An investigation of airway acidification in asthma using induced sputum: a study of feasibility and correlation

Acidification of the airways seems to be involved in asthma pathophysiology, but its assessment might be difficult.

The aim of our study is to assess the feasibility and validity of airway acidification measurement by induced sputum and its clinical significance in asthma.

Induced-sputum samples were obtained in 57 outpatients with asthma. The between-sample repeatability after 48 hours was measured in an independent population of 14 patients with asthma. pH was measured using a pH meter. The control of asthma was established by the Asthma Control Questionnaire.

The pH measurement was feasible in all samples and repeatable both within (intraclass correlation coefficient [ICC], 0.96) and between samples (ICC, 0.621). The mean pH was significantly different between healthy subjects and patients with asthma, including in those with controlled (mean pH: 7.54 in healthy subjects vs. 7.28 in subjects with controlled asthma; p = 0.0105) and uncontrolled disease (mean pH: 7.54 in healthy subjects vs. 7.06 in subjects with uncontrolled disease; p < 0.0001), and between patients with stable asthma and those with poorly controlled asthma (7.28 vs. 7.06, respectively; p = 0.0134). The validity of the method was assessed with the receiver operating characteristic curves and induced-sputum lower pH values (with a cutoff value of 7.3; sensitivity, 72.1%; specificity, 100%).

Patients with asthma show lower pH than healthy subjects. Patients with poorly controlled asthma seem to have the lowest induced-sputum pH, independent of the GINA (Global Initiative for Asthma) severity level. In conclusion, induced sputum is a feasible, repeatable, noninvasive method to measure airway pH. The pH in induced sputum may reflect a different aspect of asthma from sputum eosinophils and be related to different pathophysiologic factors.