Diagnostic Criteria for Diabetes

General Diagnostic Criteria

The American Diabetes Association recommends a fasting plasma glucose (FPG) level of ≥126 mg/dL (7.0 mmol/L) for diagnosing diabetes, with fasting defined as no caloric intake for at least 8 hours 1
A 2-hour plasma glucose level of ≥200 mg/dL (11.1 mmol/L) during an oral glucose tolerance test (OGTT) with 75 g of anhydrous glucose is also recommended for diagnosis 1, 3
A hemoglobin A1c (HbA1c) level of ≥6.5% (48 mmol/mol) is recommended, performed in a laboratory using a method certified by the National Glycohemoglobin Standardization Program (NGSP) and standardized to the Diabetes Control and Complications Trial (DCCT) assay 1, 4
In patients with classic symptoms of hyperglycemia or hyperglycemic crisis, a random plasma glucose level of ≥200 mg/dL (11.1 mmol/L) is also diagnostic 1, 3
The American Diabetes Association recommends confirming the diagnosis with a repeat test in the absence of unequivocal hyperglycemia, using the same test or a different test without delay, using a new blood sample 1, 5
If two different tests (such as HbA1c and FPG) are above the diagnostic threshold, this also confirms the diagnosis 1

Measurement of pancreatic autoantibodies (anti-GAD, anti-IA2, anti-insulin, anti-ZnT8) is recommended for diagnosing type 1 diabetes 2, 6
Evaluation of C-peptide levels (low or absent) is also recommended 7
Presence of diabetic ketoacidosis at diagnosis (approximately one-third of cases) is a characteristic feature of type 1 diabetes 8, 9

Evaluation of insulin resistance using indices such as HOMA-IR is recommended for diagnosing type 2 diabetes 2, 6
Normal or elevated C-peptide levels are characteristic of type 2 diabetes 7
Presence of metabolic syndrome and other risk factors is also a characteristic feature of type 2 diabetes 2, 6

An OGTT with 75 g of glucose between 24-28 weeks of gestation is recommended for diagnosing gestational diabetes 2, 6
Specific diagnostic criteria for pregnancy are recommended 2

In conditions associated with increased red blood cell turnover (sickle cell disease, pregnancy, hemodialysis, recent blood loss, transfusion, or erythropoietin therapy), only plasma glucose criteria should be used 1, 4
Marked discordance between measured A1C levels and plasma glucose levels should raise the possibility of interference in the A1C assay due to hemoglobin variants 1, 4
The effectiveness of interventions for primary prevention of type 2 diabetes has been demonstrated mainly among individuals with impaired glucose tolerance (IGT) with or without elevated fasting glucose 1, 9

The American Diabetes Association recommends starting with HbA1c and FPG in patients with risk factors 1, 5
If any of these tests are positive, confirmation with a second test is recommended 1
In case of discordance or high clinical suspicion with negative tests, an OGTT should be performed 1, 3