CFTR and cystic fibrosis: Meanwhile this level of functional rescue was confirmed in observational studies in F508del homozygous CF patients where effects of lumacaftor-ivacaftor therapy on in vivo CFTR function was determined by nasal potential difference and intestinal current measurements and was found to improve CFTR function in the airway and intestinal epithelium to levels of 10 to 30% of normal CFTR activity47,48.