With improved life expectancy of cystic fibrosis (CF) patients, CF-related diabetes (CFRD) has become a major complication. The oral glucose tolerance test (OGTT) is the standard test to detect it. However, the use of OGTT is controversial, in addition to being a burden for patients and the treatment team. Research to find alternative ways of testing is ongoing. While some propose that glycated hemoglobin (HbA1c) may be an effective alternative, our past results suggest otherwise. A new analysis involving the OGTT and HbA1c values of 207 patients, between 2004 and 2015, proposes that the threshold of a lower value of HbA1c of ≥5.8%(39.9 mmol/mol) gives a sensitivity of 68.2% and a specificity of 60.5%. With such sensitivity to identify patients in need of an OGTT, 31.8% of CFRD diagnosis would be missed if the suggested HbA1c value of ≥5.8% was used as a screening tool to identify patients in need of OGTTs. Considering our results, we believe the HbA1c does not possess the characteristics of a suitable screening test for CFRD.