Comparison of sputum induction using inhaled methacholine or hypertonic saline

Abstract

Hypertonic saline is recommended for the induction of sputum to assess airway inflammation. Methacholine is most commonly used to assess airway responsiveness. The aim of this study was to compare sputum induction by methacholine and hypertonic saline challenges, and the subsequent inflammatory cell differential counts. Subjects (n = 56) underwent inhaled hypertonic saline (4.5% for < or = 15 min without bronchodilator pretreatment) and methacholine (maximum cumulative dose 4 micromol) challenges 1 week apart, in random order. Sputum was processed, and on cytospins stained with May-Grunwald-Giemsa, a total of 400 inflammatory plus squamous cells were counted to obtain a differential cell count. A satisfactory sputum sample was obtained from 29 subjects following hypertonic saline and also from 10 subjects following methacholine. For these 10 subjects, the mean percent macrophages, neutrophils, eosinophils, and lymphocytes were similar after methacholine (64%, 26%, 7% and 1%) or hypertonic saline (67%, 26%, 6% and 1%). The results were similar for squamous cells (42% and 45%) and when asthmatics and nonasthmatics were compared separately. Inhaled methacholine has a much lower yield of sputum induction compared with hypertonic saline. However, the differential sputum cell count is similar following both challenge agents.