and alveolar-arterial oxygen pressure difference P(A-a)O2 during cardiopulmonary exercise testing (CPET). Whereas DLCO is a valuable tool in the assessment of the efficiency of pulmonary gas exchange, the P(A-a)O2, especially during exercise, is thought to better reflect the normality of
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