Rheology of sputum collected by a simple technique limiting salivary contamination

Rheological studies of sputum are often considered unreliable because expectorated secretions can be contaminated by saliva. The in vitro dilution of gel mucus by 50% of saliva results in a marked decrease in apparent viscosity and elasticity. A simple technique for collecting sputum "protected" from salivary contamination is described. It consists of using dental cotton-wool swabs placed immediately before expectoration between the cheek and the gum and under the tongue to absorb saliva. The rheological and transport properties of sputum, collected by the protected and unprotected technique in 20 patients with either recurrent or chronic bronchitis, are compared. The protected sputum samples are characterized by a significantly lower pourability (p = 0.02) and sol phase percentage (p = 0.01) and a higher apparent viscosity (p = 0.05). A marked, although not significant, increase in viscoelastic modulus is also noticed with the protected sputa. No difference is observed in the mucociliary transport velocity according to whether the sputum is protected or not. The reproducibility and reliability of the measurements are considerably improved when sputum is collected by the protected technique. The ratio of inter- to intrasample variability is twice to three times as high for the unprotected samples.