What are the diagnostic criteria for diabetes mellitus?

Diagnostic Criteria for Diabetes Mellitus

Diabetes mellitus is diagnosed when any one of four criteria is met: HbA1c ≥6.5% (using a certified laboratory method), fasting plasma glucose ≥126 mg/dL (7.0 mmol/L), 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during an oral glucose tolerance test, or random plasma glucose ≥200 mg/dL (11.1 mmol/L) in a patient with classic symptoms of hyperglycemia. 1

The Four Diagnostic Pathways

Any single criterion is sufficient for diagnosis, but confirmation requirements differ:

1. HbA1c ≥6.5% (≥48 mmol/mol)

• Must be performed in an NGSP-certified laboratory standardized to the DCCT assay 2, 1
• Point-of-care assays should NOT be used for diagnosis 2
• Do not use HbA1c in conditions affecting red blood cell turnover: pregnancy, hemolytic anemia, sickle cell disease, G6PD deficiency, hemodialysis, recent blood loss or transfusion, erythropoietin therapy, or HIV patients on certain medications 1, 3

2. Fasting Plasma Glucose ≥126 mg/dL (≥7.0 mmol/L)

• Fasting is defined as no caloric intake for at least 8 hours 2, 1, 3
• Samples must be centrifuged and separated immediately after collection to prevent glycolysis, which causes falsely low results 1, 3

3. 2-Hour Plasma Glucose ≥200 mg/dL (≥11.1 mmol/L) During OGTT

• Use 75 grams of anhydrous glucose dissolved in water 2, 4, 1
• Patient must consume at least 150 grams of carbohydrates daily for 3 days before testing to avoid falsely elevated results 4, 1
• Measuring glucose at 30 and 60 minutes is recommended, as values >180 mg/dL at 1 hour indicate higher risk for developing diabetes 4

4. Random Plasma Glucose ≥200 mg/dL (≥11.1 mmol/L) with Symptoms

• Classic symptoms include polyuria, polydipsia, and unexplained weight loss 2, 1
• "Random" means any time of day without regard to meals 2, 1

Confirmation Requirements

In the absence of unequivocal hyperglycemia (symptoms with glucose ≥200 mg/dL), two abnormal test results are required to confirm diabetes. 2, 1 This can be accomplished by:

• Two abnormal results from the same sample (e.g., both FPG ≥126 mg/dL AND HbA1c ≥6.5%) 1
• Two abnormal results from separate samples on different days 2, 1

Exception: A single abnormal test is sufficient if any of the following are present:

• Classic symptoms of hyperglycemia 1
• HbA1c ≥6.5% 1
• Unequivocal diabetic retinopathy 1

Critical Pitfalls to Avoid

Discordant results: When different tests give conflicting results, repeat the test that exceeded the diagnostic threshold and establish diagnosis based on the confirmed abnormal test 1, 3

OGTT reproducibility: The oral glucose tolerance test has lower reproducibility than other methods; discordant results should be repeated 4, 1

Sample handling: Failure to immediately centrifuge and separate plasma samples is a common source of error, leading to falsely low glucose values 1, 3

HbA1c limitations: While HbA1c is convenient, it should not be used as the sole diagnostic test in conditions with altered red blood cell turnover, where glucose-based criteria must be used exclusively 1, 3

Pre-Diabetes Categories

For completeness, intermediate hyperglycemia categories include:

• Impaired Fasting Glucose (IFG): FPG 100-125 mg/dL (5.6-6.9 mmol/L) 2
• Impaired Glucose Tolerance (IGT): 2-hour glucose 140-199 mg/dL (7.8-11.0 mmol/L) during OGTT 2

Both categories represent increased risk for progression to diabetes and cardiovascular disease 2