The purpose of the study was to determine the best surrogate for FVC when performing spirometry to detect obstruction or restriction. Volume-time curves from 3,539 participants in the Family Heart Study with acceptable quality test sessions were analyzed. An index of the variability of each timed volume (FEVx) from 1 to 12 s was determined for each subject. The least within-test session variability was seen for forced expired volume in 6 s (FEV(6)) and forced expired volume in 7 s (for both, mean range was 95 mL). The sensitivity and specificity for detecting obstruction and restriction when substituting the FEV(6) for the FVC were then determined before and after allowing for measurement errors of 50 mL (approximately the lower limit of spirometers ability to detect volume). Sensitivity was 76% before the 50-mL error analysis and 95% after. Specificity was 98% before the 50-mL error analysis and 99.5% after. We conclude that use of FEV(6) to replace the FVC for spirometry testing will result in improved reproducibility, with no significant loss of sensitivity or specificity, after allowing a 50-mL measurement error, for detecting obstruction or restriction.