Cystic fibrosis-related diabetes (CFRD) is a complication associated with a negative prognosis in patients with cystic fibrosis (CF). Although the oral glucose tolerance test (OGTT) is the widely recommended screening test for CFRD diagnosis, continuous glucose monitoring (CGM) is increasingly considered a useful and easy-to-perform test for diagnosis and follow-up in clinical practice. Regarding CFRD treatment, although insulin is the classic approved pharmacological option, incretins could also be a helpful alternative in early stages. CGM could be also a useful tool to measure the early response to this therapy. METHODS: We studied 25 CF patients with abnormal OGTT results and compared glucose and insulin levels during the OGTTs with CGM results as a tool for early CFRD diagnosis. In addition, we evaluated glycaemic control with CGM before and after treatment with sitagliptin. RESULTS: A correlation was found between lower plasma insulin levels during the OGTTs and higher average sensor glucose (p= 0.009) and hyperglycaemic excursions (p= 0.017). The CGM data on sitagliptin treatment (n= 25) showed an average glycaemic improvement from 124.2 to 117.2 mg/dL (p= 0.002) with a 5.6-point standard deviation of glucose decrease (p< 0.001). Hyperglycaemic excursions ≥200 mg/dL diminished 57.1% (p= 0.021). Both time in range and time above 180 mg/dL improved during treatment (p= 0.036 andp= 0.006, respectively). CONCLUSION: CGM is a useful tool that offers valuable information for both the diagnosis and the management of CFRD. Lower plasma insulin levels during OGTTs are associated with a poor ambulatory glucose profile in CGM. Sitagliptin could play an important role in the treatment of the early stages of CFRD.