Adjusting FVC for the effect of obstruction

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 FEV(1)/FVC of 0.70 or less, regression of FITC was expressed as percentage of predicted value (FVC %PRED) on potential explanatory variables, including FEV(1)/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 pars 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 FEV(1)/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.