The Role of FVC and FEV₆ in the Prediction of a Reduced TLC

¹ Dept of General Practice, University of Brussels (VUB), Laarbeeklaan 103, B-1090 Brussels, Belgium
² Respiratory Division, Academic Hospital, University of Brussels (AZ-VUB), Laarbeeklaan 101, B-1090 Brussels, Belgium

^* To whom correspondence should be addressed. E-mail: Jan.Vandevoorde{at}vub.ac.be.

	Abstract

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 <LLN) and non-obstructive subgroups, and separately for men and women.

The two-by-two tables showed generally low PPV and high NPV for FVC or FEV₆ <LLN in predicting a reduced TLC. Logistic regression analysis showed that in non-obstructive subjects, restriction can be positively predicted if FVC or FEV₆ is <55%pred (men) or <40%pred (women). Restriction can be ruled out if FVC or FEV₆ is >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).