Comparison of N-epsilon-(Carboxymethyl)Lysine levels and percentage of eosinophils in induced sputum for assessment of small airway involvements in asthma.

Background: N-epsilon-(Carboxymethyl)Lysine (CML), a major advanced glycation end product, is expressed in the lower respiratory tract. In this study, we compared the validity of measuring CML levels and percentage of eosinophils in induced sputum for assessment of airway functions, and evaluated the clinical implications of sputum CML levels in the asthmatic airways.

Material/methods: We examined CML levels and percentage of eosinophils in induced sputum from 37 asthmatic patients and 15 normal controls, and evaluated the relationships between these parameters and clinical profiles of asthmatic patients.

Results: The percentage of eosinophils in induced sputum was significantly higher in asthmatic patients than in normal controls. Similarly, CML levels were also significantly higher in asthmatic patients than in normal controls (median (range): asthmatic patients 29.3 (18.4-64.7) microg/mL; normal controls 25.8 (14.0-47.0) microg/mL, p=0.02). However, there was no significant correlation between CML level and percentage of eosinophils. In asthmatic patients, percentage of eosinophils was significantly correlated with FEV1/FVC and degree of airway hyperreactivity to methacholine. However, CML level was correlated with FEV1/FVC, but not with methacholine reactivity. In contrast, CML level, but not percentage of eosinophils, was significantly correlated with degree of small airways dysfunction.

Conclusions: Our novel, non-invasive technique of measurement of CML levels in induced sputum may prove to be important not only in the evaluation of small airway involvements, but also in helping us move toward a better understanding of the roles of the small airways in the pathogenesis of asthma.