Cystic fibrosis-related diabetes mellitus (CFRD) is the most frequent comorbidity in cystic fibrosis. Its clinical relevance is stressed by the association with increased mortality, and decreased pulmonary and nutritional status. An annual oral glucose tolerance test (OGTT) is recommended as a screening test for CFRD, but this is often not realised because of its time- and resource-consuming nature. Therefore, alternative approaches are welcome. In 2003, the American Diabetes Association (ADA) lowered the cut-off point separating normal from elevated fasting plasma glucose from <6.1 mmol x L(-1) to <5.6 mmol x L(-1), suggesting the performance of an OGTT only in those with impaired fasting glucose (IFG; range 5.6-6.0 mmol x L(-1)). The current authors tested whether this approach was reliable for the early identification of patients with CFRD. OGTTs from 1,128 patients (53% males; 47% females; median age 17.1 yrs) were available for analysis. A total of 101 (8.9%) OGTTs were classified as diabetic. The new ADA criteria for IFG increased the sensitivity to 82% (versus 65%) and decreased the specificity to 70% (versus 94%) compared with the old criteria used to identify patients with diabetic OGTTs. In conclusion, the American Diabetes Association approach of using impaired fasting glucose as an indication for performing selective oral glucose tolerance tests is definitely unsuitable when aiming at the early identification of patients with cystic fibrosis-related diabetes mellitus, and it cannot replace annual oral glucose tolerance tests.