Reference equations for the single-breath diffusing capacity. A cross-sectional analysis and effect of body size and age

A longitudinal study was implemented in an unpolluted rural area of northern Italy (near Venice), before the start of the operation of a large oil-burning thermoelectric power plant, in order to investigate the effects of the future exposure to air pollution and to elucidate the natural history of obstructive airways diseases. During the first cross-sectional survey, a sample of the general population (n = 3,300, 8 to 64 yr of age) performed several lung function tests, and information on risk factors and on the presence of respiratory symptoms were obtained by a standardized questionnaire. There were 712 subjects who were classified as normal on the basis of rigid criteria and who were able to perform satisfactorily a single-breath CO diffusion capacity (DL(COsb)) test. The DL(COsb) values showed an increase with height and age early in life and a later decrease with age. Two age groups were selected to determine an age at which DL(COsb) stopping increasing and began to decline. Reference equations were computed (using age and height) in these 2 different age groups in each of the sexes. Similar equations were calculated for the total lung capacity derived from single-breath helium dilution measurements. The DL(COsb) values in this study were higher than reported by other investigators. The method of selection of the study population, the strict criteria for normal, as well as technical, differences probably explain this finding. Reference equations for diffusing capacity corrected to lung volume (DL/VA) were computed only for adults (>= 20) in both sexes, because age and height coefficients in young subjects were insignificant. The DL/VA declined with age in adults and tended to be inversely related to height in all age groups. The decrease in DL/VA with age appears to be related to the increase in size of terminal air spaces associated with aging