What is the protocol for a modified glucose tolerance test?

Modified Glucose Tolerance Test Protocol

The modified glucose tolerance test protocol involves using a 75-g oral glucose tolerance test (OGTT) with measurements taken when fasting and at 1 and 2 hours after glucose consumption, as recommended by current guidelines for diagnosing diabetes and gestational diabetes mellitus.

Standard OGTT Protocol

• The test should be performed in the morning after an overnight fast of at least 8 hours 1
• For standard diabetes diagnosis, a 75-g glucose load dissolved in water is administered 1
• Blood samples are collected at baseline (fasting) and 2 hours after glucose consumption 1
• The diagnosis of diabetes is made when fasting plasma glucose is ≥126 mg/dL (7.0 mmol/L) or 2-hour plasma glucose is ≥200 mg/dL (11.1 mmol/L) 1

Modified OGTT Protocols

One-Step Strategy for Gestational Diabetes Mellitus (GDM)

• Perform a 75-g OGTT at 24-28 weeks of gestation in pregnant women not previously diagnosed with diabetes 1
• Blood samples are collected when fasting and at 1 and 2 hours after glucose consumption 1
• The diagnosis of GDM is made when any of the following plasma glucose values are met or exceeded 1:
  • Fasting: 92 mg/dL (5.1 mmol/L)
  • 1 hour: 180 mg/dL (10.0 mmol/L)
  • 2 hour: 153 mg/dL (8.5 mmol/L)

Two-Step Strategy for GDM

• Step 1: Perform a 50-g glucose load test (GLT) without fasting, with plasma glucose measurement at 1 hour, at 24-28 weeks of gestation 1
• If the 1-hour plasma glucose level is ≥130-140 mg/dL (7.2-7.8 mmol/L), proceed to a 100-g OGTT 1
• Step 2: The 100-g OGTT should be performed when the patient is fasting 1
• The diagnosis of GDM is made if at least two of the following plasma glucose levels are met or exceeded 1:
  • Fasting: 95 mg/dL (5.3 mmol/L)
  • 1 hour: 180 mg/dL (10.0 mmol/L)
  • 2 hour: 155 mg/dL (8.6 mmol/L)
  • 3 hour: 140 mg/dL (7.8 mmol/L)

Important Considerations

• The test should be performed using venous plasma samples for standardization 1
• For patients with conditions affecting red blood cell turnover (sickle cell disease, pregnancy, hemodialysis, recent blood loss/transfusion), only plasma glucose criteria should be used 1
• A1C testing should be performed using a method certified by the National Glycohemoglobin Standardization Program (NGSP) and standardized to the Diabetes Control and Complications Trial (DCCT) assay 1
• Different diagnostic criteria will identify different degrees of maternal hyperglycemia and maternal/fetal risk in pregnant women 1

Modified Solution Considerations

• Research suggests that dilution of the glucose solution may increase postprandial glycemia, particularly at intermediate time points (30,45, and 60 minutes) 2
• A more dilute, more palatable oral glucose solution can reduce side effects like nausea and vomiting, which is particularly beneficial during pregnancy screening 3
• When using a modified solution, the 1-hour glucose values may not be statistically different from standard solutions, but the absorption rate may be more rapid 3

Special Populations

• For pregnant women, screening for GDM should occur at 24-28 weeks of gestation 1
• Women with GDM should be screened for persistent diabetes at 6-12 weeks postpartum 1
• Women with a history of GDM should have lifelong screening for diabetes or prediabetes at least every 3 years 1
• The efficacy of interventions for primary prevention of type 2 diabetes has primarily been demonstrated among individuals with impaired glucose tolerance (IGT) 1

Remember that the OGTT should never be used to evaluate reactive hypoglycemia 4 and should be performed following standardized protocols to ensure accurate diagnosis and appropriate patient management.