ADA Diabetes and Prediabetes Diagnostic Criteria
The American Diabetes Association recognizes four distinct criteria for diagnosing diabetes, any one of which is sufficient: fasting plasma glucose ≥126 mg/dL, 2-hour plasma glucose ≥200 mg/dL during a 75-g oral glucose tolerance test, hemoglobin A1c ≥6.5%, or random plasma glucose ≥200 mg/dL with classic symptoms of hyperglycemia. 1, 2
Diabetes Diagnostic Criteria
The diagnosis of diabetes requires meeting any one of the following thresholds 1:
- Fasting plasma glucose (FPG) ≥126 mg/dL (7.0 mmol/L) after at least 8 hours of overnight fasting 1, 2
- 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during a 75-g oral glucose tolerance test (OGTT) performed after an overnight fast of at least 8 hours 1, 2
- Hemoglobin A1c ≥6.5% (48 mmol/mol), measured in a laboratory using an NGSP-certified method standardized to the DCCT assay 1, 2
- Random plasma glucose ≥200 mg/dL (11.1 mmol/L) in a patient with classic symptoms of hyperglycemia (polyuria, polydipsia, polyphagia, unexplained weight loss) 1, 2
Confirmation Requirements
Two abnormal test results are required to confirm the diagnosis of diabetes, obtained either from the same sample or from tests performed on different days. 1 The critical exception is when a patient presents with unequivocal hyperglycemia (glucose ≥200 mg/dL) accompanied by classic symptoms—this single finding confirms diabetes immediately without requiring repeat testing. 1 Alternatively, if two different tests (such as A1c and FPG) are both above their respective diagnostic thresholds, this also confirms the diagnosis. 2
Prediabetes Diagnostic Criteria
Prediabetes is identified by any one of three criteria: impaired fasting glucose (100-125 mg/dL), impaired glucose tolerance (140-199 mg/dL at 2 hours during OGTT), or A1c between 5.7% and 6.4%. 1, 2
The specific thresholds are 1, 2, 3:
- Impaired fasting glucose (IFG): FPG 100-125 mg/dL (5.6-6.9 mmol/L) 1, 3
- Impaired glucose tolerance (IGT): 2-hour plasma glucose 140-199 mg/dL (7.8-11.0 mmol/L) during a 75-g OGTT 1, 3
- A1c: 5.7-6.4% (39-47 mmol/mol) 1, 2, 3
Important Caveats for Prediabetes Diagnosis
A1c should be used only for screening for prediabetes; the actual diagnosis of prediabetes should be confirmed with glucose testing (either FPG or OGTT). 1 This distinction is critical because A1c has lower sensitivity and may miss individuals who would be identified by glucose-based criteria. 4
Critical Testing Considerations
A1c Limitations
When using A1c for diagnosis, recognize several important limitations that may affect accuracy 2:
- Hemoglobin variants can interfere with A1c measurement, though most U.S. assays are unaffected by common variants 2
- Conditions with increased red blood cell turnover (sickle cell disease, pregnancy in second/third trimesters, hemodialysis, recent blood loss or transfusion, erythropoietin therapy) require using only plasma glucose criteria for diagnosis 2
- Age and race/ethnicity may independently affect hemoglobin glycation 2
- Point-of-care A1c assays should not be used for diagnosis—only laboratory measurements using NGSP-certified methods are acceptable 5, 2
Screening Recommendations
All adults ≥35 years of age with risk factors should be screened for diabetes. 1 For adults without risk factors, screening should begin at age 45 years. 2 If results are normal, repeat testing at minimum 3-year intervals, with more frequent testing based on initial results and risk status. 2
Patients with prediabetes should be tested annually to monitor for progression to diabetes. 2
Algorithmic Approach to Diagnosis
If symptomatic (polyuria, polydipsia, weight loss): Obtain random plasma glucose—if ≥200 mg/dL, diabetes is confirmed immediately 1, 2
If asymptomatic: Obtain any one of the following tests 1, 2:
- Fasting plasma glucose (preferred for simplicity)
- A1c (convenient, no fasting required)
- 2-hour OGTT (most sensitive but cumbersome)
If first test is abnormal (but patient is asymptomatic): Confirm with a second abnormal test result, either repeating the same test on a different day or using a different diagnostic test 1, 2
If values fall in prediabetes range: Confirm with glucose testing (FPG or OGTT) rather than relying on A1c alone 1
The most practical approach in clinical practice is to use fasting plasma glucose as the initial screening test due to its simplicity, reproducibility, and patient acceptability, reserving OGTT for cases where clinical suspicion remains high despite normal FPG. 5, 6