Your doctor told you to fast before your next blood glucose (blood sugar) test for diabetes — but you’re not exactly sure what that means. While “fasting” can have slightly different meanings depending on the test, you can — and should — drink water during this time.

That said, it’s helpful to understand why your healthcare provider recommends fasting, including why drinking water matters, which diabetes-related tests require fasting, and what fasting actually involves. Knowing what to expect can help ensure your results are as accurate as possible.

Why Do Doctors Have You Fast Before Some Blood Tests for Diabetes?

Fasting helps your doctor get the most accurate picture of what’s happening in your body. What you eat and drink can temporarily affect the levels of certain substances in your blood — including your blood sugar levels. If these are the levels your healthcare provider is checking, it makes sense that food or drink could throw off the results.

Some blood tests related to diabetes require fasting, while others don’t. One common fasting test is called a fasting blood glucose test (also known as a fasting blood sugar test). This measures your blood sugar when it’s expected to be at its lowest — typically first thing in the morning, before eating. It involves a simple blood draw, and the sample is sent to a lab for testing.

Another test that involves fasting is the oral glucose tolerance test (OGTT). For this test, you’ll fast overnight, then have your blood drawn. After that, you’ll drink a sugary liquid and wait about two hours. Then, your blood will be tested again. This shows how your body handles sugar over time.

If you’re having an A1C test, a random glucose test, or a glucose challenge test, you usually won’t need to fast ahead of time. Diabetes is commonly diagnosed using a fasting blood sugar test or an A1C test, though a random glucose test can also be used in some cases.

What Exactly Does Fasting Mean?

Fasting can have different meanings depending on the test or your healthcare provider’s instructions. For example, a fasting blood sugar test typically requires you to avoid food and drinks (except water) for eight to 12 hours. Your healthcare provider will let you know exactly how long you need to fast.

During this time, most people should only drink plain water. To help make sure your test results are accurate, you should also avoid:

• Exercising
• Smoking
• Chewing gum
• Drinking coffee, water with lemon or lime, sugar-free drinks, or tea

For the OGTT — especially when used to screen for gestational diabetes — the American Diabetes Association recommends eating a moderate to high amount of carbohydrates (at least 150 grams per day) for three days before the test. This helps ensure your body’s response to glucose is measured accurately.

Eating too few carbohydrates in the days before an OGTT can cause a spike in your blood sugar after the test drink, which may lead to misleading results or an incorrect diagnosis.

All of the activities listed above can affect digestion, metabolism, or blood sugar levels — and potentially interfere with the accuracy of your test.

Unless your healthcare provider says otherwise, you can usually continue taking your regular medications while fasting. However, if you’re taking medicine that affects blood sugar, such as insulin or certain diabetes pills, check with your provider to find out whether to adjust your dose before testing.

Hydration and Blood Tests for Diabetes

It’s important to follow your doctor’s instructions when preparing for a blood test related to diabetes. As mentioned earlier, this usually means only drinking water during your fasting period — and making sure to stay well hydrated.

Drinking water helps keep your body hydrated, which is especially important when you’re about to have blood drawn. When you’re well hydrated, there’s more fluid in your veins, which can help them appear larger and easier to access. This can make it easier for the healthcare professional drawing your blood to insert the needle.

Staying hydrated is especially important for people living with type 2 diabetes. It can help keep blood sugar levels more stable, which may protect your nerves, heart, kidneys, and other parts of your body. When you’re dehydrated, the sugar in your blood becomes more concentrated, which can cause blood glucose levels to rise. This makes your kidneys work harder to filter out the extra sugar and produce more urine — increasing your risk of becoming even more dehydrated.

Using a continuous glucose monitor (CGM) or a blood glucose meter with diabetes test strips can help you keep track of your blood sugar levels during fasting. If you or your doctor is concerned about how fasting might affect your hydration or blood sugar, talk with your healthcare provider ahead of time about how to prepare and stay safe.

Manage Diabetes With Regular Blood Tests

The results of a fasting blood glucose test can give you and your doctor important information about your health. This helps your provider offer the most appropriate medical advice for your body and your needs.

These tests are commonly used to help diagnose diabetes. If your blood sugar levels are high, you may have diabetes or prediabetes or be at risk for type 2 diabetes. Sometimes, other health conditions and medications can cause high blood sugar, so your doctor may recommend additional testing based on your results.

Fasting blood sugar tests can also help you and your provider track how well your diabetes treatment plan is working. These results can show whether your blood sugar is under control or if your diabetes is getting worse — which may mean it’s time to adjust your treatment.

Because untreated diabetes can lead to serious complications, it’s important to follow your provider’s instructions about when and how to get tested. Preparing for the test the right way helps make sure the results are accurate — so you can make informed decisions about your care.

Talk With Others Who Understand

On DiabetesTeam, the social network for people with diabetes and their loved ones, members come together to ask questions, give advice, and share their stories with others who understand life with diabetes

Have you done a fasting blood test for diabetes recently? What kind of test did your doctor ask for? Share your experience in the comments below, or start a conversation by posting on your Activities page.