What diagnostic studies should be ordered for a patient with suspected Type 2 Diabetes Mellitus (T2DM)?

Diagnostic Studies for Suspected Type 2 Diabetes Mellitus

The primary diagnostic tests for suspected Type 2 Diabetes Mellitus (T2DM) should include fasting plasma glucose (FPG), hemoglobin A1c (HbA1c), and an oral glucose tolerance test (OGTT) if initial tests are inconclusive. 1

Initial Diagnostic Testing

First-Line Tests

• FPG is the preferred initial screening test due to its ease of administration, convenience, patient acceptability, and lower cost 1
• HbA1c should be ordered simultaneously with FPG as part of initial screening 1
• Both tests should be performed within a healthcare setting, not as community screening 1

Diagnostic Criteria

• Diabetes is diagnosed when any of the following criteria are met:
  • FPG ≥126 mg/dL (7.0 mmol/L) 1
  • HbA1c ≥6.5% (48 mmol/mol) 1
  • 2-hour plasma glucose ≥200 mg/dL (11.1 mmol/L) during OGTT 1
  • Random plasma glucose ≥200 mg/dL (11.1 mmol/L) with symptoms of hyperglycemia 1

Confirmatory Testing

• Any positive test result should be confirmed by repeat testing on a different day to establish the diagnosis 1
• If FPG is <126 mg/dL but there is high clinical suspicion for diabetes, an OGTT should be performed 1

When to Add OGTT

• OGTT should be added when:
  • HbA1c and FPG results are inconclusive 1
  • There is high clinical suspicion despite normal FPG 1
  • Specifically testing for impaired glucose tolerance (IGT) 1
• OGTT is the gold standard for diagnosing IGT but is less convenient than FPG 1

Testing Based on Risk Factors

• Screening should begin at age 45 for all individuals 1
• Earlier or more frequent testing is recommended for those with:
  • BMI ≥25 kg/m² (or ≥23 kg/m² in Asian Americans) 1
  • Family history of diabetes in first or second-degree relatives 1
  • High-risk ethnic groups (African Americans, Hispanic Americans, Native Americans, Asian Americans, Pacific Islanders) 1
  • History of gestational diabetes or delivery of baby >9 lbs 1
  • Hypertension (≥140/90 mmHg) 1
  • HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL 1
  • Polycystic ovary syndrome 1
  • History of cardiovascular disease 1
  • Previously identified impaired fasting glucose (IFG) or impaired glucose tolerance (IGT) 1
  • Signs of insulin resistance (acanthosis nigricans) 1

Frequency of Testing

• For individuals with normal results, repeat testing at least every 3 years 1
• More frequent testing for those with additional risk factors 1
• Testing should be repeated every 2 years for children and adolescents at risk 1

Important Considerations and Pitfalls

• Laboratory measurement of plasma glucose should be performed on venous samples with enzymatic assay techniques, not capillary blood glucose meters 1
• Certain medications can cause hyperglycemia and affect test results, including glucocorticoids and nicotinic acid 1
• A normal FPG does not exclude elevated post-load glucose levels, so OGTT may detect cases missed by FPG alone 2
• Community screening outside healthcare settings is not recommended as it has not been shown to be beneficial and may cause harm 1
• In patients with acute illness or hospitalization, diagnostic testing may be affected by stress hyperglycemia and should be interpreted with caution 1

Additional Testing for Complications

• Once diabetes is diagnosed, additional testing should be considered to evaluate for complications:
  • Urine albumin-to-creatinine ratio to screen for early diabetic nephropathy 3
  • Lipid profile to assess cardiovascular risk 1
  • Basic metabolic panel to evaluate renal function 1