Obesity is associated with insulin resistance and increased serum levels of fasting insulin and C-peptide serum levels. However, insulin levels are normal in many persons who are obese.

All patients with obesity should be screened for diabetes. Additional information is gained by using glucose and HbA1c tests together if the patient is fasting. The American Diabetes Association currently recommends using the HbA1c test not only to screen for diabetes but also to follow patients who already have the diagnosis. In contrast, the American Association of Clinical Endocrinologists recommends that HbA1c be considered an additional, optional diagnostic criterion.

Pre-diabetes is indicated by impaired fasting glucose (fasting plasma glucose levels of 100-125 mg/dL [5.6-6.9 mmol/L]) or impaired glucose tolerance (2-h oral glucose tolerance test values of 140-199 mg/dL [7.8-11.0 mmol/L]). Patients with these findings are at relatively high risk for the future development of diabetes. Type 2 diabetes is diagnosed when the fasting glucose is 126 mg/dL or greater or HbA1c is 6.5% or higher.