Comparison of lower limit of normal values with a fixed ratio assessed by spirometry in obstructive and restrictive lung diseases in Indian population

Abstract

Background and Aim: Spirometric parameters are approximated by the normal distribution and using lower limit of normal (LLN) minimize the misclassification with fixed-ratio protocol in interpretation of spirometry results. This study investigates prevalence and characterization of obstructive and restrictive lung function impairments, and dissimilarities between two guidelines interpreting spirometry. Methods: This study was conducted to assess the lung function in students and patients ( n = 74) of MGM Medical College, Kishanganj, Bihar, between September 2012 and May 2014, using computerized Spirometer. Results: Forced vital capacity (FVC), forced expiratory volume in the 1st s (FEV1)/FVC, forced expiratory flow25-75% (FEF25-75%), peak expiratory flow rate (PEFR) were normally distributed with inflection at their mean ± standard deviation. FEF25-75%, and PEFR were higher in normal individuals compared to individuals with respiratory disease. FEV1/FVC were lower, and FVC were higher in obstructive disease while the reverse results were obtained in restrictive disease category. Restrictive disease by fixed-ratio was found with 55% negative predictive value (NPV) and 5% positive predictive value (PPV), while the normal cases by fixed-ratio criteria was found with 91% NPV and 15% PPV versus LLN criteria. There were 58 individuals with normal spirometries under the LLN method but a discrepant of only 12 fixed-ratio normal results. Concordance analysis by kappa statistics yielded a κ = 0.017. Conclusion: Older adults are more pulmonary susceptible. The performance of LLN criteria in differentiating respiratory diseases is better than the fixed-ratio criteria. Poor agreement exist between the fixed-ratio and LLN protocols in interpreting spirometric results. There is the difference between the LLN and fixed-ratio criteria which need to be redressed with population-specific reference values.