The essentials

An oral glucose tolerance test (OGTT) measures your blood sugar before and after a glucose drink. Outside pregnancy, the 2-hour value matters most: below 140 mg/dL is normal, 140 to 199 mg/dL is prediabetes, 200 mg/dL or more suggests diabetes, to be confirmed. In pregnancy, thresholds are lower and one high value can be enough.

How do I read my glucose tolerance test results?

Your report shows one glucose line per blood draw, with the time since the drink. Here is a fictitious US report of a standard 2-hour, 75 g test in an adult who is not pregnant.

Fictitious example of a lab report line (United States format)
TestResultReference rangeFlag
Glucose, fasting (before drink)96 mg/dL70-99-
Glucose, 1 hour after 75 g172 mg/dLNot established-
Glucose, 2 hours after 75 g158 mg/dL<140H
  1. Fasting line: 96 mg/dL is normal (60 to 99 mg/dL)[2]; it reads like a fasting blood sugar test.
  2. "75 g": the glucose dose. It decides which thresholds apply (50 g and 100 g are pregnancy tests)[1].
  3. 1-hour line: often printed without a range, because US criteria for adults use the 2-hour value[1].
  4. 2-hour line, flag H: 140 to 199 mg/dL is impaired glucose tolerance, a form of prediabetes, not diabetes[1].

Three glucose values, a 75 g or a 100 g test, one flag? BloodSense reads your whole report line by line and helps you prepare your questions for your doctor.

Get my OGTT result explained

What is an oral glucose tolerance test used for?

After the drink, insulin should bring your blood sugar back down within about two hours. The test shows how well that works. It is used to:

  • Diagnose prediabetes and type 2 diabetes: the 2-hour value finds more cases than fasting glucose or an HbA1c test[1].
  • Screen for gestational diabetes between 24 and 28 weeks of pregnancy[1][6].
  • Check for diabetes after a transplant or in cystic fibrosis, where it is the preferred test[1].

What are normal OGTT values?

Thresholds depend on the dose and on pregnancy. Your laboratory's range prevails.

75 g OGTT in adults who are not pregnant (venous plasma glucose)
TimeNormalPrediabetesDiabetesSource
FastingBelow 100 mg/dL (5.6 mmol/L)100-125 mg/dL126 mg/dL (7.0 mmol/L) or moreADA 2026[1]
1 hourBelow 155 mg/dL (8.6 mmol/L)155-208 mg/dL209 mg/dL (11.6 mmol/L) or moreIDF 2024[8]
2 hoursBelow 140 mg/dL (7.8 mmol/L)140-199 mg/dL200 mg/dL (11.1 mmol/L) or moreADA 2026[1]

The 1-hour cut points were proposed in 2024 by the International Diabetes Federation, because this value predicts later diabetes well[8]. US criteria do not use them yet[1]. For other times of day, see our normal blood sugar levels chart.

OGTT in pregnancy: when a value counts as high
TestFasting1 hour2 hours3 hoursDiagnosis
One-step, 75 g (fasting)92 mg/dL180 mg/dL153 mg/dL-One value at or above the threshold[1][5]
Two-step, step 1: 50 g (not fasting)-130, 135 or 140 mg/dL, depending on the practice--Above it: go on to step 2[1]
Two-step, step 2: 100 g (fasting)95 mg/dL180 mg/dL155 mg/dL140 mg/dLTwo values or more; one may be enough for ACOG[1]

Because one value is enough, the one-step test finds more cases: 11.5% of pregnant people against 4.9% with two-step screening in a US review[1]. Both approaches are used in the United States[1][7].

What does a high glucose tolerance test mean?

Outside pregnancy, one high result is not a diagnosis: without clear symptoms, a second abnormal test is needed[1][4]. From the most frequent to the rarest:

  • Common Prediabetes: a 2-hour value of 140 to 199 mg/dL means a higher risk of diabetes later[2]. It is often linked to insulin resistance.
  • Common Gestational diabetes: it affects 5% to 9% of US pregnancies each year[9] and raises the risk of preeclampsia, a large baby and newborn low blood sugar[6].
  • Common Type 2 diabetes: a 2-hour value of 200 mg/dL or more, once confirmed[1].
  • Possible Test conditions: few carbohydrates in the days before, illness, acute stress, smoking or some medicines[1][2]; also surgery, a heart attack or a stroke[2].
  • Rare Hormone or pancreas conditions: an overactive thyroid, pancreatitis, or rare tumors such as Cushing syndrome or acromegaly[10].

A falsely low value can come from a sample left too long before processing, as cells keep using glucose in the tube[1]. Truly low values can follow weight loss surgery[10]; if you feel shaky or sweaty, tell the staff (see low blood sugar symptoms).

After gestational diabetes, about half of women go on to develop type 2 diabetes[9]. That is why a 75 g test is advised 4 to 12 weeks after birth, then screening every 1 to 3 years for life[1].

How do I prepare for a glucose tolerance test, and what can skew it?

The test is done in the morning. You drink the solution within five minutes, then stay at the lab for 2 hours (75 g) or 3 hours (100 g)[1][3][4]. Nausea or dizziness can occur; serious side effects are rare[4]. Check:

  • Eat normally for 3 days, with at least 150 g of carbohydrates a day: cutting them can falsely raise the result[1].
  • Fast for at least 8 hours (8 to 14 hours in pregnancy); sips of water are allowed[1][3]. The 50 g pregnancy screen needs no fasting[3].
  • If you are sick or under acute stress, ask whether to postpone: both change the result[1].
  • Do not smoke, eat or exercise during the test[1][4].
  • Bring your medicine list: some change blood sugar. Never stop one on your own[2][4].
  • If you vomit the very sweet drink, tell the staff: you may not be able to finish the test that day[12].

See also fasting before a blood test and what to expect at a blood draw.

When should I see a doctor about my OGTT result?

When to see a doctor

  • Emergency: call 911 for confusion, fainting, a seizure, or very high blood sugar with vomiting, deep fast breathing or extreme drowsiness.
  • Within 24 to 48 hours if you are pregnant with a high value, or for 200 mg/dL or more with strong thirst, frequent urination or weight loss.
  • At your next appointment for a 2-hour value between 140 and 199 mg/dL without symptoms, to plan a confirmation test and lifestyle steps.

Questions to ask your doctor

  1. Which thresholds did you use for my test: 75 g, 50 g or 100 g?
  2. Do I need a second test to confirm this result, and which one?
  3. Could my diet, an illness or a medicine have affected the result?
  4. If I had gestational diabetes: when is my test after the birth?

Frequently asked questions

Can I go to work after the test?

Usually yes. You can most often leave right after the last draw and go back to your normal activities[12]. Plan for 2 to 3 hours at the lab first[4].

Is an OGTT better than an HbA1c test?

Neither is better in every case. HbA1c reflects 2 to 3 months, without fasting; the OGTT is more sensitive but varies more from day to day[1].

Can I do an oral glucose tolerance test at home?

No. The test needs timed blood draws handled quickly by a laboratory, because glucose falls in a sample that waits[1]. A home meter can help track your sugar, not replace the test.

  1. American Diabetes Association Professional Practice Committee. 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes-2026. Diabetes Care. 2026;49(Suppl 1):S27-S49. doi:10.2337/dc26-S002 (accessed October 31, 2026)
  2. MedlinePlus Medical Encyclopedia. Glucose tolerance test - non-pregnant. US National Library of Medicine. medlineplus.gov (accessed October 31, 2026)
  3. MedlinePlus Medical Encyclopedia. Glucose screening tests during pregnancy. US National Library of Medicine. medlineplus.gov (accessed October 31, 2026)
  4. Cleveland Clinic. Glucose Tolerance Test (GTT): What It Is, Preparation & Results. my.clevelandclinic.org (accessed October 31, 2026)
  5. Metzger BE, Gabbe SG, Persson B, et al. International association of diabetes and pregnancy study groups recommendations on the diagnosis and classification of hyperglycemia in pregnancy. Diabetes Care. 2010;33(3):676-682. doi:10.2337/dc09-1848 (accessed October 31, 2026)
  6. Davidson KW, Barry MJ, Mangione CM, et al. Screening for Gestational Diabetes: US Preventive Services Task Force Recommendation Statement. JAMA. 2021;326(6):531-538. doi:10.1001/jama.2021.11922 (accessed October 31, 2026)
  7. American College of Obstetricians and Gynecologists. ACOG Practice Bulletin No. 190: Gestational Diabetes Mellitus. Obstet Gynecol. 2018;131(2):e49-e64. doi:10.1097/AOG.0000000000002501 (accessed October 31, 2026)
  8. Bergman M, Manco M, Satman I, et al. International Diabetes Federation Position Statement on the 1-hour post-load plasma glucose for the diagnosis of intermediate hyperglycaemia and type 2 diabetes. Diabetes Res Clin Pract. 2024;209:111589. doi:10.1016/j.diabres.2024.111589 (accessed October 31, 2026)
  9. Centers for Disease Control and Prevention. Gestational Diabetes. cdc.gov (accessed October 31, 2026)
  10. MedlinePlus Medical Encyclopedia. Blood sugar test - blood. US National Library of Medicine. medlineplus.gov (accessed October 31, 2026)
  11. HealthCare.gov. Preventive care benefits for women. healthcare.gov (accessed October 31, 2026)
  12. MyHealth Alberta. Oral Glucose Tolerance Test During Pregnancy: About This Test. Alberta Health Services. myhealth.alberta.ca (accessed October 31, 2026)

This page is general information. It does not replace the advice of a doctor, who interprets your results with your symptoms, history and examination.