Mucus hypersecretion is an important pathophysiologic index of airway disease. Measurement of secreted mucin in sputum has been reported in asthma, but not in chronic cough with or without increased sputum production.
We studied 49 patients with classic asthma (CA), 39 with cough-variant asthma (CVA), nine and five with chronic cough associated with sinobronchial syndrome (SBS) and gastroesophageal reflux disease (GERD), respectively, and 11 healthy controls. Seventeen patients with CA, but none from the other groups, were taking antiinflammatory medications. Mucin levels in induced sputum supernatants were measured by enzyme-linked immunosorbent assay, which detects airway mucin, probably including MUC5AC and MUC5B.
Mucin levels were higher in patients with CA (674.2 ± 548.8 µg/mL) and SBS (638.4 ± 650.7 µg/mL) than in controls (212.0 ± 167.1 µg/mL) (P = .0037 and .044). They were also higher in patients with CA than in those with CVA (350.4 ± 374.0 µg/mL) and GERD (134.3 ± 93.1 µg/mL) (P = .0016 and 0.015), but results did not differ between the latter groups and controls. When the four disease groups were combined, patients with frequent sputum production had greater mucin levels than those with occasional (P = .0023) or no sputum production (P < .0001). Patients with CA showed negative correlations of mucin levels with respiratory resistance indices on impulse oscillation and with airway sensitivity to methacholine.
Sputum mucin levels differ in various respiratory conditions when compared with controls, primarily reflecting the degree of sputum production. Airway mucin might possibly exert protective effects in asthma, at least between exacerbations, but this issue needs to be further clarified by future studies.