The 3-Hour Glucose Test: Prep, Steps and Cut-offs
The 3-hour test uses a 100 gram glucose drink and four blood samples: fasting, then at 1, 2 and 3 hours. Two or more values at or above the cut-offs meet the criteria for gestational diabetes. The cut-offs depend on which set your clinic uses. Both are shown below.
Cut-offs: Carpenter-Coustan and NDDG
| Sample | Carpenter-Coustan mg/dL (mmol/L) | NDDG mg/dL (mmol/L) |
|---|---|---|
| Fasting | 95 (5.3) | 105 (5.8) |
| 1 hour | 180 (10.0) | 190 (10.5) |
| 2 hours | 155 (8.6) | 165 (9.2) |
| 3 hours | 140 (7.8) | 145 (8.0) |
Values are “met or exceeded”. Two or more of the four meets the criteria in both sets. ACOG lists both as acceptable. NDDG stands for the National Diabetes Data Group.
Compare your values
Nothing you type is stored or sent. This compares with published numbers; it is not a diagnosis.
Preparing
- Fast first. The first sample is fasting, so you eat nothing beforehand. Your clinic will give the number of hours and tell you if water is allowed.
- Eat normally in the days before unless your clinic says otherwise. Ask if they want you to include carbohydrate at meals.
- Plan for about four hours. Bring a snack for afterward, something to read, and a ride if you may feel unwell.
What happens
- A fasting blood sample is taken.
- You drink a 100 gram glucose drink.
- Blood is taken at 1, 2 and 3 hours. You stay at the clinic and do not eat.
The test follows a 1-hour screen that was at or above the clinic’s cut-off. You can read how it compares with the 75 gram test on the glucose tolerance test page.
What the result means
If the criteria are met, your care team will explain next steps, which usually include blood sugar checks, an eating plan and sometimes medicine. See gestational diabetes numbers for the ADA targets used afterward. If the criteria are not met, ask when to be rechecked. The result is for your clinician to interpret.
Frequently asked questions
What are the 3-hour glucose test cut-offs?
Using the Carpenter-Coustan criteria: fasting 95, 1 hour 180, 2 hours 155 and 3 hours 140 mg/dL. Using the NDDG criteria: 105, 190, 165 and 145 mg/dL. Values are met or exceeded, and two or more is the usual threshold.
What is the difference between Carpenter-Coustan and NDDG?
They are two sets of cut-offs for the same 100 gram test. Carpenter-Coustan uses lower numbers, so it identifies more people. ACOG says both are acceptable, and your clinic picks one.
Do I have to fast for the 3-hour test?
Yes. The 100 gram test starts with a fasting blood sample, so you fast beforehand, usually overnight for 8 to 14 hours. Your clinic gives the exact rule and says whether water is allowed.
What if only one value is high?
Under the usual two-value rule, one high value does not meet the criteria. ACOG notes that some clinicians use a single elevated value, so ask what your clinic does.
Is the 3-hour test safe?
It is a standard test, but the glucose drink can cause nausea, and three hours without eating is long. Tell the staff if you feel faint or unwell. Ask your care team if you have concerns about it.
Information only, not medical advice. Your care team may set different targets for you, and only a lab test ordered by a clinician can diagnose diabetes or prediabetes. See our medical disclaimer.
Sources
- Merck Manual Professional: Glucose Thresholds for Gestational Diabetes Using a 3-hour Oral Glucose Tolerance Test (Carpenter-Coustan and NDDG, 100 g load)
- ADA Standards of Care in Diabetes—2026, Section 2: Diagnosis and Classification
- ACOG Practice Bulletin 190: Gestational Diabetes Mellitus (2018; cut-offs as quoted in ADA Standards 2026, Section 2)
- NIDDK: Gestational Diabetes (reviewed May 2017)
Last reviewed: by the Diabetes Blogs editorial team. Information only, not medical advice.
