Fasting blood sugar levels: what the numbers mean

The ranges

This table is printed by the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) for people who are not pregnant. NIDDK names the American Diabetes Association as its source (NIDDK).

MedlinePlus gives the same fasting ranges: 99 or below is normal, 100 to 125 is prediabetes, and 126 and above is type 2 diabetes (MedlinePlus). NIDDK's prediabetes page also puts prediabetes at a fasting glucose of 100 to 125 mg/dL and a glucose tolerance result of 140 to 199 mg/dL (NIDDK). If you are pregnant, some tests use different cut-offs, so ask your doctor what your results mean (NIDDK).

For the third common test, the A1C, see A1C levels.

How to prepare

• Fasting plasma glucose test. It measures your blood glucose at a single point in time. For the most reliable results it is done in the morning after you have fasted for at least 8 hours. Fasting means nothing to eat or drink except sips of water (NIDDK).
• Oral glucose tolerance test. You fast for at least 8 hours, give a blood sample, drink a liquid high in sugar, and give another sample 2 hours later (NIDDK). NIDDK notes it costs more, takes more time and is not as easy to give as the other tests (NIDDK) (NIDDK).
• Random plasma glucose test. Doctors sometimes use it when you have symptoms of diabetes and they do not want to wait until you have fasted. It can be done at any time (NIDDK).

Why a result is usually checked twice

NIDDK says each test uses a different measurement, and a doctor will usually use a second test to confirm diabetes (NIDDK). The fasting test shows your blood glucose at the time of the test, while the A1C shows the average over the past 3 months. In people with certain health conditions, the A1C test may miss prediabetes (NIDDK).

Who should get tested, and how often

NIDDK says a health care professional may test for prediabetes if you are age 35 or older, or if you have overweight or obesity and other risk factors for prediabetes and type 2 diabetes. If you have risk factors but tests show you do not have prediabetes, you may be retested at least every 3 years. If tests show prediabetes, you should be tested for type 2 diabetes every year (NIDDK).

Most people with prediabetes do not know they have it, because there are usually no symptoms (MedlinePlus). The diabetes risk test looks at common risk factors and can help you decide whether to ask about a blood test.

If your result is in the prediabetes range

NIDDK says people with prediabetes have a high chance of developing type 2 diabetes within 5 to 10 years, and that steps can be taken to prevent or reverse prediabetes (NIDDK). MedlinePlus lists losing weight if you are overweight, getting regular physical activity and following a healthy, reduced-calorie eating plan. In some cases a provider may also recommend diabetes medicine (MedlinePlus). More on next steps is on the prediabetes page.

Common questions

Is a fasting blood sugar of 100 prediabetes?

Yes. NIDDK prints the prediabetes range as 100 to 125 mg/dL, so 100 is the low end of it (NIDDK).

Can I drink water before a fasting blood test?

NIDDK describes fasting as nothing to eat or drink except sips of water (NIDDK). Follow the instructions you were given for your test.

How does fasting glucose relate to A1C?

They measure different things: one moment in time against a 3-month average (NIDDK). To see an A1C as an average in mg/dL, use the A1C to average glucose calculator.