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The shear modulus G of the sputum obtained by expectoration from cystic fibrosis patients is fundamental to determine the mesh size of the polymeric network pervading the sputum, a parameter related to lung functionality. The Akaike criterion revealed that in 55.2 % of the examined samples, the best approach (among those considered) to determine G relied on the mechanical spectrum fitting by the generalised Maxwell model with relaxation times scaled by a factor 10. Thus, this approach was adopted to evaluate the mesh size distribution combining G knowledge with the determination of the average magnetic relaxation time (T2m) of sputum. As G and T2m determination can be negatively affected by sputum contamination by saliva, whose presence increases T2m and depresses G, we developed a proper “decontamination” procedure to obtain more reliable T2m and G estimations (necessary in 21 % of the samples). This procedure allowed to strengthen the T2m correlation with lung functionality evaluated by FEV1 (normalised air volume emitted in the first second during a spirometry test).