Glucose tests measure the level of sugar, or glucose, in your blood.
Before some glucose tests, you’ll need to fast for at least 8 hours. Before others, you won’t have to fast at all.
If you don’t need to fast, what you eat beforehand won’t significantly affect your results.
And if you do have to fast, it makes no difference what you eat right before your fast begins.
There are several types of glucose test. They measure your blood sugar in different ways and over different periods. They can help a doctor decide whether you have diabetes or prediabetes.
The main glucose tests are the:
fasting glucose test
oral glucose tolerance test
glucose challenge test
Fasting glucose test: How to prepare
Other names for a fasting glucose test are a fasting blood sugar test or fasting plasma glucose (FPG) test. It checks for diabetes or prediabetes.
An FPG test takes place in the morning, after you’ve fasted overnight and before you eat breakfast.
You’ll need to fast for at least 8 hours before your test. You can only have small sips of water.
In the hours before your fast, you can eat as usual.
As with all the glucose tests we look at here, a healthcare professional takes a sample of blood from a vein in your arm. The needle is small. Otherwise, they may collect drops of blood by pricking your fingertip.
A technician then analyzes the sample to see whether your blood sugar levels fall into prediabetic or diabetic ranges.
HbA1C test: How to prepare
An HbA1C test is also known as an A1C, hemoglobin A1C, glycated hemoglobin, or glycohemoglobin test.
It shows your average blood sugar level for the past 3 months, and doctors use it to diagnose diabetes and prediabetes. It’s also the main test to help with diabetes management.
You don’t need to fast before the HbA1C test. You can eat and drink as usual, and you can have the test at any time of day.
If your doctor is using the test to make a diagnosis, they’ll send your sample to a lab for analysis. Once they have the results back, they’ll explain them to you.
Glucose challenge test: How to prepare
Doctors use the glucose challenge test, or glucose screening test, to look for gestational diabetes — high blood sugar during pregnancy.
The condition is temporary, but it can cause serious health problems for you and your baby. A healthcare professional will test you for it 24–28 weeks into your pregnancy.
You can have a glucose challenge test at any time of day. You don’t need to fast beforehand, so you can eat as usual.
For the test, you’ll drink a solution containing glucose. An hour later, you’ll give a blood sample.
If your blood sugar is too high, you may need a follow-up test called an oral glucose tolerance test, which we cover just below.
Oral glucose tolerance test: How to prepare
An oral glucose tolerance test helps doctors look for prediabetes, diabetes, and gestational diabetes.
It’s more complicated and expensive than glucose challenge and FPG tests.
You’ll need to fast for at least 8 hours before the test. You’ll have your last meal before bed, skip breakfast, and only have sips of water until the test. Before you start your fast, you can eat as you normally would.
At the start of the oral glucose tolerance test, a healthcare professional takes a sample of your blood to measure your blood glucose level after fasting.
You’ll then drink a glucose solution. Two hours later, you’ll have another blood test. If your blood sugar is high, it may be due to diabetes.
During the test, you shouldn’t eat or drink anything besides the solution. It’s a good idea to avoid smoking and remain relaxed.
If you’re pregnant, you’ll also give a blood sample every hour for the next 2–3 hours. If you have high blood sugar twice or more during this period, you may have gestational diabetes.
If you’re taking medication, talk to your doctor about this before the test. Some medicines can affect your results.
Before FPG and oral glucose tolerance tests, you’ll need to fast for at least 8 hours. You can eat and drink as usual until your fast begins.
Before HbA1C and glucose challenge tests, you can eat and drink as usual until the test starts — there’s no need to fast.
What should I eat the night before my test?
The fasting before certain glucose tests lasts for at least 8 hours, and this period may be overnight. So, in the evening before your fast begins, you can eat as you normally would.
If you’re taking an HbA1C or glucose challenge test, you won’t have to fast and can eat as usual.
If you have to fast, the food you eat beforehand won’t affect your blood sugar levels by the start of the test.
In general, it takes many weeks of changing your dietary habits to have a lasting impact on your blood sugar levels.
In the days before a test that requires fasting, some experts warn against making major changes to what you eat, like going on a diet. Doing so may make your test results less reliable.
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What the results can mean
A doctor can tell you if your blood glucose test results point to diabetes, prediabetes, or neither condition. Here are the next steps:
If your results suggest diabetes or prediabetes, your doctor may confirm this with a different test.
If you get a diagnosis of diabetes, you may need genetic or antibody testing to show if you have type 1, type 2, or a rare form called monogenic diabetes.
If you have gestational diabetes, your doctor will help you develop a healthy eating and exercise plan. If following the plan doesn’t bring down your blood sugar enough, you may need a medication called metformin or insulin.
Managing gestational diabetes is important because the condition increases your risk of developing preeclampsia. This can be dangerous to you and your baby, and it may lead to an early birth.
Also, if you have gestational diabetes, you and your child are more likely to develop type 2 diabetes later in life.
Test result ranges
Blood glucose readings and other indicators of diabetes vary, depending on the type of test.
The units for measuring your levels will depend on where you are in the world.
In the United States, doctors use milligrams per deciliter (mg/dL). But many other countries, including the United Kingdom, use millimoles per liter (mmol/L), which is also known as the international standard unit.
To calculate mmol/L from mg/dL, just divide by 18.
The following result ranges come from the National Institute of Diabetes and Digestive and Kidney Diseases.
|Diagnosis||FPG||HbA1C||Glucose challenge||Oral glucose tolerance|
|Normal||99 mg/dL or below||Below 5.7%||N/A||139 mg/dL or below|
|Prediabetes||100–125 mg/dL||5.7–6.4%||N/A||140–199 mg/dL|
|Gestational diabetes||N/A||N/A||140 mg/dL or above||Two high results in 3 hrs|
|Diabetes||126 mg/dL or above||6.5% or above||200 mg/dL or above||200 mg/dL or above|
Tips for managing your blood sugar levels
There are a number of strategies for managing your blood sugar responses. Some involve changing your diet.
Along with getting regular exercise, having a healthy diet is one of the best ways to reduce the risk of gestational diabetes, specifically.
Here are some ways to help manage your blood glucose levels:
Foods to eat: Opt for foods rich in fiber and healthy protein and fats, like legumes, nuts, vegetables, and whole grains. These foods also help reduce the effects of foods with a high glycemic index value if you’ve eaten them together.
Foods to limit: Refined grains like white rice and white bread can quickly increase your blood sugar levels. So can ultra-processed foods, like sodas, cookies, and several breakfast cereals. Limiting these is good for your blood sugar and overall health.
The order of foods: Changing the order in which you eat different parts of a meal can slow your digestion. And this could reduce a potential blood sugar rise by nearly 75%. So, try to eat vegetables first, then foods containing protein and fat, then carbs.
Sleep: Research from ZOE scientists shows that going to bed earlier and getting higher-quality sleep is associated with better blood sugar control the next morning.
Exercise: Unpublished ZOE research has found that exercising after a meal is associated with reduced blood sugar responses. And if you’re pregnant, regular exercise such as brisk walking is associated with a lower risk of gestational diabetes.
To find out more about how to manage your blood sugar levels, you can listen to our podcast on the topic.
Knowing how your blood sugar levels respond to what you eat can help you look after your health.
The ZOE at-home test analyzes your body’s blood sugar and blood fat responses to different foods. It also looks at the specific bacteria that make up the community of microbes in your gut.
With this information, you can discover the best foods for you and move toward your long-term health goals. If you’d like to learn more about how it works, take our free quiz.
The best way to prepare for a blood glucose test depends on the type of test.
For an FPG or oral glucose tolerance test, you’ll need to fast for at least 8 hours beforehand. It means eating no food and only taking small sips of water. Before you fast, you can eat as usual.
If you’re having an HbA1C or a glucose challenge test, you don’t need to fast, and you can eat as you normally would.
Once a glucose challenge test has started, you shouldn’t eat or drink anything during the test period.
In general, the results of these tests help show if you have prediabetes, diabetes, or gestational diabetes. Your doctor may use another type of test to confirm the diagnosis.
Definition & facts of gestational diabetes. (2017). https://www.niddk.nih.gov/health-information/diabetes/overview/what-is-diabetes/gestational/definition-facts
Diabetes tests. (2023). https://www.cdc.gov/diabetes/basics/getting-tested.html
Diabetes tests & diagnosis. (2022). https://www.niddk.nih.gov/health-information/diabetes/overview/tests-diagnosis
Food order has a significant impact on postprandial glucose and insulin levels. Diabetes Care. (2015). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4876745/
Gestational diabetes. (n.d.). https://www.niddk.nih.gov/health-information/diabetes/overview/what-is-diabetes/gestational/
Glucose tolerance tests: What exactly do they involve? (2020). https://www.ncbi.nlm.nih.gov/books/NBK279331/
Impact of insufficient sleep on dysregulated blood glucose control under standardised meal conditions. Diabetologia. (2021). https://link.springer.com/article/10.1007/s00125-021-05608-y
Prediabetes — your chance to prevent type 2 diabetes. (2022). https://www.cdc.gov/diabetes/basics/prediabetes.html
Tests & diagnosis for gestational diabetes. (2017). https://www.niddk.nih.gov/health-information/diabetes/overview/what-is-diabetes/gestational/tests-diagnosis
The A1C test & diabetes. (2018). https://www.niddk.nih.gov/health-information/diagnostic-tests/a1c-test