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.
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.