Lung function prediction equations derived from healthy South African gold miners

Abstract

Objectives - To estimate lung function prediction equations and to identify appropriate normal reference values for the population of about 250 000 of South African gold miners. Methods - Data from a lung function screening programme conducted at a large South African gold mine from 1994 to 1998 were used to estimate the lung function prediction equations. The most reliable period of lung function testing was identified in a previous study of a temporal pattern in reliabillty, and lung function tests from this period were used. Miners with a history of pulmonary tuberculosis or with radiological abnormalities were excluded from the study. The prediction equations were estimated cross sectionally on 15 772 black and 2752 white miners, and published reference equations that fitted most closely the observed data were identified. Results - The estimated prediction equations for forced vital capacity (FVC) are as follows: for black men, FVC (1)=-2.901-0.025xage+4.655xheight; and for white men, FVC(1)=-4.407-0.036xage+ 5.940xheight. For forced expiratory volume in one second (FEV1) these equations are: for black men, FEV1(1)=-l.654-0.30xage+3.665xheight; and for white men, FEV1(1)= -2.341- 0.038xage+4.314xheight. Units are years for age and metres for height. Knudson's and the European Community of Coal and Steel (ECCS) reference values provided the closest fit to the data on lung function of white miners, but the lower limits of normal from the ECCS equations were the closest to the observed one sided lower 95% confidence intervals (95% CIs). For black miners, reference equations that fitted best were derived by Louw et al on asymptomatic black South African men unexposed to occupational dust. There were significant differences between the two groups of miners in the estimated height adjusted mean lung function values for a 40 year old 1.7 m tall man (220 ml (5.2%) for FVC and 110 ml (3.2%) for FEV1); white men had bighter FVC and FEV1, but lower FEV1/FVC ratio. The ECCS reference values scaled by a conversion factor of 0.93 for the FVC and 0.95 for the FEV1 provided close fits to the data for black miners, but the rate of decline with age was higher than that in the observed data. None of the linear equations provided a good fit for the 20-29 and more than 55 years old age categories. Conclusion - The ECCS and Knudson equations provided the best fit to the data for white miners, whereas the equations by Louw et al estimated on asymptomatic black South African bank workers provided the best fit to the data for black miners. The ECCS reference values scaled by a factor of 0.93 for FVC and by 0.95 for FEV1 provided close fits, but the rate of decline with age was higher than that in the data for black miners.