Diagnostic Criteria for Diabetes
Diabetes is diagnosed when A1C is ≥6.5%, fasting plasma glucose is ≥126 mg/dL, 2-hour plasma glucose during a 75-g oral glucose tolerance test is ≥200 mg/dL, or random plasma glucose is ≥200 mg/dL with classic hyperglycemic symptoms. 1, 2
Primary Diagnostic Thresholds
The American Diabetes Association establishes four distinct pathways to diagnose diabetes in nonpregnant adults:
A1C ≥6.5% (≥48 mmol/mol) - Must be 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, 2
Fasting Plasma Glucose (FPG) ≥126 mg/dL (≥7.0 mmol/L) - Fasting is defined as no caloric intake for at least 8 hours 1, 2
2-hour Plasma Glucose ≥200 mg/dL (≥11.1 mmol/L) during a 75-g oral glucose tolerance test (OGTT) - The test should be performed as described by the World Health Organization 1, 2
Random Plasma Glucose ≥200 mg/dL (≥11.1 mmol/L) in a patient with classic symptoms of hyperglycemia (polyuria, polydipsia, unexplained weight loss) or hyperglycemic crisis (diabetic ketoacidosis or hyperglycemic hyperosmolar state) 1, 2
Confirmation Requirements
In the absence of unequivocal hyperglycemia (such as hyperglycemic crises), diagnosis requires two abnormal test results. 1, 3
These can be obtained from the same sample using two different tests (e.g., A1C and FPG measured simultaneously) 1
Alternatively, the same test can be repeated on a different day 1, 3
If two different tests are performed and both exceed diagnostic thresholds, the diagnosis is confirmed 3
If results are discordant between two different tests, repeat the test that was above the diagnostic threshold 3
No confirmation is needed when a patient presents with classic hyperglycemic symptoms or hyperglycemic crisis and random plasma glucose ≥200 mg/dL 4
Prediabetes Categories
The American Diabetes Association defines prediabetes using three criteria:
Impaired Fasting Glucose (IFG): FPG 100-125 mg/dL (5.6-6.9 mmol/L) 2, 3
Impaired Glucose Tolerance (IGT): 2-hour plasma glucose during OGTT 140-199 mg/dL (7.8-11.0 mmol/L) 2, 3
Critical Limitations of A1C Testing
Do not use A1C for diagnosis in conditions that affect red blood cell turnover. 2, 3 In these situations, use only plasma glucose criteria:
Pregnancy (second and third trimesters) 2
Glucose-6-phosphate dehydrogenase deficiency 2
HIV infection 2
A marked discrepancy between measured A1C and plasma glucose levels should raise suspicion for hemoglobin variants interfering with the assay 2, 4
Important Clinical Considerations
The concordance between FPG, 2-hour plasma glucose, and A1C is imperfect—these tests do not necessarily identify diabetes in the same individuals. 3, 5
The 2-hour plasma glucose value during OGTT diagnoses more people with prediabetes and diabetes compared to FPG and A1C cut points 1
For OGTT, patients should consume a mixed diet with at least 150g of carbohydrates in the 3 days preceding the test to avoid falsely elevated glucose levels 2, 3
Point-of-care A1C assays may be used in CLIA-certified settings that meet quality standards, but caution is advised when using them for diagnosis 4
FPG has high diurnal variation and requires an 8-hour fast 2
OGTT has low reproducibility despite being considered a first-line diagnostic test 2
Common Pitfalls to Avoid
Misdiagnosis of diabetes type occurs in up to 40% of adults with new type 1 diabetes who are incorrectly diagnosed as having type 2 diabetes 3, 4
Do not rely on a single abnormal test result when the patient does not have unequivocal hyperglycemia—always confirm with repeat testing 1, 3
Do not use A1C in conditions affecting red blood cell turnover, as this will produce unreliable results 2, 3
Recognize that even undiagnosed patients are at increased risk for developing macrovascular and microvascular complications 4, 5