How Should the Lower Limit of the Normal Range Be Defined?
Lung function parameters vary considerably with age and body size, so that, unlike many laboratory tests, the normal range of expected values must be individualized. For spirometry, only low values are considered to be abnormal, so the lower limit of normal (LLN) is taken to be equal
to the 5th percentile of a healthy, non-smoking population. Simple and commonly used “rules of thumb,” such as an FEV1/FVC < 0.70 to indicate air-flow obstruction, or assuming values < 80% of predicted to be abnormal, are inaccurate and will cause misclassification,
specifically under-diagnosis of abnormalities in younger, taller individuals and over-diagnosis in those older or shorter. A much more accurate LLN for the FEV1/FVC ratio, which recognizes the change with age of this measurement, can be easily determined by subtracting 10 (10% or
0.10) from the age specific FEV1/FVC predicted for any individual. The analysis and mathematical descriptions of reference data have become increasingly sophisticated in recent years, but the interpretation of values near the LLN continues to carry uncertainty, due to an overlap
in values between low normal values and those reflecting early disease. Among patients referred to a pulmonary function laboratory, the pre-test probability of disease may be relatively high, so that even individuals with values above the LLN may be more likely than not to have respiratory
disease. A future goal for the pulmonary community would be the development of risk stratified outcome data that would allow an estimation of the probability of disease with progressive decrements in lung function. While interpreting spirometry results near the LLN will continue to be problematic,
a more important task for the pulmonary community is to focus on finding the pool of individuals with clear-cut, but undiagnosed, COPD. And for this, good quality spirometry remains the best tool and must be widely available.
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