The non‐specific pattern of lung function: A systematic review of physiological mechanisms, clinical profiles and outcomes

BACKGROUND: The Non-Specific Pattern (NSP) is characterized by a reduced Forced Expiratory Volume in 1 s (FEV₁) and/or Forced Vital Capacity (FVC), a preserved FEV₁/FVC ratio, and a normal Total Lung Capacity (TLC). Although recognized in recent American Thoracic Society/European Respiratory Society (ATS/ERS) recommendations, this pattern remains poorly understood. OBJECTIVE: To systematically review the literature addressing the NSP in adult patients, with a focus on its physiological definition, clinical correlates, underlying mechanisms, and longitudinal outcomes. METHODS: A systematic search was conducted in PubMed and Embase from inception to 16 June 2025, following PRISMA guidelines. Eligible studies included adults presenting a physiologically defined NSP. Seven retrospective studies were included and appraised using the Joanna Briggs Institute checklist. RESULTS: Across included studies, NSP was consistently defined using core physiological criteria, though specific thresholds and diagnostic strategies varied. Etiologies included asthma, obesity, obstructive sleep apnoea, interstitial lung disease, and neuromuscular disorders. Several functional markers, such as increased Residual Volume (RV)/TLC ratios, Slow Vital Capacity (SVC)-FVC differences >100 mL, and reduced FEV₁/SVC ratios, suggested early small airway involvement or masked restriction. Two longitudinal studies revealed that NSP may evolve toward obstruction or restriction, with baseline bronchodilator responsiveness and elevated airway resistance as predictors of obstruction. Despite the clinical implications, therapeutic management remains unexplored. CONCLUSION: NSP is not a benign or incidental respiratory functional profile. It may represent an early or intermediate functional state between classic obstructive and restrictive defects. Complementary functional markers and structured follow-up may aid in clinical interpretation and risk stratification.