Comparing sputum rheology and spirometry as pathological markers in respiratory diseases

Background: Spirometry is the standard test to monitor patients with pulmonary pathologies. Sputum rheology has recently emerged as a possible biomarker of obstructive respiratory diseases in which difficulties to expectorate are related to a thickened mucus. Objective: To compare rheometry and spirometry tests as pathological markers. Methods: Healthy volunteers (HV), COPD, asthmatic, and stable CF patients (n = 11, 11, 12, 11) were tested with spirometry (FEV1, FVC, and PEF), before and after administering Beta2-adrenergic agonist. Sputum samples were also collected, and tested with oscillatory rheometry, yielding the sputum elastic and viscous moduli, G’ and G”. Under small deformations (< 5%), the sputum behaves as a soft solid (G’ > G”). Results: Expectedly, spirometry tests show the degraded pulmonary function of CF patients, but only FEV1 distinguishes the four populations with some overlap. The effect of Beta2-adrenergic agonist treatment is very weak. Rheology measurements more clearly distinguish HV, COPD, and CF patients, with a weaker population overlap. Conclusion: As expected, spirometry consistently allows to quantify a loss of pulmonary capacity in respiratory diseases. Sputum rheology offers a complementary marker of the patient condition with strong variations observed, at least for CF and COPD, which suggests its potential to monitor more finely the patient’s evolution.