Ivacaftor (Kalydeco) and Insulin in Cystic Fibrosis (CF)

Inclusion Criteria: * 6 yrs or older with cystic fibrosis * at least one G551D CFTR mutation or other non-G551D gating mutation, or residual function CFTR mutation such as, but not limited to, R117H mutation, for which ivacaftor is to be initiated. * Plan to initiate ivacaftor treatment for FDA approved indications by clinical care team or as part of an ongoing study of ivacaftor for other CFTR mutations, including gating mutations, or residual function mutations. * not pregnant Exclusion Criteria: * established diagnosis of non-CF related diabetes (ie., Type I diabetes) * history of clinically symptomatic pancreatitis in past year * prior lung or liver transplant * severe CF liver disease * fundoplication-related dumping syndrome * medical co-morbidities that are not CF-related or are unstable per the Investigator opinion * acute CF pulmonary exacerbation within 4 weeks prior to study procedures * treatment with oral or intravenous corticosteroids within 4 weeks of study * hemoglobin \<10g/dL within 90 days of GPA test or at Screening * abnormal renal function within 90 days of GPA test or at Screening * long-standing CFRD with fasting hyperglycemia, elevated HbA1C (\>8) beyond time surrounding diagnosis of CFRD, significant basal insulin requirement * inability to perform study specific procedures (MMTT, GPA).