Diabetes is a common complication of Friedreich ataxia, requiring sensitive diagnostic methods. Here, we compared the performance of different tests that assess glucose tolerance, insulin sensitivity, and β-cell function in Friedreich ataxia patients, heterozygous FXN mutation carriers and controls. We find that diabetes is underdiagnosed with fasting glucose alone. The oral glucose tolerance test (OGTT) provides 1.2- to 3.5-fold more diagnoses of impaired glucose homeostasis and diabetes, and adequately measures insulin sensitivity, insulin secretion, and β-cell function. Clinicians in charge of Friedreich ataxia patients and researchers should incorporate the OGTT as an accurate diagnostic and research tool.