The primary way in which spirometry results are assessed is by comparing them to normal or reference values. There is a relationship between the FVC, FEV1 and FEV1/FVC ratio, and the height, gender, age and ethnicity of individuals, but this relationship is inexact.  A long-standing problem for all Pulmonary Function testing facilities is selecting reference equations not only for the population they serve as a whole but also for individual patients.

Much has been written about the choice of reference equations and their implication on interpretation.  The important points are:

• Reference equations should match the population of the individual being tested.
• Reference equations should be derived from equipment similar to that used for testing.
• That it should be possible to accurately estimate a lower limit of normal for the reference equations.
• That the reference values used for FVC, FEV1 and the FEV1/FVC ratio should all come from the same source.

Much has also been written about the differences in reference values between ethnicities, but it is important to keep in mind that:

• There are no clear or consistent definitions of what constitutes an ethnicity.
• Even within what appears to be a single, well-defined ethnicity the range of reference values can be as great as it is between ethnicities.
• That the probable causes of the differences between ethnicities, i.e. genes, diet and environment, are in a state of flux throughout the world.

Many institutions have used the same reference equations for decades.  There is some value in continuity but it should be recognized that these older reference equations came from studies that often had significant limitations in their populations, test equipment and statistical analysis. For this reason the ATS/ERS recommends that reference equations should be reviewed at least once every 10 years and replaced by whenever more relevant reference equations become available.

When reviewing test results there will be subjects whose results as defined by the selected reference equations are clearly normal and subjects whose results are clearly abnormal, but there will also a gray area in between these where a clear determination may not be possible.  For this reason individuals whose results are on the borderline should not be considered abnormal without accompanying symptoms.  To some extent though, most pulmonary function tests are performed on subjects who are symptomatic to one degree or another so there is at least some reason to shade interpretations accordingly.

Even though reference equations and the framework that has been established for assessing test results have limitations, experience has shown that they nevertheless work reasonably well and even when their limitations are apparent they are still a necessary starting point.

Recommendation:

At the present time the NHANESIII and GLI are the most widely used spirometry reference equations worldwide. This is partly due to their large and ethnically diverse populations and partly to their sophisticated statistical analysis.  Nevertheless, a number of studies have criticized the inability of these equations to meet the needs of specific populations and this factor has to be a consideration.

Therefore, when there are spirometry reference equations that clearly match the population served by a pulmonary test facility, then these should be selected.  Otherwise, NHANESIII or GLI are equally likely to be adequate choices.

PFT Interpretation by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License15