Quantifying severity of airway obstruction in COPD
The European Respiratory Society, British Thoracic Society and American Thoracic Society (see international recommendations) have recommended to use the FEV1/(F)VC ratio to decide whether there is or is not airway obstruction, and to quantify its severity on the basis of the FEV1. In spite of ethnic differences in FEV1 and (F)VC, the FEV1/(F)VC ratio differs little, if at all, between ethnic groups, from childhood to old age. This was recently corroborated in the large multi-ethnic study carried out by the Global Lung Function Initiative. Therefore, FEV1/FVC offers a robust solution to diagnosing airway obstruction independent of ethnic or sex-related differences in ventilatory function. The world is divided as to what constitutes airway obstruction: according to the GOLD group an FEV1/FVC ratio < 0.70, but according to common sense, scientific rigor and clinical evidence the appropriate lower limit of normal is the 5th percentile.
The level of FEV1 is used to grade severity of airways obstruction; this is fraught with difficulties:
- The recommendations relate to COPD and do not take account of coexisting restrictive ventilatory defects.
- Restrictive lung disease associated with an FEV1/(F)VC ratio below a borderline leads to overestimating the severity of airway obstruction.
- Each assessment is very sensitive to how well the reference values used fit the population.
- General practitioners tend to underestimate FEV1 by up to 280 mL, depending on equipment used and previous training (see FEV1 underestimated). In general this is to be expected if spirometric measurements are not performed by professionally trained personnel and do not conform to international recommendations. If FEV1/(F)VC is below ‘normal limits’, airway obstruction then tends to be systematically overestimated when based on the level of FEV1.
In keeping with international recommendations the severity of airway obstruction is based on the level of FEV1%predicted. Recommendations on the scaling of severity varied in time and between respiratory societies.
|FEV1%FVC post bronchodilator||FEV1%pred||ATS/ERS 2004||GOLD||NICE 2010|
|< 0.70||>80||mild||mild (1)||mild (1) *|
|< 0.70||50-79||moderate||moderate (2)||moderate (2)|
|< 0.70||30-49||severe||severe (3)||severe (3)|
|< 0.70||< 30||very severe||very severe (4)||very severe (4)|
|* diagnosis of COPD requires the presence of respiratory symptoms|
|< LLN||50-59||moderately severe|
|< LLN||< 35||very severe|
As argued earlier, the use of percent of predicted is misguided because it leads to age-related bias, due to which the severity of airway obstruction will be systematically overestimated in elderly people. Only an FEV1/FVC ratio < LLN and FEV1 < LLN was found to be associated with elevated risk of having respiratory symptoms and elevated risk of death (PubMed); therefore it would be better to abandon using FEV1 % predicted and adopting the LLN for FEV1 in defining mild airway obstruction.
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NOTE: Although the summary of the publication suggests otherwise, the authors misrepresented their findings: the adjusted hazard ratio for premature death was elevated only if the FEV1/FVC ratio was < LLN.
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