What are the diagnostic criteria for diabetes?

Diagnostic Criteria for Diabetes Mellitus

Diabetes mellitus is diagnosed when any of the following criteria are met on two separate occasions: Hemoglobin A1C ≥ 6.5%, Fasting Plasma Glucose (FPG) ≥ 126 mg/dL, 2-hour Plasma Glucose ≥ 200 mg/dL during OGTT, or Random Plasma Glucose ≥ 200 mg/dL with symptoms of hyperglycemia. 1

Diagnostic Tests and Criteria

Primary Diagnostic Criteria

• Fasting Plasma Glucose (FPG): ≥ 126 mg/dL (7.0 mmol/L) after no caloric intake for at least 8 hours 2, 1
• 2-hour Plasma Glucose (2-h PG): ≥ 200 mg/dL (11.1 mmol/L) during an oral glucose tolerance test (OGTT) using 75g of glucose 2, 1
• Hemoglobin A1C: ≥ 6.5% (48 mmol/mol) using a method certified by the National Glycohemoglobin Standardization Program (NGSP) and standardized to the Diabetes Control and Complications Trial (DCCT) assay 2, 1
• Random Plasma Glucose: ≥ 200 mg/dL (11.1 mmol/L) in a patient with classic symptoms of hyperglycemia or hyperglycemic crisis 2, 1

Confirmation Requirements

• In the absence of unequivocal hyperglycemia with acute metabolic decompensation, diagnosis requires two abnormal test results from:

  • The same test on two different days, OR
  • Two different tests (e.g., FPG and A1C) on the same day 2, 1

• If a single test shows "diabetic type" hyperglycemia, diabetes can be diagnosed immediately if one of these conditions is present:

  • Classic symptoms of diabetes (polyuria, polydipsia, unexplained weight loss)
  • Unequivocal diabetic retinopathy
  • Hyperglycemic crisis 2, 1

Prediabetes Diagnostic Criteria

Prediabetes (increased risk for diabetes) is diagnosed when test results fall within these ranges:

• A1C: 5.7-6.4% 1
• FPG: 100-125 mg/dL (5.6-6.9 mmol/L) - Impaired Fasting Glucose (IFG) 2, 1
• 2-h PG during OGTT: 140-199 mg/dL (7.8-11.0 mmol/L) - Impaired Glucose Tolerance (IGT) 2, 1

Special Considerations and Limitations

A1C Testing Limitations

• A1C should not be used for diagnosis in conditions that alter the relationship between A1C and glycemia:
  • Hemoglobinopathies (sickle cell disease, etc.)
  • Pregnancy (second and third trimesters)
  • Glucose-6-phosphate dehydrogenase deficiency
  • HIV infection
  • Hemodialysis
  • Recent blood loss or transfusion
  • Erythropoietin therapy 2, 1

Discordant Results

• If two different tests yield discordant results (one diagnostic, one not):
  • The test with the result above the diagnostic threshold should be repeated
  • Diagnosis is confirmed based on the repeated test 2
• Marked discordance between A1C and plasma glucose levels should raise suspicion of hemoglobin variants or A1C assay interference 2

Common Pitfalls to Avoid

• Failure to confirm diagnosis: Not repeating abnormal results when required can lead to misdiagnosis 1
• Inappropriate use of A1C: Using A1C in conditions where it's contraindicated can lead to false results 2, 1
• Overlooking preanalytical variables: Delayed processing of glucose samples can cause falsely low results due to glycolysis 1
• Relying on a single test: Different tests may not identify the same individuals with diabetes; using multiple tests may be necessary in borderline cases 2
• Ignoring symptoms: Classic symptoms with a single random glucose ≥ 200 mg/dL is diagnostic and doesn't require confirmation 2, 1

Screening Recommendations

• Testing should be considered in all adults who are overweight (BMI ≥ 25 kg/m²) with additional risk factors 2, 1
• In those without risk factors, testing should begin at age 45 years 2, 1
• If results are normal, repeat testing at minimum 3-year intervals 2, 1
• In those identified with prediabetes, assess and treat other cardiovascular risk factors 2

By following these diagnostic criteria and being aware of the potential pitfalls, clinicians can accurately diagnose diabetes mellitus and initiate appropriate management to reduce morbidity and mortality associated with this chronic disease.