Preparation for a glucose test depends on the type of test you’re getting, but the most common requirement is fasting for at least 8 hours beforehand. Some tests require dietary changes in the days leading up to them, while others need no preparation at all. Here’s what you need to know for each scenario so your results are accurate.
Know Which Test You’re Getting
There are several types of glucose tests, and each has different prep rules. The main ones are:
- Fasting blood glucose test: A single blood draw after an overnight fast. Used to screen for diabetes and prediabetes.
- Oral glucose tolerance test (OGTT): You drink a sugary solution, then have your blood drawn at timed intervals (usually one and two hours later). Requires fasting.
- Glucose challenge test (pregnancy screening): A one-hour test where you drink a 50-gram glucose solution. No fasting required.
- Three-hour glucose tolerance test (pregnancy diagnostic): A follow-up if the one-hour screening is abnormal. Requires fasting for at least 8 hours.
If you’re unsure which test you’re having, call your provider’s office and ask. The prep is meaningfully different between them, and getting it wrong can mean retaking the test.
How to Fast Correctly
For any test that requires fasting, you should have no food or caloric drinks for 8 to 14 hours before your appointment. “Fasting” is defined as no caloric intake at all. Most people schedule their test first thing in the morning and stop eating after dinner the night before.
Plain water is not only allowed but encouraged. Staying well hydrated keeps your veins fuller, which makes the blood draw easier and faster. However, plain water is the only safe option. Coffee, juice, soda, tea, and even flavored or lemon-infused sparkling water can affect your results. Flavored waters often contain sugars or artificial sweeteners that interfere with the test. Stick to flat, unflavored water.
If your test is scheduled for later in the day, count backward at least 8 hours to figure out when to stop eating. A light, balanced meal at your cutoff time is fine. You don’t need to eat anything special the night before a standard fasting glucose test.
The Three-Day Carb Rule for Tolerance Tests
If you’re getting an oral glucose tolerance test, particularly the diagnostic version during pregnancy, your provider may give you specific dietary instructions for the three days before the test. The typical recommendation is to eat at least 150 grams of carbohydrates per day during those three days.
This might sound counterintuitive when you’re being tested for blood sugar problems, but there’s a reason for it. If you’ve been eating very low-carb in the days before the test, your body may temporarily lose some of its ability to process a large sugar load efficiently. That can produce a falsely high result. Eating a normal amount of carbs ensures your body responds to the glucose drink in a way that reflects your actual metabolic health.
For reference, 150 grams of carbs is roughly what you’d get from a normal mixed diet: a couple of servings of bread or rice, some fruit, a glass of milk, and a serving of pasta or potatoes throughout the day. You don’t need to go overboard, just don’t restrict carbs in the days leading up to the test.
Keep Your Activity Level Normal
Maintain your typical exercise routine for at least three days before the test. Don’t skip workouts entirely, but also don’t do anything unusually intense. A hard workout you’re not accustomed to can temporarily change how your muscles absorb glucose, which may skew your results in either direction.
If you normally walk for 30 minutes a day, keep doing that. If you don’t usually exercise, don’t start a new routine the week of your test. The goal is for the results to reflect your body’s normal baseline.
What to Know About Medications
Certain medications can raise or lower blood sugar independently, which may affect your test results. Steroids, some blood pressure medications, and even high doses of common over-the-counter products like acetaminophen or vitamin C can interfere. Don’t stop taking any prescribed medication on your own, but do tell your provider about everything you’re taking, including supplements, so they can interpret the results in context or adjust the timing.
Pregnancy Glucose Screening: One Test, No Fasting
If you’re pregnant and getting the initial one-hour glucose challenge test (typically done between weeks 24 and 28), you do not need to fast. You can eat normally before the appointment. You’ll drink a solution containing 50 grams of sugar, then have your blood drawn one hour later.
If that screening comes back elevated, your provider will schedule the follow-up three-hour test. That one does require fasting for at least 8 hours. You’ll drink a stronger solution containing 100 grams of sugar, and your blood will be drawn at one, two, and three hours afterward. This is the test where the three-day carbohydrate loading guideline applies.
Many women find the glucose drink easier to tolerate when it’s cold. Ask the lab if they can refrigerate it, or whether you can pick it up ahead of time and chill it yourself.
What Your Results Mean
For a standard fasting blood glucose test, a normal result is below 100 mg/dL. A reading between 100 and 125 mg/dL falls into the prediabetes range, and 126 mg/dL or higher on two separate tests indicates diabetes. These thresholds come from the American Diabetes Association’s current diagnostic criteria.
For the two-hour oral glucose tolerance test used to screen for type 2 diabetes, you drink a 75-gram glucose solution and have blood drawn at one and two hours. Your provider will compare your readings against established cutoffs at each time point.
A single elevated result doesn’t always mean a diagnosis. Providers typically confirm with a repeat test or a different type of test before drawing conclusions. If you didn’t fast properly or were sick on the day of the test, mention that, because it may explain an unexpected number and warrant retesting rather than a diagnosis.
Quick Prep Checklist
- Confirm the test type so you know whether fasting is required.
- Fast 8 to 14 hours if instructed, stopping after dinner the night before a morning test.
- Drink plain water freely. Skip coffee, tea, juice, and flavored water.
- Eat at least 150 grams of carbs daily for three days before an oral glucose tolerance test, if your provider advises it.
- Keep your exercise routine normal for three days before the test.
- Tell your provider about all medications and supplements you’re taking.
- Schedule the test early in the morning if you’re fasting, so most of the fast happens while you sleep.