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 blood sugar levels and is used to diagnose 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 over age 45, tested every three years. It is also performed outside of those circumstances if you have symptoms of diabetes or multiple risk factors for diabetes.
Prolonged fasting triggers a hormone called glucagon, which is produced by the pancreas. It 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.
In the diagnosis of diabetes, the FPG test may be performed alone, or it may be requested by your provider in conjunction with one of these other assays:
- Random glucose tolerance test
- Oral glucose tolerance test (OGTT)
- Hemoglobin A1c
If your physician is recommending the FPG test because you’ve been having symptoms of diabetes, you can expect that the test will need to be repeated on a different day to confirm the results.
Your doctor also may recommend a different assay, the OGTT or the haemoglobin A1C test, to be performed instead of a second FPG test.
The FPG test is held in high regard as an accurate test and is considered more sensitive than the A1C, though it is not quite as sensitive as the gold standard of glucose tolerance, the OGTT. One major difference between the FPG test and the OGTT is that the OGTT cannot be used to monitor or measure the effectiveness of a diabetes management program in those who have already been diagnosed with the disease.
The hemoglobin A1C test lends a look at the average of your blood glucose levels over the past three months, rather than the day-of glimpse provided by the FPG test.
Risks and Contraindications
The FPG test is a standard blood draw performed in a lab and is generally considered low risk. However, there are several potential risks correlated with any blood work:
- Multiple venipuncture wounds if the technician has trouble locating a vein
- Excessive bleeding
- Dizziness, light-headedness, or fainting
- Bruising or blood collecting under the skin (hematoma)
Before the Test
Once your doctor 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 doctor at this time.
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.
The test will usually take place in a lab, hospital, or possibly even in your doctor’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 doctor’s recommendation.
Cost and Health Insurance
While the price may vary depending on where the test is performed, the FPG test is not an expensive test. Most health insurances cover this test 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 discover which benefits they offer.
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 just in case you get stuck in the waiting room.
A quick web search will bring up numerous FPG testing kits you can order at home. While these may be helpful for monitoring your condition, they are not recommended for diagnosing or screening diabetes.
During the Test
Your blood sample will be taken by a lab technician, nurse, or phlebotomist (a person trained in venipuncture).
Once you arrive, check with the front desk if you need to fill out any insurance billing forms or consent forms. You may also want to mention if you are feeling 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, non-invasive 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 as they try 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, and you may feel a sharp pain for a brief moment.
Blood will start to collect into a vial, and the technician will remove the tourniquet to encourage more blood flow. Notify the technician if you start to feel woozy or lightheaded. It can help to look away from the blood collection if necessary.
Try not to cross your legs or tense your body, as stress can raise your glucose levels and constriction can make extracting blood more difficult.
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 really painful.
You’re usually free to go as long as you’re not feeling faint or weak. If you’re feeling a little dizzy, you may be asked to sit in the waiting room for a few minutes. It can be helpful to have a snack on hand 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, and notice if there’s any bruising, swelling, or infection. Bruising should go away on its own in a few days, but swelling and infection may be a sign of something more serious and may require medical attention.
Depending on if your test took place in your physician’s office or in a lab, results may take anywhere from the same day to up to a week to arrive.
Doctors interpret FPG test results by looking at glucose levels in the blood. Diagnosis categories include the following, measured in milligrams per deciliter (mg/dL):
- A reading of 70 mg/dL to 99mg/dL is considered within the normal range.
- A reading of 100 mg/dL to 126 mg/dL suggests prediabetes, indicating an increased risk in developing full-blown diabetes.
- A reading above 126 mg/dL is the threshold at which diabetes is diagnosed.
- Blood glucose levels lower than 70mg/dL imply an episode of hypoglycemia, in which blood sugar is dangerously low.
If your FPG test results are borderline or elevated and indicating diabetes, the test will need to be repeated on the second day in the very 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.
Of course, 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.
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 physician to confirm your diagnosis of diabetes. Speak with your doctor about setting up a treatment plan depending on the type of diabetes you have, but your plan may include regular glucose monitoring, medication, and supplements, along with lifestyle changes, such as a whole-foods, low-carb diet, exercise, and stress reduction techniques.
There are several other reasons for having a high FPG that may not be due to diabetes:
- Cushing’s syndrome
- Myocardial infarction (heart attack)
Your doctor will be able to track your symptoms and perform more tests to rule these other conditions out if necessary.
If instead, your FPG reading is between 100 mg/dL to 126 mg/dL, the American Diabetes Association recommends that you continue to be screened for diabetes by repeating this test every one to two years. 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
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
- Malabsorption syndrome
These conditions will also require further testing and symptom tracking by your doctor.
Depending on your results, if you’re using the FPG test to monitor your fasting glucose levels, your doctor may want to adjust your medication or treatment plan according to how your levels present.
If you’re diagnosed with diabetes based on confirmed readings from an FPG test, your doctor 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 a cause for despair.
Keep an open dialogue with your doctor about your results from the FPG test and which next steps you should take.
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.
- American Diabetes Association. 2. Classification and Diagnosis of Diabetes: Standards of Medical Care in Diabetes—2020. Diabetes Care. 2020;43(Suppl 1):S14-S31. doi:10.2337/dc20-S002
- American Diabetes Association. Hypoglycemia (low blood sugar).
- American Diabetes Association. Diabetes overview: Diagnosis.
- National Institute of Diabetes and Digestive and Kidney Diseases. Diabetes and prediabetes tests. Updated April 2014.
- American Diabetes Association. 6. Glycemic Targets: Standards of Medical Care in Diabetes—2020. Diabetes Care. 2020;43(Suppl 1):S66-S76. doi:10.2337/dc20-S006
- National Institute of Diabetes and Digestive and Kidney Diseases. Cushing’s syndrome. Updated May 2018.
- Hyperglycemia and Adverse Pregnancy Outcome (HAPO) Study Cooperative Research Group. Hyperglycemia and Adverse Pregnancy Outcome (HAPO) study: preeclampsia. Am J Obstet Gynecol. 2010;202(3):255.e1-7. doi:10.1016/j.ajog.2010.01.024
- Kikuta K, Masamune A, Shimosegawa T. Impaired glucose tolerance in acute pancreatitis. World J Gastroenterol. 2015;21(24):7367-74. doi:10.3748/wjg.v21.i24.7367
- Wernly B, Lichtenauer M, Hoppe UC, Jung C. Hyperglycemia in septic patients: an essential stress survival response in all, a robust marker for risk stratification in some, to be messed with in none. J Thorac Dis. 2016;8(7):E621-4. doi:10.21037/jtd.2016.05.24
- Ishihara M. Acute hyperglycemia in patients with acute myocardial infarction. Circ J. 2012;76(3):563-71. doi:10.1253/circj.cj-11-1376
- Galati SJ, Rayfield EJ. Approach to the patient with postprandial hypoglycemia. Endocr Pract. 2014;20(4):331-40. doi:10.4158/EP13132.RA
- Desimone ME, Weinstock RS. Non-diabetic hypoglycemia. In: Feingold KR, Anawalt B, Boyce A, et al., editors. Endotext [Internet]. South Dartmouth (MA): MDText.com, Inc.; 2000-. Updated September 23, 2017.