Comparison of spirometric thresholds in diagnosing smoking-related airflow obstruction
- Surya P Bhatt1,
- Jessica C Sieren2,
- Mark T Dransfield1,
- George R Washko3,
- John D Newell Jr2,
- Douglas A Stinson4,
- Gideon K D Zamba5,
- Eric A Hoffman2,
- for the COPDGene Investigators
- 1Division of Pulmonary, Allergy and Critical Care Medicine, University of Alabama at Birmingham, Birmingham, Alabama, USA
- 2Department of Radiology, University of Iowa, Iowa City, Iowa, USA
- 3Division of Pulmonary and Critical Care Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts, USA
- 4Division of Pulmonary and Critical Care Medicine, National Jewish Health, Denver, Colorado, USA
- 5Department of Biostatistics, University of Iowa College of Public Health, Iowa City, Iowa, USA
- Correspondence to Dr Surya P Bhatt, Division of Pulmonary, Allergy and Critical Care Medicine, University of Alabama at Birmingham, THT 422, 1720, 2nd Avenue South, Birmingham, AL 35294, USA;
- Received 2 October 2012
- Revised 18 February 2013
- Accepted 27 February 2013
- Published Online First 23 March 2013
Background Diagnosis of chronic obstructive pulmonary disease is based on detection of airflow obstruction on spirometry. There is no consensus regarding using a fixed threshold to define airflow obstruction versus using the lower limit of normal (LLN) adjusted for age. We compared the accuracy and discrimination of the Global Initiative for Chronic Obstructive Lung Disease (GOLD) recommended fixed ratio of forced expiratory volume in the first second/forced vital capacity<0.70 with LLN in diagnosing smoking-related airflow obstruction using CT-defined emphysema and gas trapping as the disease gold standard.
Methods Data from a large multicentre study (COPDGene), which included current and former smokers (age range 45–80 years) with and without airflow obstruction, were analysed. Concordance between spirometric thresholds was measured. The accuracy of the thresholds in diagnosing emphysema and gas trapping was assessed using quantitative CT as gold standard.
Results 7743 subjects were included. There was very good agreement between the two spirometric cutoffs (κ=0.85; 95% CI 0.83 to 0.86, p<0.001). 7.3% were discordant. Subjects with airflow obstruction by fixed ratio only had a greater degree of emphysema (4.1% versus 1.2%, p<0.001) and gas trapping (19.8% vs 7.5%, p<0.001) than those positive by LLN only, and also smoking controls without airflow obstruction (4.1% vs 1.9% and 19.8% vs 10.9%, respectively, p<0.001). On follow-up, the fixed ratio only group had more exacerbations than smoking controls.
Conclusions Compared with the fixed ratio, the use of LLN fails to identify a number of patients with significant pulmonary pathology and respiratory morbidity.