Use of Lambda-Mu-Sigma-Derived Z Score for Evaluating Respiratory Impairment in Middle-Aged Persons

BACKGROUND: The lambda-mu-sigma (LMS) method calculates the lower limit of normal for spirometric values as the 5th percentile of the distribution of Z scores. Conceptually, LMS-derived Z scores account for normal age-related changes in pulmonary function, including variability and skewness in reference data. Evidence is limited, however, on whether the LMS method is valid for evaluating respiratory impairment in middle-aged persons. OBJECTIVE: To evaluate the association of LMS-defined respiratory impairment (airflow limitation and restrictive pattern) with mortality and respiratory symptoms. METHODS: We analyzed spirometric data from white participants ages 45-64 years in the Third National Health and Nutrition Examination Survey (NHANES III, n = 1,569) and the Atherosclerosis Risk in Communities study (ARIC, n = 8,163). RESULTS: LMS-defined airflow limitation was significantly associated with mortality (adjusted hazard ratios: NHANES III 1.90, 95% CI 1.32-2.72, ARIC 1.28, 95% CI 1.06-1.57), and respiratory symptoms (adjusted odds ratios: NHANES III 2.48, 95% CI 1.75-3.51, ARIC 2.27, 95% CI 1.98-2.62). LMS-defined restrictive-pattern was also significantly associated with mortality (adjusted hazard ratios: NHANES III 1.98, 95% CI 1.08-3.65, ARIC 1.38, 95% CI 1.03-1.85), and respiratory symptoms (adjusted odds ratios: NHANES III 2.34, 95% CI 1.44-3.80, ARIC 1.89, 95% CI 1.46-2.45). CONCLUSIONS: In white middle-age persons, LMS-defined airflow limitation and restrictive-pattern were significantly associated with mortality and respiratory symptoms. Consequently, an approach that reports spirometric values based on LMS-derived Z scores might provide an age-appropriate and clinically valid strategy for evaluating respiratory impairment. Key words: spirometry; airflow limitation; restrictive-pattern; mortality; respiratory symptoms. [Respir Care 2011;56(11):1771-1777. (C) 2011 Daedalus Enterprises]