75-g Oral Glucose Tolerance Test (OGTT)

75-g OGTT statistics at our clinic

See anonymized two-hour glucose proportions and comparisons of age and HbA1c. These figures do not represent rates of confirmed diabetes diagnosis.

What the test can show

A 75-g OGTT may be recommended when fasting glucose or HbA1c alone does not clearly establish a diagnosis, or when impaired glucose tolerance is suspected. It is not required for every patient.

This page describes testing in nonpregnant adults. Testing during pregnancy uses different diagnostic criteria.

The day before

• Continue your usual diet for at least three days before the test. Do not start an extreme low-carbohydrate diet.
• Finish your usual dinner by 9:00 p.m.
• After 9:00 p.m., do not eat or consume caloric drinks, candy or gum.
• Avoid alcohol and strenuous exercise.
• A small amount of plain water is allowed.

The morning of the test

• Do not eat breakfast. Please arrive by the instructed time.
• Do not drink tea, coffee, milk, juice or sports drinks. Plain water is allowed in a small amount.
• Do not smoke until the test is complete.
• Do not stop prescribed medication on your own. Follow the instructions given when booking.
• Tell staff if you use diabetes medication or insulin, or if you are unwell or have a fever.

Test-day schedule

A fasting blood sample is usually taken at 9:00 a.m. You then drink the glucose solution; completion of the drink is time zero. Five blood samples are taken in total, and the visit takes approximately two and a half hours.

• Before the drink: blood glucose and insulin are measured.
• 30 minutes: blood glucose and insulin are measured to assess the early insulin response.
• 60 minutes: blood glucose is measured.
• 90 minutes: blood glucose is measured to help show the shape of the glucose curve. This is an additional measurement used by our clinic and is not a required diagnostic time point.
• 120 minutes: blood glucose is measured for the standard diagnostic interpretation.

During the test

• Please remain seated and avoid unnecessary walking or exercise.
• Do not eat, smoke or drink anything other than the water permitted by staff.
• Tell staff immediately if you feel nauseated, dizzy, sweaty, faint or otherwise unwell.
• If you vomit, the test may need to be stopped and rescheduled.

How results are interpreted

Under the Japanese criteria used by our clinic, a normal pattern requires both fasting plasma glucose below 110 mg/dL and the 120-minute value below 140 mg/dL. A fasting value of 126 mg/dL or higher, or a 120-minute value of 200 mg/dL or higher, is in the diabetic range. Results between these ranges are classified as borderline.

A value in the diabetic range does not always establish the diagnosis from a single OGTT. HbA1c, symptoms, previous results and confirmatory testing may also be required.

Fasting and 30-minute insulin values help us assess the early insulin response. The 60- and 90-minute glucose values help describe the glucose curve, but the standard diagnostic classification mainly uses the fasting and 120-minute glucose values.

Frequently asked questions

• How long does it take? The final sample is taken about two hours after the drink. Please allow approximately two and a half hours for reception, preparation and the final condition check.
• Can I drink water? A small amount of plain water is allowed; other drinks can affect the result.
• Why is there a 90-minute sample? It helps show whether glucose is still rising or is falling toward the 120-minute value. It is supplementary and not a required diagnostic point.
• Should I take my usual medication? Instructions differ by medicine. Do not stop treatment without speaking with us first.

English references

The preparation and diagnostic explanation on this page was checked against the following English-language clinical sources.