Cystic fibrosis (CF) is a genetic, multi-system disease due to mutations in the cystic fibrosis conductance regulator (CFTR) gene, leading to ineffective anion channel activity [1]. The resulting impaired mucociliary clearance permits initial acquisition of Pseudomonas aeruginosa and, if untreated, the establishment of persistent infection in the CF airways. It has long been recognized that chronic infection, often characterized by a mucoid P. aeruginosa phenotype, is associated with more rapid lung function decline and earlier death in individuals with CF [[2], [3], [4]]. Defining chronic P. aeruginosa infection is therefore an important step in identifying CF patients most at risk of lung disease progression. Traditionally, the Leed's criteria has been used to define chronicity (as having >50% of sputum cultures being P. aeruginosa positive in the preceding 12 months), as it is the only clinically validated definition [5]. However, the Leed's criteria are difficult to implement in young children unable to provide sputum and further limited by the required number of sputum samples and follow-up time [6].