What Is the Fasting Plasma Glucose Test?

The fasting plasma glucose (FPG) test, also known as the fasting blood glucose test (FBG) or fasting blood sugar test, measures the levels of glucose (sugar) in the blood. Used to screen for diabetes, it is a relatively simple, accurate, and inexpensive test that exposes problems with insulin functioning.

Purpose of Test

The FPG test is recommended as a screening test for people 35 or older to be repeated every three years. It may also be used outside of those parameters for people who have symptoms of diabetes or multiple risk factors for diabetes.

Prolonged fasting triggers a hormone called glucagon, which is produced by the pancreas and causes the liver to release glucose (blood sugar) into the bloodstream. If you don’t have diabetes, your body reacts by producing insulin, which prevents hyperglycemia (high blood sugar). However, if your body cannot generate enough insulin or cannot appropriately respond to insulin, fasting blood sugar levels will stay high.

Lack of insulin or insulin resistance causes higher than normal levels of glucose in the blood.

In the diagnosis of diabetes, the FPG test may be performed alone or along with one of these other assays:

• Random glucose tolerance test
• Oral glucose tolerance test (OGTT), which measures blood glucose levels after the ingestion of a highly-sugared beverage following a period of fasting
• Hemoglobin A1C, a measurement of average blood glucose levels over the course of two or three months

If your healthcare provider is recommending the FPG test because you've been having symptoms of diabetes, you can expect it will need to be repeated on a different day to confirm the results. Or, your healthcare provider may recommend the OGTT or the hemoglobin A1C test instead of a second FPG test.

The FPG test is regarded as accurate and more sensitive than the A1C, though it is not quite as sensitive as the gold standard of glucose tests, the OGTT.

Risks and Contraindications

As a standard blood draw performed in a lab, the FPG test is considered safe outside of a handful of potential risks associated with any blood work:

• Multiple venipuncture wounds if the technician has trouble locating a vein
• Excessive bleeding
• Dizziness, lightheadedness, or fainting
• Bruising or the accumulation of blood under the skin (hematoma)
• Infection

Before the Test

Once your healthcare provider orders an FPG test, they should be able to let you know if you'll need to repeat the test a second time on a different day or perform an OGTT or A1C test. If you have any questions or qualms about the test, ask your healthcare provider at this time.

Timing

The FPG test requires that you be in a fasted state—having not eaten anything for at least eight hours—so the test is usually scheduled for earlier in the morning. Once the lab technician is prepped to draw your blood, the test takes all of five minutes.

Location

The test will usually take place in a lab, hospital, or even your healthcare provider's office.

What to Wear

Short sleeves are helpful for a blood draw in your arm, but not absolutely necessary. Dress comfortably—you can always push up your shirt sleeve or remove a layer of clothing.

Food and Drink

This is a fasted test, which means no food or drink for eight to 12 hours before testing. The ideal way to prep for this is to fast overnight. An occasional sip of water is OK, but drinking a full glass is usually not allowed. Follow your healthcare provider's directions.

Cost and Health Insurance

While the price may vary depending on where the test is performed, the FPG test is not expensive. Most health insurance policies cover the FPG for routine examinations, monitoring, or for the purpose of diagnosing diabetes. You may be responsible for a co-pay or co-insurance fee. Call your insurance company to determine if you will have any out-of-pocket expenses for the test.

What to Bring

Bring your insurance card, identification, and a copy of the lab request/referral form. You might also want to bring a book or magazine in case you get stuck in the waiting room.

Other Considerations

A quick web search will bring up numerous FPG testing kits you can order at home. While these may be helpful for monitoring diabetes after it's been diagnosed, they are not recommended for diagnosing or screening for diabetes.

During the Test

Your blood sample will be taken by a lab technician, nurse, or phlebotomist (a person trained in venipuncture).

Pre-Test

Once you arrive, check in at the front desk. You may need to fill out insurance billing forms or consent forms. Speak up if you're nervous about having blood work done or have a history of feeling faint after medical procedures so that the lab tech or nurse can take special precautions.

Throughout the Test

The test consists of a simple, noninvasive blood test. The technician or person administering the test will collect a small vial of blood from your arm (if being used for screening or diagnosis) or a drop from a finger prick (if being used for monitoring purposes). Your blood sample will be sent to a lab for analysis.

The technician will ask you which arm you prefer to use for the test—most people choose their non-dominant arm. You'll be seated, with your chosen arm resting on a flat surface. If necessary, roll up your sleeve above your elbow. The technician will place a rubber band as a tourniquet just below your bicep in order to find a vein in the crook of your arm. They will clean the area with an alcohol wipe and press a small, fine needle into your vein: You may feel some pain for a brief moment.

Blood will start to collect into a vial, at which time the technician will remove the tourniquet to encourage more blood flow. Tell the technician if you start to feel woozy or lightheaded. It can help to look away from the blood collection if necessary.

The needle will be removed from your arm and the technician will place a cotton ball or cotton gauze square over the puncture site to stop the bleeding, along with a bandage.

If instead, you're having your fasting glucose levels monitored, you may just need a finger prick in the office. This point-of-care test usually gets quick results and is only slightly uncomfortable, not painful.

Post-Test

As long you're not feeling faint or weak, you should be free to leave as soon as the test is complete. If you're feeling dizzy, you may be asked to sit in the waiting room for a few minutes. This would be the time to have a snack to replenish low blood sugar levels as a result of fasting.

After the Test

After your blood draw, you can head home or get back to your day.

Managing Side Effects

Keep an eye on the puncture site to make sure the bleeding has stopped; note if there's any bruising, swelling, or infection. Bruising should go away on its own in a few days, but swelling and infection may require medical attention.

Interpreting Results

Depending if your test took place in your healthcare provider's office or in a lab, results may take anywhere from the same day to up to a week to arrive.

Healthcare providers interpret FPG test results by looking at glucose levels in the blood. Diagnosis categories include the following, measured in milligrams per deciliter (mg/dL).

If your FPG test results are borderline or elevated and indicate diabetes, the test will need to be repeated a second time in the near future or other tests might be performed, such as the hemoglobin A1C test, oral glucose tolerance test, or the postprandial plasma glucose test.

Results may vary from lab to lab—or even in the same lab—from day to day. As a result, two abnormal results from tests taken on two different days are required to confirm a diagnosis.

Results may be lower if blood is drawn in the afternoon rather than in the morning. A glucose level sometimes can be falsely low if too much time passes between when the blood is drawn and the lab processes the sample. The results can also be affected by previous or current medical conditions or by personal habits, such as smoking and exercise.

Abnormal test results may also indicate diabetes. A healthcare professional should consider a person’s full medical history when conducting this test and interpreting the results. Keep in mind that this blood test is used not only to diagnose diabetes but also to prevent it. Higher values are likely to reflect diet and lifestyle issues as well as poor insulin functioning.

Follow Up

Depending on your confirmed results, there are several courses of action.

High FPG and Symptoms of High Blood Sugar

If you are experiencing symptoms of elevated blood sugar and your FPG reading is above 126 mg/dL, that may be enough evidence for your healthcare provider to confirm your diagnosis of diabetes.

In that case, you and your healthcare provider will discuss setting up a treatment plan. Depending on the type of diabetes you have, it may include regular glucose monitoring, medication, and supplements, along with lifestyle changes, such as a whole foods, low-carb diet, exercise, and stress reduction practices.

There are several reasons for having a high FPG that may not be due to diabetes:

• Cushing's syndrome
• Preeclampsia
• Pancreatitis
• Sepsis
• Myocardial infarction (heart attack)

Your healthcare provider will be able to track your symptoms and perform more tests to rule these other conditions out if necessary.

Borderline/Prediabetes

If instead, your FPG reading is between 100 mg/dL to 126 mg/dL, the American Diabetes Association recommends regular screening for diabetes be repeated yearly. Having prediabetes doesn't mean you'll automatically develop type 2 diabetes, however. Prediabetes (and type 2 diabetes, for that matter) are reversible conditions.

Low FPG and Symptoms of Low Blood Sugar

An FPG reading below 55mg/dL in a person without diabetes is considered hypoglycemia; the cutoff for people with diabetes is below 70mg/dL

If your FPG reading is below 70 mg/dL, this is indicative of hypoglycemia and could mean you've developed one of the following conditions:

• Reactive (postprandial) hypoglycemia
• Kidney or liver insufficiency
• Pancreatic tumors like insulinomas (if FPG is below 55mg/dL and symptoms are present)
• Hypopituitarism, a rare disorder characterized by inadequate production of hormones
• Malabsorption syndrome

These conditions will also require further testing and symptom tracking by your healthcare provider.

Monitoring

Depending on your results, if you're using the FPG test to monitor your fasting glucose levels, your healthcare provider may want to adjust your medication or treatment plan according to how your levels present.

Other Considerations

If you're diagnosed with diabetes based on confirmed readings from an FPG test, your healthcare provider will then attempt to define which type.

• Type 1 diabetes is an autoimmune condition in which the pancreas may no longer be producing insulin. Additional tests may be required to check for autoimmune antibodies, such as the C-peptide test.
• Type 2 diabetes is a chronic disease in which the body becomes insulin resistant and can no longer efficiently process sugar for energy.
• Gestational diabetes occurs during pregnancy.

Whether a person has type 1, type 2, or gestational diabetes, a healthy lifestyle helps insulin to work better. In this sense, the fasting plasma glucose test is a signal for action, not necessarily a cause for anxiety.

Keep an open dialogue with your healthcare provider about your results from the FPG test and what steps you should take next.

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A Word From Verywell

The fasting blood glucose test is a noninvasive blood test that can be used to determine diabetes risk, diagnose diabetes, and evaluate blood sugar and insulin function in those people who have diabetes. Abnormal results can help guide you to make lifestyle changes and determine medication adjustments if needed.