Early smoking injury surfaced on more sensitive lung tests before spirometry moved
Conventional spirometry missed most of the earliest smoking-related lung injury in this cohort, but more sensitive physiology did not. In 170 healthy adults, investigators compared current smokers, former smokers, and never smokers using self-report, exhaled carbon monoxide, and a broader biomarker-of-exposure panel to verify smoking status rather than relying on questionnaire data alone. Baseline and repeat testing 7 ± 3 days later included spirometry, forced oscillation technique (FOT), and single-breath diffusing capacity for carbon monoxide (DLCO), a combination designed to detect subtle airway and gas-transfer abnormalities before overt obstruction appears.
The central finding was simple and clinically uncomfortable: standard spirometric indices and traditional FOT resistance and reactance measures separated poorly across groups, while within-breath expiratory–inspiratory differences did not. In current smokers, Δexp–insp R5 was higher than in never smokers, with median values of 0.57 versus 0.23, and higher than in former smokers, with median values of 0.57 versus 0.33. Those differences point to early ventilatory heterogeneity and small-airway dysfunction that can be invisible when a patient still blows an apparently normal forced expiratory volume in one second.
Within-breath oscillometry exposed airway asymmetry that spirometry missed
The physiology matters. Smoking injures the small airways and peripheral lung long before forced expiratory flows fall below conventional thresholds, and within-breath oscillometry is better positioned to catch that distortion because it probes airway mechanics during both expiration and inspiration. A larger expiratory–inspiratory gap in resistance suggests dynamic airway narrowing, flow-dependent airway closure, or altered airway wall properties that are not captured by a single forced maneuver. In practice, that means a smoker can appear “normal” on routine spirometry while already showing mechanical fingerprints of early lung injury.
Repeatability strengthens the case for these metrics. Spirometry was highly repeatable, and FOT metrics were described as good on repeat testing over the short interval. A marker that is sensitive but unstable has little value in cessation or switching studies; a marker that is both reproducible and responsive can serve as a much better endpoint when investigators want to measure whether exposure reduction is changing pulmonary physiology over time. This study was built around that question, and the repeatability data support using these measures in longitudinal work.
One sentence captures the bedside implication: normal spirometry does not mean normal smoking-related lung biology.
Gas-transfer abnormalities appeared before airflow obstruction
Diffusion capacity told a parallel story. DLCO% and KCO% were lower in current smokers than in never smokers, and KCO% also differed between current and former smokers. Median KCO% was 94% in current smokers versus 106% in never smokers, with a significant difference, and 94% versus 102% in former smokers. Those reductions point toward impaired alveolar-capillary gas transfer, which may reflect early emphysema-like change, vascular injury, or smoking-related alterations in the alveolar membrane long before imaging or spirometry would necessarily force the issue.
Because KCO adjusts DLCO for alveolar volume, it can help separate pure lung size effects from inefficient transfer across the gas-exchange interface. In a cohort of otherwise healthy adults, lower KCO% in current smokers implies that smoking exerts measurable physiologic cost even when routine clinical screening would often classify the lung as preserved. Former smokers landing between current and never smokers fits a biologically plausible gradient: removal of exposure can reduce ongoing injury, but prior damage may not fully normalize.
Biochemical verification sharpened the exposure signal
The use of biochemical verification is not a minor technical detail. Self-report alone can blur the distinction between current, former, and never smoking, especially in research settings where participants may understate use or misclassify recent cessation. By combining self-report, exhaled CO, and a comprehensive biomarker-of-exposure panel, the analysis anchored the exposure groups more firmly than studies that lean on interview data alone. When the question is whether a subtle lung-function signal truly tracks smoke exposure, the exposure definition has to be credible.
That design choice likely increased confidence in the observed gradient across smoking categories. The cohort was limited to healthy adults, so the findings describe early functional change rather than established chronic obstructive pulmonary disease. Even so, the signal on within-breath oscillometry and KCO% argues that injury begins earlier than most routine testing detects. For clinicians following smokers who still look “normal,” these measures may offer a way to detect physiologic change before a patient crosses the threshold into diagnosable obstruction.
Why these endpoints matter for cessation and switching studies
The study points toward a practical research direction: if the goal is to measure whether quitting or switching away from combustible cigarettes reduces harm, the outcome variable has to move when biology changes. Conventional spirometry may be too blunt for that job in early disease, while within-breath oscillometry and KCO% appear more responsive to smoking-related injury. That makes them attractive as repeatable endpoints in cessation and switching studies, especially when investigators want to capture small improvements or early deterioration over relatively short intervals.
The implications for day-to-day respiratory practice are more restrained but still important. A normal spirometry result should not be interpreted as proof of pulmonary safety in a current smoker. The more sensitive signals in this study suggest that airway mechanics and gas exchange can already be abnormal at a stage when the patient feels well and office testing remains unrevealing.
Funding came from an investigator-initiated project sponsored by Altria Client Services LLC, although the sponsor had no role in study design, implementation, analysis, or interpretation. The article’s own disclosures describe a broad set of competing interests among senior investigators, which places a premium on independent replication. The most useful next step is straightforward: larger longitudinal studies should test whether these same oscillometry and diffusion markers predict who progresses, who stabilizes after cessation, and which exposure-reduction strategies produce measurable physiologic benefit.