Regional variations in vital capacity in adult males in India: comparison of regression equations from four regions and impact on interpretation of spirometric data.

ABSTRACT

BACKGROUND: Information on variations in vital capacity predicted from various regional equations for adult males in India and on their impact on interpretation of spirometric data is not available. MATERIAL AND METHODS: In a retrospective study, spirometry data of 1672 male patients, aged 15 years and above were studied. Predicted values of forced vital capacity (FVC), labelled as FVC(North), FVC(East), FVC(West) and FVC(South) were calculated from the available regional prediction equations. A prediction equation for FVC for south Indian adults above 40 years of age is not available. Spirometry data was interpreted using these and the extent of agreement between pairs of equations in detection of pattern of abnormality was analysed. RESULTS: The FVC(North) and FVC(East) were close and greater than FVC(West) and FVC(South), which were in turn, close to each other. Up to the age of 40 years, the FVC(North) exceeded FVC(East), FVC(West) and FVC(South) by 2.4%, 11.8% and 13.3%, respectively, while in the above 40 years age group, it exceeded FVC(East) and FVC(West) by 5.1% and 9.67%, respectively. The differences, however, decreased substantially with increasing FVCs and even reversed at higher values with FVC(East) tending to exceed FVC(North) in both the age groups, and FVC(West) tending to exceed FVC(North) in the above 40 years age group. While northern and eastern, and, western and southern equations gave acceptable differences (less than 5%) in interpretation of abnormality in spirometric data in patients up to 40 years of age, differences between other pairs of equations in this age group, and between all pairs in the above 40 years age group were large and unacceptable. CONCLUSIONS: Substantial variations exist in vital capacity predicted from various regional equations in adult males in India. In general, northern and eastern equations, and, western and southern equations yield closer values. While the northern Indian equation gives the highest predicted vital capacity, this is true only for lower values of vital capacities and at higher values, this may be less than that predicted from eastern or western equations. The regional differences may result in unacceptable errors in interpretation of spirometry data, if inappropriate prediction equations are used.