Within- and between- subject variability of indices from the closing volume and flow volume traces.

The within-subject variability of consecutive measurements of indices derived from the closing volume (CV) trace and from the maximal expiratory flow volume (MEFV) curve was studied in 24 subjects. The variability of the closing volume and of the maximal expiratory flow rates at 50 percent (Vmax. 50) and 75 percent (Vmax. 75) of the expired vital capacity was about three to eight times greater than that of the FEV1, FVC or FEV1 percent. The lung volume measured from total lung capacity to the onset of airway closure (OAC) was about five times more reproducible than the CV. The coefficients of variation for the CV (as a percentage of the vital capacity), the Vmax. 75, and the OAC, both in litres and as a percentage of the vital capacity, were significantly correlated with age. No difference in the mean coefficients of variation for the CV, OAC, Vmax. 50 or Vmax. 75 were found with respect to sex, smoking habit or previous experience with the test routines. The between-subject variability of the FEV1, FVC, FEV1 percent, transfer factor, diffusion coefficient, Vmax. 50, Vmax. 75, CV and OAC was evaluated from a study of 75 asymptomatic lifetime non-smokers. The variability of the Vmax. 50, Vmax. 75 and CV was about two to eight times greater than that of the other tests used, irrespective of sex. The OAC (percent VC) was three to four times less variable than the CV. The variability of the Vmax. 50 and Vmax. 75 was reduced by, on average, 7 percent when these flow rates were expressed per litre of FVC.