A blood glucose test measures the amount of glucose, or sugar, in your blood.

When you eat carbohydrates, your body converts them into glucose to use as energy. Having too much or too little glucose in your blood could mean you have a serious medical condition.

Doctors often order a blood glucose test to help diagnose diabetes. People already diagnosed with diabetes might use a blood glucose test to manage their condition.

To measure blood glucose levels, a doctor will collect a sample of blood from your vein using a small needle. If you already have diabetes, you can perform a blood glucose test at home using a device that quickly pricks your finger in order to collect a drop of blood.

Glucose testing is primarily done to diagnose or manage type 1 diabetes, type 2 diabetes, and gestational diabetes. Diabetes is a condition that causes your blood glucose levels to rise.

The amount of sugar in your blood is usually controlled by a hormone called insulin. But if you have diabetes, your body either doesn’t make enough insulin or the insulin produced doesn’t work properly. This causes sugar to build up in your blood.

If left untreated, chronically elevated levels of blood sugar can lead to other serious conditions including kidney disease, blindness, and heart disease.

In some cases, blood glucose testing may also be used to test for hypoglycemia. This condition occurs when the levels of glucose in your blood are too low, usually lower than 70 milligrams per deciliter (mg/dL).

Hypoglycemia can occur in people with diabetes if they take too much of their medication, like insulin, exercise more than usual, or skip a meal. Less commonly, hypoglycemia can be caused by other underlying conditions or medications.

Doctors consider very low blood sugar to be a medical emergency, as it can lead to seizures, coma, and even death.

Blood glucose tests fall into several categories, including:

  • fasting
  • random (non-fasting)
  • oral glucose tolerance test (OGTT), a 2-hour test
  • post-prandial, which means testing after consuming food or drink

Doctors use fasting, random, and OGTT tests to diagnose diabetes.

Post-prandial tests can help show how well a person with diabetes is managing their blood sugar levels.

Before your test, tell your doctor about the medications you’re taking, including prescriptions, over-the-counter drugs, and herbal supplements. Certain medications can affect blood glucose levels. Your doctor may ask you to stop taking a particular medication or to change the dosage before your test temporarily.

Medications that can affect your blood glucose levels include:

Severe stress can also cause a temporary increase in your blood glucose. You should tell your doctor if you’ve recently had any of these:

Fasting test preparation

For a fasting blood glucose test, you can’t eat or drink anything except water for 8 hours before your test. You may want to schedule a fasting glucose test first thing in the morning so that you don’t have to fast during the day.

Fasting before a blood glucose test is important because it’ll provide more accurate results that are easier for your doctor to interpret.

Random testing preparation

Random (non-fasting) blood glucose tests don’t require you to not eat or drink before the test.

Your doctor may have you take several random measurements throughout the day to see how your glucose levels change.

Post-prandial testing preparation

A test given 2 hours after starting a meal is used to measure postprandial plasma glucose. This test is most often done at home when you have diabetes.

It can help you understand if you’re taking the right amount of insulin with meals. You must do this test 2 hours after you start eating a meal.

If you already have diabetes, this information may also help your doctor understand if you need to adjust your medications.

If you’re still in the diagnosis process, your doctor will likely recommend a blood sample test. But if you’re managing your diabetes at home, you’ll likely administer a finger-prick test yourself.

Blood-drawn tests

To perform a blood glucose test, a healthcare professional will most likely draw blood from a vein on the inside of your elbow. The procedure is fairly simple:

  1. They clean the area with an antiseptic, like alcohol, to kill any germs.
  2. They tie an elastic band around your upper arm, causing your veins to swell with blood.
  3. They insert a sterile needle into a vein. You may feel slight to moderate pain when the needle goes in, but you can reduce the pain by relaxing your arm.
  4. Your blood is then drawn into a tube attached to the needle.
  5. When they’re finished drawing blood, the healthcare professional removes the needle and places a bandage over the puncture site.
  6. Pressure will be applied to the puncture site for a few minutes to prevent bruising.

The sample of blood is then sent to a lab for testing. Your doctor will follow up with you to discuss the results.

Finger-prick tests

If you have diabetes, your doctor may have instructed you to monitor your glucose levels at home using a blood glucose meter or a continuous glucose monitor (CGM). Your test kit should include directions for how to take blood from your finger. In general, you’ll follow these steps:

  1. Wash your hands thoroughly with soap and water. Dry well.
  2. Use the lancet device that came with your kit to prick the side of a fingertip.
  3. Touch the edge of a test strip to the drop of blood that arises after the finger prick.
  4. Place the strip in the meter.
  5. Track and record your results.
  6. Dispose of the lancet and the meter.

Aftercare tips

You might experience slight pain and bruising following the blood draw or finger prick, but this will go away on its own. These tips can help you recover more quickly:

  • Continue to wear the bandage for a few hours or until the bleeding stops, unless it’s irritating your skin.
  • Avoid strenuous exercise or heavy lifting.
  • Avoid wearing tight or restrictive clothing on your arm.
  • If bleeding occurs, apply firm pressure with your fingers directly over the puncture site.
  • If you experience any bruising, apply a cloth-covered ice pack to the area for about 20 minutes.
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Blood glucose targets aren’t the same for everyone. While the American Diabetes Association provides recommended targets for most non-pregnant adults, target ranges might vary depending on many factors. These factors include:

  • your age
  • any underlying conditions
  • whether you have diabetes and for how long
  • medications you’re taking

Normal results

The implications of your results will depend on the type of blood glucose test used and when you last ate.

Type of testTarget blood glucose range for people without diabetesTarget blood glucose range for people with diabetes
Fasting testless than 99 mg/dLbetween 80 and 130 mg/dL
2 hours after beginning a meal (postprandial plasma glucose)less than 140 mg/dLless than 180 mg/dL
Random testN/AN/A

There are no random blood glucose targets, but a doctor will diagnose diabetes if a random test shows blood sugar levels are 200 mg/dL or higher.

Abnormal results

If you had a fasting blood glucose test and the results are above normal, it may indicate that you have either prediabetes or diabetes:

Fasting testPrediabetesDiabetes
Blood glucose range100–125 mg/dL126 mg/dL or above

If you had a random blood glucose test, a level of 200 mg/dL or higher often means you have diabetes. Your doctor will probably order a fasting blood glucose test to confirm the diagnosis or another test like an A1c or a glucose tolerance test.

If you had a post-prandial blood glucose test, a level above 180 mg/dL could mean that your diabetes isn’t well controlled, and your doctor may need to adjust your medications.

In some cases, people who are pregnant will develop high blood sugar during their pregnancy. This is called gestational diabetes.

Most pregnant people are given a blood glucose test between weeks 24 and 28 of their pregnancy to screen for this condition, but you may be screened earlier if you’re at a higher risk of having gestational diabetes.

To diagnose gestational diabetes, a doctor may have you participate in a test called an oral glucose challenge test. During this test, you’ll be asked to drink a sugary drink. You’ll then wait an hour before your blood is drawn. You don’t have to fast before this test.

Normal results for a gestational diabetes oral glucose challenge test are a level of 140 mg/dL or lower.

If you have an abnormal result on the oral glucose challenge test, you’ll need to have a follow-up test to confirm a diagnosis. This test is called an oral glucose tolerance test (OGTT).

First, you’ll have a fasting glucose test. Remember not to eat or drink anything for at least 8 hours before this test.

After this test, you’ll drink a sugary drink and have your blood drawn to obtain your glucose levels three times — after 1, 2, and 3 hours. You’ll be diagnosed with gestational diabetes if two or more glucose values fall at or above the typical glucose threshold.

High blood glucose levels are a sign of diabetes. This could be type 1, type 2, or another type of diabetes called secondary diabetes, which stems from an underlying condition.

A doctor may carry out further investigations to find the reason for high blood glucose levels. This will enable them to provide suitable treatment.

Causes of secondary diabetes include:

High blood glucose levels (hyperglycemia) can cause damage to your organs and nerves and increase your risk of heart attack and stroke, even if you don’t have type 1 or type 2 diabetes.

It’s also possible to have levels of blood glucose that are too low. But this isn’t as common. Low blood glucose levels, or hypoglycemia, may be caused by:

There’s a very low chance that you’ll experience a problem during or after a blood test. The possible risks are the same as those associated with all blood tests. These risks include:

  • multiple puncture wounds if it’s difficult to find a vein
  • excessive bleeding
  • lightheadedness or fainting
  • hematoma, or blood collecting under your skin
  • infection

A doctor may recommend another test or diagnose diabetes if the person’s fasting blood sugar is 126 mg/dL or higher, if non-fasting glucose is 200 mg/dL or higher, or if their A1C result is 6.5 percent or above.

A doctor may do a fasting blood glucose test, a random (non-fasting) blood test, an oral glucose tolerance test, or an A1C test.

The A1C shows glucose levels over time, while the others provide a snapshot of glucose levels. Post-prandial tests show how eating or drinking affects a person’s blood sugar levels and can be used to monitor diabetes and blood sugar management.

What is the best time to test for blood sugar?

Doctors usually do a fasting blood sugar test in the morning, after fasting for 8 hours. People can do a random test or an OGTT at any time, without fasting, but the OGTT takes 2 to 3 hours to complete.

A post-prandial test is specifically for use after eating or drinking. An A1C test shows how glucose levels have fluctuated over the last 3 months.

A blood glucose test is an important test often used to diagnose diabetes or to help people with diabetes manage their condition. Your doctor may order a blood glucose test as part of an annual wellness checkup.

But if you’re experiencing any new symptoms of diabetes, like increased thirst, frequent urination, or blurred vision, talk with your doctor about blood glucose testing.

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