Oral glucose tolerance test (OGTT)

Principle

• Measures the body's ability to clear a standard oral glucose load; used when fasting glucose is borderline or to diagnose gestational diabetes.

Specimen

• Venous plasma in fluoride-oxalate tubes, collected fasting and at timed intervals after the load.

Procedure

• Patient eats an unrestricted diet (at least 150 g carbohydrate/day) for 3 days before the test.
• Overnight fast of 8–14 hours; water allowed.
• Collect fasting sample; if fasting glucose is already clearly diabetic, the load is usually not given.
• Give 75 g anhydrous glucose (82.5 g monohydrate) dissolved in 250–300 mL water, drunk within 5 minutes.
• Children: 1.75 g/kg body weight, maximum 75 g.
• Patient stays seated, no smoking, eating or exercise; collect a 2-hour sample (WHO/ADA standard test).

Interpretation

• Normal (non-pregnant): 2-hour <140 mg/dL (7.8 mmol/L).
• Impaired glucose tolerance: 2-hour 140–199 mg/dL (7.8–11.0 mmol/L).
• Diabetes: fasting ≥126 mg/dL (7.0 mmol/L) or 2-hour ≥200 mg/dL (11.1 mmol/L).
• Gestational diabetes, one-step 75 g test at 24–28 weeks (IADPSG/WHO 2013/ADA): any one of 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 approach (ADA alternative): 50 g non-fasting screen, then 100 g 3-hour OGTT if the screen is positive.

Quality control and pitfalls

• Acute illness, stress, prolonged bed rest, recent carbohydrate restriction and drugs (glucocorticoids, thiazides) impair tolerance.
• Vomiting after the drink invalidates the test.
• Delayed separation without fluoride gives falsely low glucose.

Clinical use

• Diagnosis of diabetes, impaired glucose tolerance and gestational diabetes.