Fasting plasma glucose

What is it?

According to MedlinePlus, the fasting plasma glucose test measures glucose after fasting (no food or drink except water) for at least 8 hours.

The NIDDK describes it as measuring blood glucose level at a single point in time, and it is usually done in the morning.

Why is it done?

According to MedlinePlus, it is used to screen for or monitor prediabetes and diabetes, to diagnose other conditions that affect glucose, and to check the side effects of some drugs.

MedlinePlus notes that routine screening for diabetes begins at age 35 with a fasting plasma glucose test, repeated every 3 years for those with no risk factors, and more often for those with several risk factors. The NIDDK lists people who should be tested, including those with symptoms of diabetes, those with risk factors for type 2, and those who have had gestational diabetes.

Preparation

According to the NIDDK and MedlinePlus: fast for at least 8 hours before the draw (water only), and the test is usually done in the morning.

How is it done?

According to MedlinePlus, a professional takes a blood sample from a vein in the arm with a small needle, and it is collected in a tube. You may feel a prick or mild to moderate pain, and a bruise may appear afterwards.

Understanding the result

According to the NIDDK and CDC, for non-pregnant adults (unit mg/dL): normal 99 or lower, prediabetes 100 to 125, diabetes 126 or higher.

MedlinePlus gives the same values with their mmol/L equivalents: normal 70 to 99 mg/dL (3.9 to 5.5 mmol/L), prediabetes 100 to 125 mg/dL (5.6 to 6.9), and possible diabetes at 126 mg/dL or higher (7 mmol/L).

Laboratories and guidelines differ, so what counts is your laboratory's report and your doctor's interpretation.

The NIDDK notes that the doctor usually uses a second test to confirm the diagnosis.

According to MedlinePlus, high values may be linked to causes other than diabetes such as thyroid disorders, pancreatic diseases, severe stress and rare tumours, and low values may be linked to an underactive pituitary or thyroid, pancreatic tumours, too little food, too much insulin, and liver or kidney disease. This is a general list and not a diagnosis.

Risks and side effects

According to MedlinePlus, the risks are small, and may include excessive bleeding, fainting, difficulty reaching the vein, blood collecting under the skin, or infection.

Limits of the test

This test reflects the glucose level at a single moment (NIDDK), unlike A1C, which reflects the average over recent months.

According to MedlinePlus (on the glucose tolerance test page), drugs and severe stress such as injuries, stroke, heart attack, surgery and strenuous exercise can affect glucose levels. Tell the doctor about any such circumstance.

After the test

According to the NIDDK, people with prediabetes are retested every year, and those with normal results every 3 years. You and your doctor decide the next step.

Common questions