How accurate is spirometry at predicting restrictive pulmonary impairment?

Objective: To determine the accuracy with which spirometric measurements of FVC and expiratory flow rates can diagnose the presence of a restrictive i mpairment. Design: The pulmonary function tests of 1,831 consecutive white adult patie nts who had undergone both spirometry and lung volume measurements on the s ame visit over a 2-year period were analyzed. The probability of restrictiv e pulmonary impairment, defined as a reduced total lung capacity (TLC) belo w the lower limit of the 95% confidence interval, was determined for each o f several categoric classifications of the spirometric data, and additional ly for each of several interval levels of the FVC and the FEV1/FVC ratio. Setting: A large clinical laboratory in a university teaching hospital usin g quality-assured and standardized spirometry and lung volume measurement t echniques according to American Thoracic Society standards. Results: Two hundred twenty-five of 1,831 patients (12.3%) had a restrictiv e defect. The positive predictive value of spirometry for predicting restri ction was relatively low; of 470 patients with a low FVC on spirometry, onl y 41% had restriction confirmed on lung volume measurements. When the analy sis was confined to the 264 patients with a restrictive pattern on spiromet ry (ie, low FVC and normal or above normal FEV1 (FVC ratio), the positive p redictive value was 58%. Conversely, spirometry had a very favorable negati ve predictive value; only 2.4% of patients (32 of 1,361) with a normal vita l capacity (VC) on spirometry had a restrictive defect by TLC measurement. The probability of it restrictive defect was directly and linearly related to the degree of reduction of FVC when the FVC was < 80% of predicted (p = 0.002). Combining the FVC and the FEV1/FVC ratio improved the predictive ab ility of spirometry; for all values of FVC < 80% of the predicted amount, t he likelihood of restrictive disease increased as the FEV1/FVC ratio increa sed. Conclusions: Spirometry is very useful at excluding a restrictive defect. W hen the VC is within the normal range, the probability of a restrictive def ect is < 3%, and unless restrictive lung disease is suspected a priori, mea surement of lung volumes can be avoided. However, spirometry is not able to accurately predict lung restriction; < 60% of patients with a classical sp irometric restrictive pattern had pulmonary restriction confirmed on lung v olume measurements. For these patients, measurement of the TLC is needed to confirm a true restrictive defect.