What are the recommended diagnostic tests for type 2 diabetes?

Diagnostic Tests for Type 2 Diabetes

The primary diagnostic tests for type 2 diabetes are fasting plasma glucose (FPG), 2-hour plasma glucose during oral glucose tolerance test (OGTT), and hemoglobin A1C, with FPG being the preferred initial screening test due to its convenience, lower cost, and patient acceptability. 1

Diagnostic Criteria for Type 2 Diabetes

Any one of the following criteria confirms diabetes diagnosis (requires confirmation with a second test on a different day unless symptoms are present with unequivocal hyperglycemia):

• Fasting Plasma Glucose (FPG): ≥126 mg/dL (7.0 mmol/L) after no caloric intake for at least 8 hours
• 2-hour Plasma Glucose (2-h PG): ≥200 mg/dL (11.1 mmol/L) during OGTT using 75g glucose load
• Hemoglobin A1C: ≥6.5% (using NGSP-certified method standardized to DCCT assay)
• Random Plasma Glucose: ≥200 mg/dL (11.1 mmol/L) in patients with classic symptoms of hyperglycemia 1

Test Selection Algorithm

1. First-line test: Fasting Plasma Glucose (FPG)

   • Preferred for initial screening due to ease of administration, convenience, patient acceptability, and lower cost 1, 2
   • Requires 8-hour fast
   • Interpretation:
     • Normal: <100 mg/dL
     • Impaired Fasting Glucose (IFG): 100-125 mg/dL
     • Diabetes: ≥126 mg/dL

2. Second-line test: Hemoglobin A1C

   • Advantages: No fasting required, reflects average glycemia over 2-3 months, less day-to-day variability
   • Must use NGSP-certified method standardized to DCCT assay
   • Interpretation:
     • Normal: <5.7%
     • Prediabetes: 5.7-6.4%
     • Diabetes: ≥6.5%
   • Caution: Not valid in conditions with abnormal red blood cell turnover (hemoglobinopathies, pregnancy, recent blood loss/transfusion, hemolysis, hemodialysis) 1

3. Third-line test: Oral Glucose Tolerance Test (OGTT)

   • Indicated when:
     • FPG is normal but suspicion for diabetes remains high
     • Evaluating for gestational diabetes
     • Diagnosing impaired glucose tolerance
   • Procedure: 75g glucose load with plasma glucose measurement at baseline and 2 hours
   • Interpretation:
     • Normal: 2-h PG <140 mg/dL
     • Impaired Glucose Tolerance (IGT): 2-h PG 140-199 mg/dL
     • Diabetes: 2-h PG ≥200 mg/dL

Confirmation Requirements

• In the absence of unequivocal hyperglycemia with acute metabolic decompensation, diagnosis requires confirmation with a second test, preferably the same test repeated on a different day 1
• When results from two different tests are available and both exceed diagnostic thresholds, diagnosis is confirmed
• When results are discordant, the test with result above diagnostic threshold should be repeated 1

Screening Recommendations

• Who to screen:

  • Adults aged ≥45 years, especially with BMI ≥25 kg/m² (≥23 kg/m² for Asian Americans)
  • Adults of any age with BMI ≥25 kg/m² (≥23 kg/m² for Asian Americans) plus one or more additional risk factors:
    • First-degree relative with diabetes
    • High-risk race/ethnicity (African American, Latino, Native American, Asian American, Pacific Islander)
    • History of cardiovascular disease
    • Hypertension (≥140/90 mmHg or on therapy)
    • HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL
    • Physical inactivity
    • Polycystic ovary syndrome
    • History of gestational diabetes or delivery of baby >9 lbs
    • Previous prediabetes diagnosis (A1C ≥5.7%, IGT, or IFG) 1, 2

• Screening frequency:

  • Every 3 years if results normal
  • Annually if prediabetes detected 2

Common Pitfalls and Caveats

1. A1C limitations:

   • Not valid in hemoglobinopathies, pregnancy, recent blood loss/transfusion, hemolysis
   • May underestimate glycemia in some ethnicities
   • Marked discordance between A1C and plasma glucose should prompt consideration of hemoglobin variants 1

2. FPG limitations:

   • Requires proper fasting (8 hours)
   • Day-to-day variability
   • May miss some cases of diabetes that would be detected by OGTT 3

3. OGTT limitations:

   • Time-consuming and inconvenient
   • Greater intraindividual variability
   • Affected by acute illness, medications, activity, and diet

4. Medication interference:

   • Glucocorticoids and nicotinic acid can produce hyperglycemia and affect test results 1

5. Community screening outside healthcare settings:

   • Not recommended due to lack of evidence for benefit and potential harm 1

6. Optimal cutoffs:

   • Recent research suggests optimal cutoffs for screening may be lower than diagnostic thresholds (A1C ~6.0%, FPG ~104 mg/dL) 3

7. Combined testing:

   • Using both FPG and A1C may improve prediction of future diabetes 4, 5