Associate Professor, Pulmonary Medicine Postgraduate Institute of Medical Education and Research, Chandigarh E-mail:gro.tsehcaidni@hsotuhsa
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Associate Professor, Pulmonary Medicine Postgraduate Institute of Medical Education and Research, Chandigarh E-mail:gro.tsehcaidni@hsotuhsa
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Demonstration of a reduced FEV1/VC ratio on spirometry remains the universally accepted criterion for diagnosis of airway obstruction in routine clinical practice. Despite this, there is no consensus as to how this “reduction” should be defined. It has been an age old practice to use a fixed ratio as the cut-off for this purpose. Most commonly, a FEV1/VC ratio less than 0.70 or 0.75 is used to interpret an obstructive defect on spirometry. Even though there is no statistical or epidemiological basis for choosing 0.70 [or for that matter any other similar ratio] as a cut-off for this purpose, this practice remains engrained in usage worldwide.
The problems and errors of using fixed percentages of reference values were pointed out more than four decades ago. Subsequently it was suggested that statistically derived lower limits of normal should be preferred over fixed percentages while interpreting spirometry data. We have earlier shown that use of fixed percentage based cut-offs introduce unacceptable misclassification rates in interpretation of spirometry results. In an effort to standardize interpretation of lung function tests, American Thoracic Society [ATS] proposed its guidelines in 1991, and recommended that airway obstruction should be defined by a FEV1/VC [or FEV1/FVC] ratio below a certain lower limit of normal [LLN]. This LLN could either be the value below the fifth percentile or the lower 95 percent confidence limit of the values from a reference population. The more recent ATS/European Respiratory Society [ERS] guidelines published in 2005 have largely reiterated this stand. However, several old and recent international initiatives, mostly focusing on chronic obstructive pulmonary disease [COPD], still recommend use of fixed percentages for this purpose. The ATS had earlier defined airway obstruction as a FEV1/FVC ratio less than 0.75. British Thoracic Society recommends documentation of both reduced FEV1 [