A GTT blood test, short for glucose tolerance test, measures how well your body processes sugar. It involves drinking a sugary liquid and then having your blood drawn at timed intervals to track how quickly your blood sugar rises and falls. The test is primarily used to diagnose diabetes, prediabetes, and gestational diabetes during pregnancy.
How the Test Works
The glucose tolerance test checks whether your body can move sugar out of your bloodstream at a normal rate. After you drink a concentrated glucose solution, your pancreas releases insulin to push that sugar into your cells. If your blood sugar stays elevated longer than expected, it signals that something in that process isn’t working properly, whether it’s your pancreas not producing enough insulin or your cells not responding to it well.
Your doctor might order a GTT when a standard fasting blood sugar test comes back borderline, when you have risk factors for type 2 diabetes, or as a routine screen during pregnancy (typically between weeks 24 and 28). It’s considered more sensitive than a simple fasting blood draw because it reveals how your body handles sugar in real time rather than capturing a single snapshot.
What to Expect During the Test
You’ll need to fast for at least eight hours before the test, so most people schedule it first thing in the morning after skipping breakfast. In the days leading up to the test, eat and drink as you normally would. There’s no special diet to follow beforehand.
When you arrive, a technician draws blood to measure your fasting glucose level. Then you drink a sweet liquid containing a standardized amount of glucose, usually 75 grams for a standard two-hour test. The drink tastes like flat, very sweet soda, and you’ll typically need to finish it within five minutes.
After that, you wait. Blood is drawn again at set intervals, most commonly at the one-hour and two-hour marks. During the waiting periods, you’ll sit in the lab or clinic. You can’t eat, drink, or exercise, since any of those could alter your results. For pregnant women being screened with the longer version, the test uses 100 grams of glucose and includes blood draws at one, two, and three hours, making it a roughly four-hour commitment from start to finish.
Understanding Your Results
Results are measured in milligrams per deciliter (mg/dL). For the standard two-hour test, the key numbers break down like this:
- Normal: fasting blood sugar below 100 mg/dL, and two-hour reading below 140 mg/dL
- Prediabetes: fasting blood sugar between 100 and 125 mg/dL, or two-hour reading between 140 and 199 mg/dL
- Diabetes: fasting blood sugar at 126 mg/dL or above, or two-hour reading at 200 mg/dL or above
A result in the prediabetes range is sometimes called “impaired glucose tolerance.” It means your body is struggling to clear sugar from your blood efficiently but hasn’t crossed into full diabetes. This is the stage where lifestyle changes like diet and exercise can make the biggest difference in preventing progression.
Gestational Diabetes Screening
Pregnant women often go through a two-step process. The first step is a shorter screening test where you drink 50 grams of glucose and have blood drawn one hour later. If that result comes back at 135 to 140 mg/dL or higher, you’ll be asked to return for the full three-hour test with 100 grams of glucose.
The three-hour test has tighter thresholds than the standard version. A gestational diabetes diagnosis requires two or more values that exceed the cutoffs: fasting below 95 mg/dL, one-hour below 180 mg/dL, two-hour below 155 mg/dL, and three-hour below 140 mg/dL. Exceeding any two of those four values points to gestational diabetes.
Common Side Effects
The glucose drink can be hard on your stomach. Nausea is the most frequently reported side effect, followed by dizziness, weakness, and occasionally diarrhea. Some people experience shakiness, sweating, or a racing heartbeat, particularly if their blood sugar drops quickly after the initial spike. These symptoms are temporary and typically resolve once you eat after the test is complete.
Vomiting can occasionally happen, and if it does, the test may need to be rescheduled since losing any of the glucose drink makes the results unreliable. Letting the lab staff know if you’re feeling unwell during the test is important so they can monitor you.
What Can Affect Your Results
Several factors can throw off a glucose tolerance test. Being sick, even with a mild cold, can temporarily raise blood sugar and produce a falsely high reading. Physical or emotional stress has a similar effect because stress hormones interfere with insulin’s ability to do its job.
Many common medications also influence glucose metabolism. Steroids, certain blood pressure drugs, and some psychiatric medications can push blood sugar higher. Others may lower it. If you’re taking any prescription or over-the-counter medications, let your doctor know before the test so they can decide whether to adjust the timing or account for the effect when interpreting results. Your overall nutritional status, alcohol use, and even how active you’ve been in prior days can all play a role.
If your results come back abnormal and your doctor suspects something skewed the numbers, they may ask you to repeat the test under more controlled conditions or confirm with an A1C blood test, which reflects your average blood sugar over the previous two to three months and isn’t affected by a single day’s circumstances.