Low spirometric forced vital capacity(FVC) in conjunction with a normal or high ratio of the forced expiratory volume at 1 second to the forced vital capacity(FEV₁/FVC%) has traditionally been classified as a restrictive abnormality. However, the gold-standard diagnosis of a restrictive pulmonary impairment requires a measurement of the total lung capacity (TLC). This study was performed to determine the predictive value of spirometric measurements of the FVC for diagnosing a restrictivepulmonary abnormality.

Test results from 1,371 adult patients who undertook both spirometry and lung volume measurements on the same visit from January 1999 to December 2000 were enrolled in this study. The test values for the FVC, the TLC that was below 80% of predicted value, and a FEV₁/FVC% that was below 70%, were classified as being abnormal.

Of the 1,371 patients, 353 patients had a reduced a FVC. Of these patients, 186 patients had a reduced TLC. Therefore, the positive predictive value was 52.7%. Of the 196 patients with a normal FEV₁/FVC% and a reduced FVC, 148(75.5%) patients had a lower TLC. Thirty eight (24.2%) patients out of 157 patients with a low FEV₁/FVC% and a low FVC showed a restrictive defect.

Spirometry is useful to rule out a restrictive pulmonary abnormality, but a restrictive pattern on the spirometry dose not mean there is a true restrictive disease. For the patients with a low FVC, TLC measurements are essential for diagnosing a restrictive pulmonary impairment.