Comparison of Indian reference equations for spirometry interpretation

Background and Objectives: It would be desirable in a large country such as India that a single set of reference equations be used to interpret lung function tests performed across the entire country. This study compared north, west and south reference equations in interpreting spirometry results in north Indian patients. Methods: Spirometric records of 27383 patients aged 16-65 years were assessed. Spirometric values for FVC, FEV1 and FEV1%FVC values derived from north, west and south Indian reference equations were compared. Differences in the lower limit of normal (LLN) were studied across the age and height range of the study group to determine if there was any clinically significant difference in the three derived values. Results: The north and west Indian equations was discordant in 22.1% instances, and the north and south Indian equations in 12.9% instances, with kappa estimates of agreement being 0.626 and 0.781, respectively. Most of the patients with abnormal spirometry using north Indian equations were erroneously interpreted to have normal spirometry using west or south Indian equations. The south Indian equations underpredicted LLN for FVC and FEV1 for most men and women. The west Indian equations underpredicted LLN for FVC and FEV1 in all men, and in younger and short statured women. Conclusions: North, west and south Indian reference equations do not yield equivalent results for spirometry interpretation in north Indian patients.