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Adjusting FVC for the effect of obstruction
J J Lefante1, H W Glindmeyer, H Weill
1Department of Medicine, Tulane University School of Medicine, New Orleans, LA 70112-2699, USA.
Objective:
To derive a method of taking into account the effects of obstruction on the FVC, allowing more accurate assessment of other negative ("restrictive") influences.
Design:
In 656 subjects with airways obstruction, defined as FEV1/FVC of 0.70 or less, regression of FVC was expressed as percentage of predicted value (FVC %PRED) on potential explanatory variables, including FEV1/FVC. The resulting equation was used to adjust FVC %PRED in 530 other obstructed men, to test whether the adjustment resulted in a different relationship of FVC %PRED to body weight.
Setting:
A large occupational respiratory surveillance program utilizing rigorously quality-assured and standardized spirometry.
Subjects:
The study included 5,188 men aged 45 to 65 years who denied fibrogenic dust exposures, including 656 with airways obstruction.
Results:
There was a significant (p < 0.0001) linear relationship between more severe obstruction (lower FEV1/FVC) and lower FVC %PRED. Depending on which of several predictive equations is used, obstruction explains 15 to 17% of variability in FVC %PRED over the entire range of severity of obstruction. Adjusting for obstruction in the separate group of 530 subjects showed a significantly larger effect of body weight on FVC %PRED.
Conclusion:
A simple equation can be used to discount the negative effects of obstruction on FVC. This allows more accurate clinical interpretation and can be useful in the analysis of epidemiologic data.