What is the typical interval between measurements for a Glucose Tolerance Test (GTT)?

Glucose Tolerance Test Measurement Intervals

The standard interval for a Glucose Tolerance Test (GTT) is fasting, 1 hour, 2 hours, and 3 hours (for the 100g test), with measurements taken at these specific time points after glucose load administration. 1

One-Step Strategy (75g OGTT)

• Measurements are taken at three time points: fasting, 1 hour, and 2 hours after a 75g glucose load 1
• The test should be performed in the morning after an overnight fast of at least 8 hours 1
• Diagnosis of gestational diabetes mellitus (GDM) is made when any of these values are met or exceeded:
  • Fasting: 92 mg/dL (5.1 mmol/L) 1
  • 1 hour: 180 mg/dL (10.0 mmol/L) 1
  • 2 hours: 153 mg/dL (8.5 mmol/L) 1

Two-Step Strategy

• Step 1: 50g glucose load test (GLT) with measurement at 1 hour only (nonfasting) 1
• Step 2: If 1-hour value is elevated (≥130,135, or 140 mg/dL depending on protocol), proceed to 100g OGTT with measurements at four time points 1:
  • Fasting
  • 1 hour
  • 2 hours
  • 3 hours

Important Considerations for GTT Timing

• Exact timing of sample collection is critical for accurate interpretation 2
• Studies show glucose values can fluctuate significantly within a 10-minute window around the standard collection time 2
• Two patients in one study had 60-minute values >140 mg/dL but 50 or 70-minute values <140 mg/dL, demonstrating how timing affects results 2

Diagnostic Criteria for 100g OGTT (Carpenter-Coustan)

• Fasting: 95 mg/dL (5.3 mmol/L) 1
• 1 hour: 180 mg/dL (10.0 mmol/L) 1
• 2 hour: 155 mg/dL (8.6 mmol/L) 1
• 3 hour: 140 mg/dL (7.8 mmol/L) 1

Potential for Shortened Testing

• Some research suggests that the 3-hour measurement may not be necessary for diagnosing GDM 3
• A study comparing 2-hour versus 3-hour 100g GTT found that omitting the third hour would have changed the diagnosis in only 2% of cases 3
• However, current guidelines still recommend the complete protocol with all time points 1

Clinical Pearls

• The 75g OGTT is considered the reference standard for diabetes diagnosis 1
• For non-pregnant adults, the 2-hour OGTT (75g) is sufficient with measurements at fasting and 2 hours 1, 4
• For gestational diabetes screening, the full protocol (either one-step or two-step) is recommended at 24-28 weeks gestation 1
• Accurate timing is crucial - even 10 minutes' difference can affect results and potentially lead to misdiagnosis 2