Delineation of lower limits of normal (LLN) is important in the clinical utilization of reference values. In a cross-sectional multi-center study to derive the reference spirometric values of adult Chinese, we compared two methods of its determination in lung function parameters.
The study was carried out between January 2001 and March 2003. Healthy non-smoking Chinese subjects were recruited from the community by random digit dialing to undergo spirometry in lung function laboratories of eight hospitals of Hong Kong. Prediction equations for lung function parameters were developed using multiple regression analysis. LLN were determined by: (1) traditional method assuming constant difference between the mean and fifth percentile throughout the age range (mean-1.645 residual standard deviation); or (2) distribution-free estimation of age-related centiles described by Healy & Rabash (Annals of Human Biology 1988).
Evaluable data of 1089 (494 males and 595 females) subjects aged 18 to 80 years were analysed. In deriving LLN, distribution-free estimation, when compared with the traditional equation, yielded better approximation to the fifth percentiles. To illustrate, 4.88% vs 4.05%, 4.66% vs 3.64% and 4.86% vs 3.85% of men were considered to have subnormal values of FEV1, FVC and FEV1/FVC ratio respectively. Equations from distribution-free estimation also yielded more stable age-related profiles of LLN. To illustrate, for FEV1, the proportions of subjects in four age stratifications who were classified as having subnormal values ranged from 3.33% to 6.58%, compared with 0.67% to 7.55% obtained with the traditional method.
In determining LLN for spirometric values, the distribution-free estimation of age-related centiles, compared with another traditional method, yielded better proximity to fifth percentiles and more stable profiles in various age groups.
Application of distribution-free estimation of age-related centiles was more appropriate in this study cohort.
Johnny Chan, Grant monies (from sources other than industry) This study was supported by a grant from Hong Kong Pneumoconiosis Fund Board; Other The study was conducted under the auspices of Hong Kong Thoracic Society and American College of Chest Physicians (HK and Macau Chapter).