Four Methods to Determine Expiratory Time Constants from Spirometry and Their Sensitivity to Detect Airway...

The expiratory time constant (TC), reflecting the rate of lung emptying, has emerged as a marker of early small airway disease. Although TC is traditionally used in mechanical ventilation, several calculation methods are also applicable to spirometry. This study aimed to evaluate the feasibility of four spirometry-based approaches for determining TC and to compare their sensitivity to detect airway obstruction.

In this multicenter, cross-sectional study of 17,988 adult flow/volume curves, the TC was directly measured (TC_M: time to exhale 63% of forced vital capacity), calculated as proposed by Ikeda (TC_I) and Brunner (TC_B), or derived using the slicing method (TC_S). Stability was assessed by the 5th (lower limit of normal, LLN) and 95th (upper limit of normal, ULN) percentile ranges. Diagnostic performance to detect obstruction was quantified by the area under the receiver operating characteristic curve (AUC).

All four TC methods were feasible but not interchangeable across spirometric categories. In the normal group, TC_I (ULN at 0.7s) and TC_B (ULN at 0.76) were most stable and had values below or at the 0.76s obstruction threshold. In the obstruction group, only TC_M was most closely aligned with the obstruction cutoff (TC_M LLN 0.75s). AUC analysis showed the TC_M was most sensitive (AUC = 0.977) for detecting obstruction, outperforming TC_I (AUC = 0.965), TC_S (AUC = 0.890), and TC_B (AUC = 0.822) (DeLong's test, p < 0.001 for all comparisons). This finding was most pronounced in younger individuals.

TC_M demonstrated the highest overall accuracy for detecting obstruction.