How is type 2 Diabetes Mellitus (DM) diagnosed?

Diagnosis of Type 2 Diabetes Mellitus

Type 2 diabetes is diagnosed when a patient has a fasting plasma glucose ≥126 mg/dL (7.0 mmol/L), or a 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during an oral glucose tolerance test, or an A1C ≥6.5% (48 mmol/mol), with any abnormal test confirmed by repeat testing on a subsequent day. 1

Diagnostic Criteria

• Fasting plasma glucose (FPG) ≥126 mg/dL (7.0 mmol/L) after at least an 8-hour fast 1
• 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during a 75-g oral glucose tolerance test (OGTT) 1
• A1C ≥6.5% (48 mmol/mol) using a method certified by the National Glycohemoglobin Standardization Program (NGSP) 1
• Random plasma glucose ≥200 mg/dL (11.1 mmol/L) in a patient with classic symptoms of hyperglycemia (polyuria, polydipsia, weight loss, blurred vision, fatigue) 1

Confirmation Requirements

• In the absence of unequivocal hyperglycemia with acute metabolic decompensation, results should be confirmed by repeat testing on a subsequent day 1
• A single random plasma glucose level of ≥200 mg/dL with typical symptoms of hyperglycemia is sufficient for diagnosis without confirmation 1

Risk Assessment

The American Diabetes Association (ADA) recommends screening for type 2 diabetes in:

• Adults aged 35 years and older 1
• Adults with BMI ≥25 kg/m² (or ≥23 kg/m² in Asian Americans) who have one or more risk factors 1:
  • First-degree relative with diabetes 1
  • High-risk race/ethnicity (African American, Latino, Native American, Asian American, Pacific Islander) 1
  • History of cardiovascular disease 1
  • Hypertension (≥140/90 mmHg or on therapy) 1
  • HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL 1
  • Women with polycystic ovary syndrome 1
  • Physical inactivity 1
  • Other conditions associated with insulin resistance (severe obesity, acanthosis nigricans) 1

Screening Tests

• The ADA Risk Test can be used to identify individuals at high risk (score ≥5) who should undergo diagnostic testing 1
• For asymptomatic individuals, screening should be repeated every 3 years if results are normal, or annually in those with prediabetes 1

Prediabetes Identification

Prediabetes is diagnosed when test results fall within these ranges:

• FPG 100-125 mg/dL (5.6-6.9 mmol/L) - impaired fasting glucose (IFG) 1
• 2-h PG during OGTT 140-199 mg/dL (7.8-11.0 mmol/L) - impaired glucose tolerance (IGT) 1
• A1C 5.7-6.4% (39-47 mmol/mol) 1

Special Considerations

• In patients without traditional risk factors for type 2 diabetes and/or of younger age, consider islet autoantibody testing (e.g., GAD65 autoantibodies) to exclude type 1 diabetes 1
• Asian Americans have increased diabetes risk at lower BMI levels (approximately 15 pounds lower than the general population) 1
• Women with history of gestational diabetes should have lifelong testing at least every 3 years 1

Common Pitfalls to Avoid

• Relying on a single test without confirmation (except in cases with classic symptoms and random glucose ≥200 mg/dL) 1
• Failing to recognize that type 2 diabetes frequently goes undiagnosed for many years because hyperglycemia develops gradually 1
• Not considering alternative diagnoses like type 1 diabetes, especially in younger patients or those without typical risk factors 1
• Overlooking the need for more frequent screening in high-risk individuals 1
• Not accounting for conditions that may affect A1C accuracy (hemoglobinopathies, anemia, recent blood transfusion) 1

Diagnostic Algorithm

1. Assess risk factors and symptoms
2. Perform one of the diagnostic tests (FPG, OGTT, A1C, or random glucose if symptomatic)
3. If results are abnormal, confirm with repeat testing on a different day (unless patient has classic symptoms and random glucose ≥200 mg/dL)
4. If confirmed, diagnose type 2 diabetes and begin appropriate management
5. If results indicate prediabetes, implement preventive measures and rescreen annually 1

Type 2 diabetes accounts for 90-95% of all diabetes cases and is characterized by insulin resistance and relative (rather than absolute) insulin deficiency 1. Early diagnosis is crucial as even undiagnosed patients are at increased risk of developing macrovascular and microvascular complications 1.