FEV₆ AS A SURROGATE FOR FVC: AUTHORS SHOULD HAVE INCLUDED ROC-CURVE ANALYSES

To the Editor :

We have read with interest Swanney and colleagues' recent article (1) on forced expiratory volume in 6 seconds (FEV₆) as an acceptable surrogate for forced vital capacity (FVC). Simplifying pulmonary diagnosis has positive implications both clinically and in population-based epidemiological studies. However, we are concerned with a possible deficiency in the development of a simplified measure of FVC. The issue of cutpoint selection is important in the evaluation of a diagnostic test, where a continuous variable has been dichotomized. In this case, we are concerned with FEV after a certain time. The traditional approach to the selection of a cutpoint is the evaluation of a receiver operating characteristic curve (2, 3). The selection of a cutpoint of 6 seconds is practical but somewhat arbitrary. It is possible that a cutpoint of 3, 4, 5, or 7 seconds (FEV₃, FEV₄, FEB₅, FEV₇) yields more information. Forced expiratory volume at arbitrary time points should be available with little modification of existing spirometry software. For reference values, the spirometric curves from NHANES III were digitized (4), facilitating the same analysis, and hence could be made available. Additionally, 33% of spirometries were technically inadequate. The authors state that n = 65 (40%) of these had "normal lung function." Presumably, these should have been included in the analysis. Furthermore, a cutpoint of less than 6 seconds may increase the proportion of acceptable spirometries. We hope that the promise of an simplified and adequate measure of FVC will be further strengthened if the present and future investigators incorporate this perspective.

University of Bergen, Bergen, Norway

1. Swanney MP, Jensen RL, Crichton DA, Beckert LE, Cardno LA, Crapo RO. FEV₆ is an acceptable surrogate for FVC in the spirometric diagnosis of airway obstruction and restriction. Am J Respir Crit Care Med 2000; 162: 917-919 [Abstract/Free Full Text].

2. Altman DG. Diagnostic tests. In: Practical statistics for medical research, 1st ed. London: Chapman & Hall; 1991. p. 409-419.

3. Rosner B. ROC curves. In: Fundamentals of biostatistics, 5th ed. Boston: PWS-Kent Publishing Company; 2000. p. 63-65.

4. Hankinson JL, Odencrantz JR, Fedan KB. Spirometric reference values from a sample of the general U.S. population. Am J Respir Crit Care Med 1999; 15: 179-187 .

From the Authors :

We thank Dr. Brøgger and colleagues for their comments on our study (1). Our paper was intended as a starting point for defining an optimal endpoint.

FEV₆ was not chosen at random. It is an acceptable end-of-test criterion in the American Thoracic Society guidelines. The commercial spirometer we used reported only FVC and FEV₆. In addition, we only have reference data for FVC and FEV₆ from the NHANES III study (2). We understand waveform data in the NHANES study were stored, but the data are not currently available to the public.

We are reassured that FEV₆ is a reasonable choice by the fact that it performs so well (3). Previous work, published only in abstract form, suggests the minimum variation in a FVC surrogate occurred at about 6 seconds of exhalation (4). Shorter times are less attractive because FEV₆ can be projected from much shorter times (abstract) and longer times would begin to defeat our goal of a simple, practical endpoint.

We look forward to other studies to assess the accuracy and clinical validity of this and other endpoints in clinical spirometry.

Christchurch Hospital, Christchurch, New Zealand

Robert O. Crapo, and Robert L. Jensen

University of Utah School of Medicine, Salt Lake City, Utah

1. Swanney MP, Jensen RL, Crichton DA, Beckert LE, Cardno LA, Crapo RO. FEV₆ is an acceptable surrogate for FVC in the spirometric diagnosis of airway obstruction and restriction. Am J Respir Crit Care Med 2000; 162: 917-919 .

2. Hankinson JL, Odencrantz JR, Fedan KB. Spirometric reference values from a sample of the general U.S. population. Am J Respir Crit Care Med 1999; 159: 179-187 [Abstract/Free Full Text].

3. Jensen RL, Shipp ND, Crapo RO. FEV₁/FEV₆ is an acceptable surrogate for FEV₁/FVC for the diagnosis of airway obstruction [abstract]. Am J Respir Crit Care Med 2000; 161: A275 .

4. Jensen RL, Crapo RO. FEV₆ can be estimated when expiratory times are as short as three seconds [abstract]. Am J Respir Crit Care Med 2000; 161: A275 .