Forced vital capacity and forced expiratory volume in six seconds as predictors of reduced total lung capacity.

-The present study aims to derive guidelines that identify patients for whom spirometry can reliably predict a reduced total lung capacity (TLC).
A total of 12,693 lung function tests were analysed on Caucasian subjects, aged 18-70 yrs.
Restriction was defined as a reduced TLC. Lower limits of normal (LLN) for TLC were obtained from ERS recommended reference equations. For FVC and FEV6 reference equations from NHANES III were used. The performance of FVC and FEV6 to predict the presence of restriction was studied (a) using two-by-two tables, and (b) by logistic regression analysis. Both analyses were performed in obstructive (defined as FEV1/FVC or FEV1/FEV6 The two-by-two tables showed generally low PPV and high NPV for FVC or FEV6 100%pred (men) or >85%pred (women). In obstructive patients, spirometry cannot reliably diagnose a concomitant restrictive defect, but it can rule out restriction for patients with FVC or FEV6 >85%pred (men) or >70%pred (women).