The purpose of this study is to support the clinical validation of a new assay to measure the levels of ivacaftor, tezacaftor, and elexacaftor (the components of Trikafta) in the bloodstream in order to achieve greater understanding of the effectiveness of this medication in all people with cystic fibrosis. Blood will be drawn at 3.0, 4.5, and 6.0 hours after taking the medication in the morning.

Blood samples will be collected to measure steady state blood levels for participants who are being treated with elexacaftor/tezacaftor/ivacaftor (ETI). This study is being conducted to evaluate drug dosing in people with cystic fibrosis. The aim of this study is to support clinical validation of pharmacokinetics launch of the elexacaftor/tezacaftor/ivacaftor blood serum level assay to demonstrate instrument capabilities to pass ADx validation criteria.

Inclusion Criteria:

• Written informed consent obtained from subject
• Diagnosis of CF
• Aged 18 years and older
• Use of ETI that has reached steady state dosing levels of ETI, defined as a continuous dosing regimen for a minimum of 14 days.

Exclusion Criteria:

• A person with CF (pwCF) that is either not taking ETI or dose not follow a continuous dosing regimen for a minimum of 14 days.
• Use of prohibited medications as outlined by package insert
• Pregnant or lactating people with CF
• Acute, severe deterioration in health requiring intensive care unit admission

Up to 100 people with CF who attend the Adult CF Program of Colorado at National Jewish Health and are actively taking ETI hat has reached steady state dosing levels of ETI (defined as a continuous dosing regimen for a minimum of 14 days) will be recruited and enrolled.