Glucose Tolerance Tests

GTT Basics - Sweet Preamble

• Purpose: Assesses glucose metabolism; diagnoses Diabetes Mellitus (DM), Gestational DM (GDM), Impaired Glucose Tolerance (IGT).
• Principle: Measures blood glucose response to an oral glucose load after fasting.
• Patient Prep:
  • Unrestricted diet (≥150g CHO/day) for 3 days.
  • Overnight fast (8-14 hours).
  • No smoking; minimal activity during test.
  • Hold interfering drugs.

⭐ Standard adult oral glucose load: 75g anhydrous glucose in 250-300 ml water, consumed in 5 min_

OGTT Protocol - The Sugar Sip

• Patient Preparation:

  • Ensure overnight fast (8-12 hours).
  • Maintain unrestricted carbohydrate diet (≥150g/day) for 3 days prior.
  • Avoid smoking, strenuous exercise before and during test.

• Glucose Load & Administration:

  • Adults: 75g anhydrous glucose (or 82.5g glucose monohydrate) in 250-300ml water, consumed over 5 minutes.
  • Children: 1.75g/kg body weight (maximum 75g).
  • Pregnancy (DIPSI single-step): 75g glucose, 2-hr plasma glucose. Can be non-fasting.

• Test Steps: 📌 Sip Sugar, Sit Still, Sample Sharp (timed).

• Sample: Venous plasma glucose.

⭐ The standard glucose load for OGTT in non-pregnant adults and for GDM screening (WHO/DIPSI) is 75g oral anhydrous glucose. For children, it's 1.75g/kg (max 75g).

Result Interpretation - Decoding Sweetness

OGTT (75g glucose load) values in mg/dL:

• DM by OGTT: FPG ≥ 126 mg/dL OR 2-h PG ≥ 200 mg/dL.
• Confirmation: Usually 2 abnormal results needed if asymptomatic (e.g., repeat FPG, or FPG + 2hPG from same OGTT).

⭐ High-Yield Fact: Isolated IGT (Normal FPG, 2-h PG 140-199 mg/dL) often indicates more severe insulin resistance and β-cell dysfunction than isolated IFG, thus carrying a higher risk for cardiovascular disease (CVD) and progression to overt diabetes.

GDM Screening - Baby's Sugar Watch

• Universal screening for all pregnant women.
• Method: 75g oral glucose (non-fasting), per DIPSI.
• Test: Plasma glucose at 2 hours.
• Diagnosis: Value ≥140 mg/dL.
• Timing: 1st visit; if negative & early, re-screen at 24-28 weeks.

  ⭐ India primarily uses the single-step DIPSI 75g OGTT, simplifying GDM diagnosis.

Influencing Factors & Caveats - Sweet Spoilers

• Diet: Ensure adequate carbohydrate intake (≥150g/day for 3 days prior).
• Medications: Corticosteroids, thiazides, β-blockers, OCPs can ↑glucose.
• Illness/Stress: Acute illness, infection, or trauma can ↑glucose.
• Activity: Avoid strenuous exercise before the test.
• Fasting: Strict 8-14 hours overnight fast crucial.
• ⚠️ Test unreliable in acutely ill or hospitalized patients.

⭐ Certain drugs, notably corticosteroids and thiazide diuretics, can significantly impair glucose tolerance, potentially leading to misinterpretation of GTT results if not considered.

High‑Yield Points - ⚡ Biggest Takeaways

• OGTT is crucial for diagnosing Gestational Diabetes Mellitus (GDM).
• Standard adult dose: 75g anhydrous glucose; children: 1.75 g/kg (max 75g).
• Diabetes diagnosis: FPG ≥ 126 mg/dL or 2-hr PG ≥ 200 mg/dL post-OGTT.
• GDM (WHO, 75g OGTT): FPG ≥ 92, 1-hr ≥ 180, or 2-hr ≥ 153 mg/dL.
• Impaired Glucose Tolerance (IGT): 2-hr PG 140-199 mg/dL.
• Patient prep: 3-day unrestricted carb diet (≥150g/day), 8-14 hr fast crucial for accuracy.