How do I get Impaired Fasting Glucose (IFG) and Impaired Glucose Tolerance (IGT) results?

How to Obtain IFG and IGT Results

To diagnose Impaired Fasting Glucose (IFG) and Impaired Glucose Tolerance (IGT), you need to perform specific laboratory tests: either fasting plasma glucose (FPG), 2-hour oral glucose tolerance test (OGTT), or hemoglobin A1c (HbA1c). 1

Diagnostic Tests and Criteria

Fasting Plasma Glucose (FPG)

• Procedure: Patient fasts for at least 8 hours
• IFG Diagnostic Criteria:
  • American Diabetes Association (ADA): 100-125 mg/dL (5.6-6.9 mmol/L) 1
  • World Health Organization (WHO): 110-125 mg/dL (6.1-6.9 mmol/L) 1
• Best time: Morning testing is preferred as afternoon values tend to be lower 1

Oral Glucose Tolerance Test (OGTT)

• Procedure:
  • Morning test after 8-14 hour overnight fast
  • Patient consumes 75g anhydrous glucose dissolved in water
  • Blood samples taken at baseline and 2 hours after glucose consumption
  • Patient should consume a mixed diet with at least 150g carbohydrates for 3 days prior to testing 2
• IGT Diagnostic Criteria: 2-hour plasma glucose 140-199 mg/dL (7.8-11.0 mmol/L) 1

Hemoglobin A1c (HbA1c)

• Procedure: No fasting required
• Prediabetes Criteria: 5.7-6.4% 1, 2
• Advantages: More convenient than FPG or OGTT; not affected by acute changes in glucose levels due to stress or illness 1

Testing Recommendations

1. Initial Screening Approach:

   • Start with HbA1c and FPG as initial screening tests 1
   • Add OGTT if HbA1c and FPG results are inconclusive 1

2. Who Should Be Tested:

   • Adults ≥45 years of age, particularly those with BMI ≥25 kg/m² 1
   • Adults <45 years with risk factors:
     • Physical inactivity
     • First-degree relative with diabetes
     • High-risk ethnic population (African American, Latino, Native American, Asian American, Pacific Islander)
     • History of gestational diabetes or delivering a baby >9 lbs
     • Hypertension (≥140/90 mmHg or on therapy)
     • HDL cholesterol <35 mg/dL and/or triglycerides >250 mg/dL
     • Polycystic ovarian syndrome
     • Other conditions associated with insulin resistance
     • History of cardiovascular disease 1
   • Asian Americans should be considered for screening at lower BMI (≥23 kg/m²) 1

3. Testing Frequency:

   • If results are normal: Repeat testing at least every 3 years 1
   • Consider more frequent testing depending on initial results and risk status 1

Important Considerations

• Confirmation of Results: Diagnosis of IFG, IGT, or diabetes should be confirmed with repeated testing using the same test on a different day 1

• Testing Location: Testing should be carried out within healthcare settings rather than community screening to ensure appropriate follow-up and discussion of abnormal results 1

• Clinical Significance: Both IFG and IGT are risk factors for:

  • Future development of type 2 diabetes 3, 4
  • Cardiovascular disease 3, 5
  • Risk is greater when both IFG and IGT occur together 3

• Testing Limitations:

  • Different diagnostic criteria between ADA and WHO identify different at-risk populations 6
  • In acute coronary syndromes, OGTT should not be performed earlier than 4-5 days after the event to minimize false-positive results 1

• Pathophysiology: IFG and IGT share similar underlying pathophysiological mechanisms related to insulin resistance and impaired β-cell function 7

By following these testing protocols, you can accurately identify patients with IFG and IGT, allowing for early intervention to reduce the risk of progression to diabetes and cardiovascular complications.