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Correspondence |

Lung Function Data InterpretationLung Function Data Interpretation FREE TO VIEW

Martin R. Miller, MD
Author and Funding Information

From the Institute of Occupational and Environmental Medicine, University of Birmingham.

Correspondence to: Martin R. Miller, MD, Institute of Occupational and Environmental Medicine, University of Birmingham, Edgbaston, Birmingham, West Midlands B15 2TT England; e-mail: m.r.miller@bham.ac.uk


Financial/nonfinancial disclosures: The author has reported to CHEST the following conflicts of interest: Dr Miller has presented lectures for BioMedSys on spirometry in research.

Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (http://www.chestpubs.org/site/misc/reprints.xhtml).


© 2012 American College of Chest Physicians


Chest. 2012;141(3):832-833. doi:10.1378/chest.11-2718
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To the Editor:

The article by Mannino and Diaz-Gutzman1 in CHEST (January 2012) reiterates an argument previously published by Mannino et al2 suggesting that the use of a fixed ratio of 0.7 for FEV1/FVC is justified in defining the presence of airflow obstruction because it identifies people who are at increased risk of dying. However, this misconstrues the true relationship between lung function and risk of death, which is a continuum.3 For FEV1 data, people with lung function just above their predicted value have an increased mortality when compared with those with the very best function. For FEV1/FVC, the risk is increased as soon as the level of function goes below predicted. If one was justified in using lung function to support the definition of a disease state on the basis of associated increased mortality, then at least one-half of the population would be diagnosed with the disease, which is clearly not meaningful. Furthermore, the approach by Mannino and Diaz-Gutzman1 of looking at those considered “abnormal” by the fixed ratio, but above the lower limit of normal (LLN), identifies an older age group (Table 3 of the article, mean age 67.9 years, SD 0.9), and this population will consequently have a higher mortality because age was not included in their model. The findings presented in this article could be demonstrated in endless other data sets, but this does not mean the argument by Mannino and Diaz-Gutzman1 is correct because it conveniently ignores the true relationship between lung function, age, and mortality.

The difference of opinion about how to define airflow obstruction from lung function data splits neatly into the groups of experts who have set down guidelines on how to record and use lung function data, who all favor lower limits of normal, and the groups of experts who have set down guidelines on the management of COPD who advocate the fixed ratio of 0.7.4 I have yet to see any recommendations about pulmonary disease management coming from the experts on lung function data, and yet the experts on COPD management are happy to adopt their own rules on the use of lung function data, which differ from the accepted methodology applied for all other routine laboratory tests.5 Mannino and Diaz-Gutzman1 have suggested that the use of LLN in my earlier article6 as the “gold standard” was unfounded, and the fixed ratio and percent-of-predicted method used by GOLD (Global Initiative for Chronic Obstructive Lung Disease) is, in fact, better. The LLN is based on statistical and scientific methodology that has been accepted worldwide by the medical and scientific community and published in journals as the best way for ensuring fairness and equity when analyzing data and drawing conclusions. The fixed ratio and % predicted method is not based on any scientific methodology and discriminates on the basis of both sex and age because of the biases they introduce. In the area of employment, it is illegal in many countries to discriminate on the basis of age and sex, and I see no reason why such discrimination should be seen as acceptable when it comes to making diagnoses on individual subjects.

It has been argued by some that the difference of opinion about the use of the fixed ratio is not important, but for individual patients it is extremely important. When a clinician tells a patient that unfortunately he or she has a particular disease, it is beholden on the clinician to be as certain as possible that this is correct. Using the fixed ratio means subjects whose lung function is within population norms for their age, sex, and height may be deemed to have airflow obstruction and be diagnosed with COPD. The psychologic impact on a patient from receiving this diagnosis can be profound in that it is a progressive disease with high mortality and, as yet, there is very little treatment that is effective. Research into this disease should concentrate on patients who definitely have the disease and not be clouded by adding subjects whose lung function is within the accepted normal limits for their age and sex. Adding these subjects to studies will add noise to any possible signal and so may prevent studies and drug trials from finding a clear result. If this noise was avoided, such studies would then not need to be so large and expensive (a point that may be of interest to the pharmaceutical industry) and patients would stand a greater chance of benefiting from therapeutic discoveries that are so desperately needed in this condition.

Mannino DM, Diaz-Gutzman E. Interpreting lung function data using 80% predicted and fixed thresholds identifies patients at increased risk of mortality. Chest. 2012;1411:73-80. [PubMed] [CrossRef]
 
Mannino DM, Sonia Buist A, Vollmer WM. Chronic obstructive pulmonary disease in the older adult: what defines abnormal lung function? Thorax. 2007;623:237-241. [PubMed]
 
Chinn S, Gislason T, Aspelund T, Gudnason V. Optimum expression of adult lung function based on all-cause mortality: results from the Reykjavik study. Respir Med. 2007;1013:601-609. [PubMed]
 
Swanney MP, Ruppel G, Enright PL, et al. Using the lower limit of normal for the FEV1/FVC ratio reduces the misclassification of airway obstruction. Thorax. 2008;6312:1046-1051. [PubMed]
 
NCCLSNCCLS How to Define and Determine Reference Intervals in the Clinical Laboratory; Approved Guideline. 2000;202nd ed13 Wayne, PA National Committee for Clinical Laboratory Standards:C28-A2
 
Miller MR, Quanjer PH, Swanney MP, Ruppel G, Enright PL. Interpreting lung function data using 80% predicted and fixed thresholds misclassifies more than 20% of patients. Chest. 2011;1391:52-59. [PubMed]
 

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Tables

References

Mannino DM, Diaz-Gutzman E. Interpreting lung function data using 80% predicted and fixed thresholds identifies patients at increased risk of mortality. Chest. 2012;1411:73-80. [PubMed] [CrossRef]
 
Mannino DM, Sonia Buist A, Vollmer WM. Chronic obstructive pulmonary disease in the older adult: what defines abnormal lung function? Thorax. 2007;623:237-241. [PubMed]
 
Chinn S, Gislason T, Aspelund T, Gudnason V. Optimum expression of adult lung function based on all-cause mortality: results from the Reykjavik study. Respir Med. 2007;1013:601-609. [PubMed]
 
Swanney MP, Ruppel G, Enright PL, et al. Using the lower limit of normal for the FEV1/FVC ratio reduces the misclassification of airway obstruction. Thorax. 2008;6312:1046-1051. [PubMed]
 
NCCLSNCCLS How to Define and Determine Reference Intervals in the Clinical Laboratory; Approved Guideline. 2000;202nd ed13 Wayne, PA National Committee for Clinical Laboratory Standards:C28-A2
 
Miller MR, Quanjer PH, Swanney MP, Ruppel G, Enright PL. Interpreting lung function data using 80% predicted and fixed thresholds misclassifies more than 20% of patients. Chest. 2011;1391:52-59. [PubMed]
 
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