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Glucose tolerance test (OGTT)
The 75 g glucose challenge: detects impaired glucose tolerance and diabetes when fasting glucose is still normal. Standard for gestational diabetes.
Other names: Oral glucose tolerance test, OGTT, 75 g glucose loadSection: Blood chemistry
Updated: August 31, 2026·MedProtect editorial team Reference information — not a diagnosis or a prescription
In shortThe OGTT measures glucose fasting, then 1 and 2 hours after a 75 g glucose drink. Normal at 2 hours is below 140 mg/dL (7.8 mmol/L); 140–199 mg/dL (7.8–11.0 mmol/L) means impaired tolerance; 200 mg/dL (11.1 mmol/L) and above means diabetes. It is the standard test for gestational diabetes at 24–28 weeks.
Reference ranges
Group
Reference range
Fasting — typical range (international reference, not a national standard)
60–99 mg/dL (3.3–5.5 mmol/L)
1 hour — typical range (international reference, not a national standard)
< 200 mg/dL (< 11.1 mmol/L)
2 hours — typical range (international reference, not a national standard)
< 140 mg/dL (< 7.8 mmol/L)
Impaired tolerance (2 h) — typical range (international reference, not a national standard)
140–199 mg/dL (7.8–11.0 mmol/L)
Diabetes (2 h) — typical range (international reference, not a national standard)
≥ 200 mg/dL (≥ 11.1 mmol/L)
Thresholds from the MedlinePlus Medical Encyclopedia (international reference, not a national standard). Pregnancy uses separate, stricter criteria set by the obstetrician. Reference ranges vary between laboratories and assays — use the range printed on your own lab report.
Check your value
Enter the number from your lab report — see how it compares with the typical range. Mind the units shown in the chosen row.
A rough comparison with a typical range, not a diagnosis. Reference ranges vary between laboratories — check the range on your own report and let a clinician interpret the result.
What this marker is
Some people keep normal fasting glucose but fail the challenge — the 2-hour value unmasks diabetes that fasting glucose and even HbA1c miss.
Three days of normal carbohydrate eating precede the test; during it, eating, smoking, and walking around are not allowed, because all three distort the curve.
When it is high
Impaired glucose tolerance (prediabetes)An elevated 2-hour value with normal fasting glucose — an early stage of carbohydrate metabolism disturbance.
Type 2 diabetesA diagnostic 2-hour value of 200 mg/dL (11.1 mmol/L) or more is confirmed by a repeat test or HbA1c.
Gestational diabetesPregnancy applies separate thresholds; exceeding even one point requires follow-up.
When it is low
Reactive hypoglycemiaIn some people glucose drops below baseline after the load, with symptoms — discussed with the provider.
What can skew the result
These factors can shift the value without any disease — worth considering before a retest:
Carbohydrate intake in the previous 3 daysMovement and smoking during the testMedicines (steroids, diuretics)
How to prepare
Three days of normal carbohydrate-rich eating; fasting 8–12 hours; during the test — no eating, smoking, or active movement.
When a retest makes sense
A diagnostic result is confirmed with a repeat test on another day or with HbA1c.
Who usually reviews this result
A result outside the reference range is usually reviewed by: an endocrinologist; your GP or primary care doctor; an obstetrician-gynecologist in pregnancy. Bring your lab report and any previous results to the appointment.
Names used on lab reports
Your report may use any of these names: OGTTGlucose tolerance test75 g glucose testOral glucose tolerance test
Units and conversion
Measured in: mmol/L
mmol/L=mg/dL
mg/dL = mmol/L × 18. Results are rounded to 2 decimal places.
Several markers on your report?
See how MedProtect explains results in the app: the whole report, a check of the recognized values, and a clear breakdown.
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