Diagnostic Criteria for Diabetes

Primary Diagnostic Tests

The American Diabetes Association recommends a fasting plasma glucose (FPG) test with a diagnostic threshold of ≥126 mg/dL (7.0 mmol/L) after at least 8 hours of fasting, or alternatively, an HbA1c test with a diagnostic threshold of ≥6.5% which requires no fasting 1, 2
For FPG tests, blood should be drawn in the morning after an overnight fast, and sample handling should use a tube with a rapidly effective glycolytic inhibitor (granulated citrate buffer) to prevent glycolysis 1, 2
The American Diabetes Association recognizes Hemoglobin A1c (HbA1c) as a diagnostic test with a threshold of ≥6.5% (48 mmol/mol) and no fasting requirement, but it must be NGSP-certified and standardized to DCCT assay 3, 4, 5
The 2-Hour Oral Glucose Tolerance Test (OGTT) requires a fasting period of at least 8 hours, a glucose load of 75 grams anhydrous glucose dissolved in water, and a diagnostic threshold of 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) 3, 5
Random Plasma Glucose testing has a diagnostic threshold of ≥200 mg/dL (11.1 mmol/L) but requires the presence of classic symptoms (polyuria, polydipsia, unexplained weight loss) or hyperglycemic crisis 3, 5

The American Diabetes Association recommends that all positive results must be confirmed on a subsequent day with repeat testing, unless the patient presents with unequivocal hyperglycemia (≥200 mg/dL with symptoms) 3, 5
Confirmation can use the same test or a different diagnostic test to prevent false-positive diagnoses due to laboratory error or biological variation 1, 2, 3, 5

The American Diabetes Association recommends screening asymptomatic high-risk individuals using HbA1c, FPG, or 2-hour OGTT, with normal screening values of HbA1c <5.7%, FPG <100 mg/dL (5.6 mmol/L), and 2-hour OGTT <140 mg/dL (7.8 mmol/L) 1, 2
Screening should be repeated every 3 years if results are normal 1, 2

Venous plasma must be used for diagnostic testing, and capillary blood glucose meters are inadequate for diagnosis 1, 2
An accredited laboratory is required for all diagnostic and screening tests, and proper glycolysis prevention is essential to avoid falsely low glucose readings 1, 2
Laboratory glucose measurement should have analytical imprecision ≤2.4%, bias ≤2.1%, and total error ≤6.1% to minimize misclassification at diagnostic thresholds 1, 2

For Gestational Diabetes Mellitus (GDM), screening should occur at 24-28 weeks gestation with an 8-hour fasting requirement for diagnostic OGTT, and two approaches are available: one-step 75g OGTT or two-step 50g non-fasting screen followed by 100g fasting OGTT if positive 5
For children and adolescents, the same diagnostic criteria as adults apply, and screening should be performed on overweight children (BMI ≥85th percentile) with two additional risk factors 3, 7