Detection of Reductions in Cystic Fibrosis Airway Inflammation While Using Aztreonam Lysine Solution

Inclusion Criteria: * Documented diagnosis of cystic fibrosis * Age 18 years old or greater * FEV1 percent predicted greater than 25% * Ability to perform reproducible pulmonary function tests and produce sputum spontaneously * Chronic bacterial colonization with Pseudomonas aeruginosa with 2 positive cultures in previous 2 years. * Chronically stable pulmonary condition without evidence of acute pulmonary exacerbation within 14 days prior to screening * Starting Cayston cycle as part of clinical care. Exclusion Criteria: * Presence of a condition or abnormality that, in the opinion of the Principal Investigator (PI), would compromise the safety of the patient or the quality of the data. * Aztreonam allergy, bronchospasm or other contraindication to use of aztreonam. * Signs and symptoms of acute pulmonary exacerbation at the time of enrollment or during study. * Active infection and treatment for non-tuberculous mycobacteria. * Concomitant use of systemic steroids. * Use of inhaled antimicrobial agents with activity against Pseudomonas aeruginosa within 28 days prior to Visit 1.