Morning-to-Evening Decline in Pulmonary Diffusing Capacity and Spirometry.

Pulmonary function tests are performed at varying times of day, yet whether time of day affects spirometry and pulmonary diffusing capacity for nitric oxide (DLNO) and carbon monoxide (DLCO) is poorly characterized. The diurnal behavior of DLNO is unknown. We examined whether these measurements vary throughout the day in healthy men and derived empirical within-day limits of change. Thirty-eight healthy men (median age 29.8 years) underwent up to six testing sessions across a single waking day. Each session comprised spirometry (FVC, FEV1, SVC, FEV1/FVC) followed by single-breath DLNO/DLCO, yielding DLCO, DLNO, alveolar volume (VA), transfer coefficients KCO and KNO, and the DLNO/DLCO ratio. Time-of-day effects were assessed using linear mixed-effects models with subject as a random intercept; total within-subject variability was quantified as the within-subject standard deviation. FVC, FEV1, SVC, DLNO, DLCO, and VA decreased across the day (p ≤ 0.005), whereas FEV1/FVC, KCO, KNO, and DLNO/DLCO did not. Total observed within-day variability ranged from 1.5% (SVC) to 4.0% (DLCO). At the one-sided 95% level, empirical within-day change limits were ∼11 mL/min/mmHg for DLNO (z-score change = 0.59), 3.2 mL/min/mmHg for DLCO (z-score = 0.66), 0.28 L for VA (z-score = 0.35), and 0.34 for the DLNO/DLCO ratio. Spirometry volumes and absolute diffusing capacities declined from morning to evening in a pattern consistent with falling VA, while ratio-based indices remained stable. Perform serial measurements at consistent times of day. The empirical limits reflect measurement error plus short-term physiological or procedure-based variation and are specific to this device, protocol, and population.