ORIGINAL ARTICLES

Limits of longitudinal decline for the interpretation of annual changes in FEV₁ in individuals

Eva Hnizdo¹, Kanta Sircar¹, Tieliang Yan², Philip Harber³, James Fleming⁴, Henry W Glindmeyer⁵

¹ Division of Respiratory Disease Studies, National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, Morgantown, WV, USA
² Constella Group, Morgantown, WV, USA
³ Division of Occupational and Environmental Medicine, Department of Family Medicine, University of California Los Angeles, Los Angeles, CA, USA
⁴ Phoenix Fire Department, Phoenix, AZ, USA
⁵ Section of Pulmonary, Critical Care, and Environmental Medicine, Department of Medicine, Tulane Medical School, New Orleans, LA, USA

Dr E Hnizdo, Division of Respiratory Disease Studies, National Institute for Occupational Safety and Health, 1095 Willowdale Road, Morgantown, WV 26505, USA; ehnizdo{at}cdc.gov

Objective: Spirometry-based screening programmes often conduct annual assessment of longitudinal changes in forced expiratory volume in 1 second (FEV₁) to identify individuals with excessive rates of decline. Both the American Thoracic Society (ATS) and the American College of Occupational and Environmental Medicine (ACOEM) recommend a reference limit value of 15% for excessive annual decline. Neither the ATS nor the ACOEM adjust this limit for the precision of the existing spirometry data. The authors propose an improved method of defining the reference limit of longitudinal annual FEV₁ decline (LLD) based on the precision of the spirometry data.

Method: The authors used data from four monitoring programmes and measured their data precision using a pair-wise within-person variation statistic. They then derived programme- and gender-specific absolute and relative LLD values and validated these against the 95th percentiles for observed yearly changes in FEV₁.

Results: The relative limit for annual decline was more practical than the absolute limit as it adjusted for gender differences in the magnitude of FEV₁. The programme-specific relative limit values were in good agreement with 95th percentiles for year-to-year FEV₁ changes and ranged from 6.6% to 15.8%. For individuals with COPD and bronchial hyperreactivity the 95th percentiles for year-to-year changes were about 15% and higher.

Conclusions: The relative longitudinal limit for annual FEV₁ decline based upon precision of measurements is valid and can be generalised to different gender and population groups. A relative limit of approximately 10% appears appropriate for good quality workplace monitoring programmes, whereas a limit of about 15% appears appropriate for clinical evaluation of individuals with an obstructive airway disease. Computer software based on the method described is available from the corresponding author.

Abbreviations: ACOEM, American College of Occupational and Environmental Medicine; ATS, American Thoracic Society; BHR, bronchial hyper-reactivity; COPD, chronic obstructive pulmonary disease; ERS, European Respiratory Society; FEV₁, forced expiratory volume in 1 second; LLD, limit of longitudinal decline; LLN, lower limit of normal; NIOSH, National Institute for Occupational Safety and Health

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