A glucose tolerance test measures how well your body processes sugar by tracking your blood sugar levels after you drink a standardized sugar solution. It’s one of the primary tools used to diagnose prediabetes, type 2 diabetes, and gestational diabetes. The test works by giving your body a controlled dose of glucose and then checking, through blood draws over the next one to three hours, whether your system can bring blood sugar back down to a normal range.
How the Test Works
The basic idea is simple: drink a measured amount of sugar, then see how your body responds. When you consume glucose, your pancreas releases insulin to shuttle that sugar out of your bloodstream and into your cells for energy. If your body doesn’t produce enough insulin, or your cells have become resistant to it, blood sugar stays elevated longer than it should. The glucose tolerance test captures that response in real time.
There are different versions of the test depending on what your doctor is screening for, but they all follow the same principle: establish a baseline blood sugar reading, introduce a known quantity of glucose, and measure how quickly your body clears it.
What to Expect During the Test
For the standard version used to screen for type 2 diabetes and prediabetes, you’ll need to fast for at least eight hours beforehand, meaning no food or drinks. When you arrive, a blood sample is drawn to measure your fasting blood sugar. Then you drink a syrupy glucose solution containing 75 grams of sugar. Blood samples are taken again at the one-hour and two-hour marks.
The glucose drink is very sweet, and some people feel nauseated after drinking it. Vomiting can happen, and if it does, the test typically can’t be completed that day and will need to be rescheduled. You may also feel lightheaded from having multiple blood draws in a short window. Otherwise, the test is straightforward: you sit and wait between draws.
In the days leading up to the test, some clinics recommend eating at least 150 grams of carbohydrates per day for three days beforehand. This ensures your body’s insulin response hasn’t been artificially suppressed by a very low-carb diet, which could skew results. Your doctor’s office should give you specific prep instructions.
Reading Your Results
The key number is your blood sugar at the two-hour mark after drinking the glucose solution. The American Diabetes Association sets these thresholds:
- Normal: below 140 mg/dL
- Prediabetes (impaired glucose tolerance): 140 to 199 mg/dL
- Diabetes: 200 mg/dL or higher
A result in the prediabetes range means your body is struggling to process sugar efficiently but hasn’t crossed into diabetes. This isn’t a benign finding. Even at the prediabetes stage, damage to blood vessels, the heart, and the kidneys can already be underway, including a higher risk of silent heart attacks. The good news is that prediabetes is often reversible with changes to diet, physical activity, and weight.
If your result falls in the diabetes range, your doctor will likely confirm the diagnosis with a repeat test or an A1C blood test, which reflects your average blood sugar over the previous two to three months.
Glucose Testing During Pregnancy
Pregnant women are routinely screened for gestational diabetes between 24 and 28 weeks. If you have risk factors like obesity, a family history of diabetes, or a previous pregnancy with gestational diabetes, screening may happen earlier.
Most practices in the U.S. use a two-step approach. The first step is a one-hour screening test that doesn’t require fasting. You drink a solution with 50 grams of sugar, and your blood is drawn one hour later. If your blood sugar exceeds the screening threshold, you move to the second step: a three-hour diagnostic test. For this one, you fast for eight hours, then drink a solution with 100 grams of sugar. Blood is drawn at the one-hour, two-hour, and three-hour marks. A diagnosis of gestational diabetes requires two or more elevated values across those draws.
Some providers use a one-step approach instead: a single fasting test with 75 grams of sugar and blood draws at one and two hours. Only one elevated value is needed for diagnosis. Because the bar is lower, this method identifies roughly 18% to 20% of pregnant women as having gestational diabetes, about two to three times the rate found with the two-step method. Both approaches are considered acceptable, and which one your provider uses depends on their clinical guidelines.
Why It’s Used Instead of Other Tests
You might wonder why the glucose tolerance test exists when simpler blood tests, like fasting blood sugar or A1C, can also diagnose diabetes. The answer is that the glucose tolerance test catches problems the others can miss. Some people have normal fasting blood sugar but an abnormal response after eating, a pattern that only shows up when the body is challenged with a glucose load. This makes the test particularly useful for detecting prediabetes and for pregnancy screening, where catching elevated blood sugar early has a direct impact on outcomes for both parent and baby.
The trade-off is convenience. The test takes two to three hours in a clinic, requires fasting, and involves multiple blood draws. A fasting blood sugar test or an A1C test can be done with a single blood draw and no special preparation. For routine screening in non-pregnant adults, many doctors start with those simpler tests and reserve the glucose tolerance test for cases where results are borderline or more detail is needed.