Normal variations in spirometry in health following placebo inhalation

Normal i.e., chance variations in spirometry were evaluated in 80 healthy Indian adults by the measurement of base-line spirometric indices (FVC, FEV1 and FEF25-75%) and the per cent changes following placebo administration of aerosolized saline. There was a mean per cent change of 0.7, 0.4 and 0.3 per cent over the base-line values in FVC, FEV1 and FEF25-75% respectively following placebo inhalation. The cut off responses for 95 per cent confidence limits were 11.2 per cent for FVC and 12.7 per cent for FEV1 following placebo inhalation. Since the test sample represents a healthy Indian population, a per cent change greater than 12.7 per cent in FEV1 is more than that observed in 95 per cent of healthy Indians. It is suggested that for both delta FEV1 per cent and delta FVC per cent, the lower limit should be fixed at more than 15 per cent to define 'abnormality' in clinical practice.