CHaractErizing CFTR Modulated Changes in Sweat Chloride and Their Association With Clinical Outcomes

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)