This is a multicenter, cross-sectional, cohort study which will collect contemporary sweat chloride (SC) values from approximately 5000 Cystic Fibrosis (CF) patients prescribed and currently receiving commercially approved Cystic Fibrosis transmembrane conductance regulator (CFTR) modulator therapies.

Eligible subjects who have been prescribed and chronically taking a commercially approved CFTR modulator for at least 3 months will be enrolled for a single visit to collect sweat to be analyzed for SC at their local site laboratory. Limited clinical data obtained at this visit will be augmented by retrospective and prospective data obtained from the Cystic Fibrosis Foundation Patient Registry (CFFPR). Study subjects who have been prescribed and switch to an alternative commercially approved CFTR modulator will be approached to re-enroll in the study after being on the alternative modulator for at least 3 months so that a new SC value can be obtained.

Inclusion Criteria:

1. Written informed consent (and assent when applicable) obtained from subject or subject's legal representative
2. Enrolled in the CFFPR
3. Male or female ≥ 4 months of age on day of study visit
4. Diagnosis of CF.
5. Current treatment with a prescribed commercially approved CFTR modulator for at least 90 days prior to enrollment
6. Able to perform the testing and procedures required for this study, as judged by the investigator

Additional Inclusion Criteria for CHEC-PKPD Sub-Study:

1. Male or female ≥ 6 years of age on day of study visit.
2. Current treatment with elexacaftor/tezacaftor/ivacaftor for at least 90 days prior to enrollment.
3. Last dose of elexacaftor/tezacaftor/ivacaftor taken at least 24hours and last dose of ivacaftor taken at least 12 hours prior to trough blood draw on day of visit.

Exclusion Criteria:

1. Presence of a condition or abnormality that, in the opinion of the Investigator, would compromise the safety of the patient or the quality of the data
2. Currently enrolled in an investigational trial (including open-label follow-on studies and Early Access Programs (EAP) of an agent expected to have an impact on sweat chloride (refer to current list provided on study website)

Cystic Fibrosis Care Centers