The many ways sputum flows - Dealing with high within-subject variability in cystic fibrosis sputum rheology

We evaluated test-retest reliability of sputum viscoelastic properties in clinically stable patients with cystic fibrosis (CF). Data from a prospective, randomized crossover study was used to determine within-subject variability of sputum viscoelasticity (G', storage modulus and G", loss modulus at 1 and 10 rad s^-1) and solids content over three consecutive visits. Precision of sputum properties was quantified by within-subject standard deviation (SD_ws), coefficient of variation (CV) and intraclass correlation coefficients (ICC). Fifteen clinically stable adults with CF (FEV₁ range 24-94% predicted) were included. No differences between study visits (mean ± SD 8 ± 2 days) were observed for any sputum rheology measure. CV's for G', G" and solids content ranged between 40.3-45.3% and ICC's between 0.21-0.42 indicating poor to fair test-retest reliability. Short-term within-subject variability of sputum properties is high in clinically stable adults with CF. Investigators applying shear rheology experiments in future prospective studies should consider using multiple measurements aiming to increase precision of sputum rheological outcomes.