Fasting blood glucose: normal levels and prediabetes

Fasting blood glucose levels: ADA prediabetes and diabetes cutoffs in mg/dL and mmol/L, what skews the result, how to prepare and what to do when it is high.

Fasting blood glucose: normal levels and prediabetes

8 min - BioSense Team - 2025-01-09

  • level:understanding
  • glycémie
  • prédiabète
  • diabète
  • jeûne
  • HbA1c

Fasting blood glucose: normal levels, prediabetes, diabetes

In the US, the American Diabetes Association (ADA) considers a fasting blood glucose normal below 100 mg/dL (5.6 mmol/L). A level of 100 to 125 mg/dL (5.6 to 6.9 mmol/L) falls in the prediabetes range, and 126 mg/dL (7.0 mmol/L) or higher meets the criterion for diabetes, which normally has to be confirmed by a second abnormal result.

Fasting glucose is one of the most common blood tests, and part of the routine metabolic panel. Yet the same number can be labeled "prediabetes" by one organization and not by another, and an unusual result sometimes comes down to an infection, a medication or a tube left too long before processing. This article focuses on that number: what it measures, the cutoffs used in the US and elsewhere, what can skew it, and what to do when it falls outside the normal range.

What a fasting glucose test measures

Blood glucose is the concentration of glucose (the main sugar) in the blood. After a night without food, it mainly reflects what the liver produces to fuel the brain and organs, and how well insulin restrains that production. A rising fasting glucose therefore signals that this brake is working less well.

It is a snapshot of a single moment, sensitive to the conditions of the day. Hemoglobin A1c (HbA1c), on the other hand, sums up average blood glucose over the past two to three months: the two tests complement each other. Our article on HbA1c, prediabetes and diabetes covers this second marker and its own pitfalls.

US labs report glucose in milligrams per deciliter (mg/dL); many other countries use millimoles per liter (mmol/L). To convert, divide mg/dL by 18: 126 mg/dL is 7.0 mmol/L and 100 mg/dL is 5.6 mmol/L.

Normal fasting glucose levels: ADA, WHO and NICE cutoffs

The diabetes cutoff is the same for the ADA and the World Health Organization (WHO). The prediabetes cutoff is not, which explains much of the confusion when you compare websites from different countries.

| Fasting glucose | In mmol/L | US (ADA 2026) | WHO (2006) | |---|---|---|---| | below 100 mg/dL | below 5.6 | normal | normal | | 100 to 109 mg/dL | 5.6 to 6.0 | prediabetes | normal | | 110 to 125 mg/dL | 6.1 to 6.9 | prediabetes | impaired fasting glucose | | 126 mg/dL or higher | 7.0 or higher | diabetes | diabetes |

MedlinePlus, the National Library of Medicine's consumer site, gives 70 to 99 mg/dL (3.9 to 5.5 mmol/L) as the normal fasting range, and each lab prints its own range on the report.

In the ADA 2026 Standards of Care, a fasting glucose of 126 mg/dL (7.0 mmol/L) or higher, without clear symptoms, needs two abnormal test results to establish a diagnosis. These can be two different tests from the same sample, such as fasting glucose and HbA1c, or the same test on two separate samples. In someone with classic symptoms (intense thirst, passing large amounts of urine, unexplained weight loss), a random glucose of 200 mg/dL (11.1 mmol/L) or higher is enough.

In the UK, NICE considers that a fasting glucose of 5.5 to 6.9 mmol/L (99 to 125 mg/dL) signals a high risk of type 2 diabetes. Risk rises gradually, and each organization sets its cutoff according to its prevention strategy. A glucose of 105 mg/dL (5.8 mmol/L) is therefore not a disease, but it is not a number to forget either.

Fasting glucose: why a single result is not enough

Several situations raise blood glucose temporarily, without diabetes:

  • an infection, a fever, recent surgery or significant acute stress; the ADA lists recent illness and stress among the factors that affect glucose levels;
  • certain medications, corticosteroids first of all, but also thiazide diuretics, statins, some HIV medications and some antipsychotics, which the ADA identifies as raising the risk;
  • an incomplete fast: a sweetened coffee, a juice or a meal eaten less than 8 hours before is enough to skew the result.

Conversely, glucose can be falsely low if the sample waits too long before processing. Blood cells keep consuming glucose in the tube: the ADA points out that plasma must be separated promptly, or the sample kept on ice, otherwise the result underestimates the true value.

Finally, glucose naturally varies from one day to the next. A single value of 98 or 102 mg/dL (5.4 or 5.7 mmol/L) does not carry the same weight as a series of tests that rise steadily.

How to prepare for a fasting blood glucose test

For the number to be interpretable:

  • fast for at least 8 hours with no calorie intake, according to the ADA definition; in practice, many labs suggest an overnight fast and a morning blood draw;
  • plain water is allowed, but not sweetened coffee or tea, juice or sugary chewing gum;
  • avoid intense exercise just before, and ideally have a normal night's sleep;
  • mention your medications, especially a course of corticosteroids, and any ongoing infection;
  • if you already take treatment for diabetes, follow your doctor's instructions for taking your medications on the morning of the test.

Our article on preparing for a fasting blood test covers these rules for the whole lab panel.

Who should get a blood glucose test, and how often?

US recommendations do not all cover the same people.

The ADA (2026) suggests considering testing at any age in adults with overweight or obesity (BMI of 25 or higher, 23 or higher in people of Asian ancestry) and at least one risk factor, and starting at age 35 for everyone else. Risk factors include:

  • a first-degree relative with diabetes;
  • high-risk race, ethnicity or ancestry (for example African American, Latino, Native American, Asian American);
  • a history of cardiovascular disease;
  • high blood pressure, or HDL below 35 mg/dL (0.9 mmol/L), or triglycerides above 250 mg/dL (2.8 mmol/L);
  • polycystic ovary syndrome, physical inactivity or other conditions linked to insulin resistance.

If the result is normal, the ADA repeats the test at least every three years. With prediabetes, it calls for a test every year. After gestational diabetes, it recommends testing at least every one to three years.

The USPSTF (2021, grade B) is narrower: it recommends screening adults aged 35 to 70 who have overweight or obesity, and says that every 3 years may be a reasonable interval after a normal result.

High triglycerides are among the signals to take into account: our article on triglycerides, sugar and alcohol explains this link.

Slightly high fasting glucose: what to do next

A fasting glucose of 100 to 125 mg/dL (5.6 to 6.9 mmol/L), or one that climbs from one test to the next even below this range, is a window for action. The CDC estimates that 115.2 million US adults have prediabetes. Your doctor may suggest:

  1. rechecking fasting glucose under good conditions;
  2. measuring HbA1c, for a view over several months;
  3. in some cases, an oral glucose tolerance test (OGTT): you drink 75 g of glucose and blood glucose is measured two hours later. A value of 200 mg/dL (11.1 mmol/L) or higher indicates diabetes, and 140 to 199 mg/dL (7.8 to 11.0 mmol/L) indicates prediabetes;
  4. reviewing blood pressure, the lipid panel, waist size and liver enzymes, as these often go together.

The link between glucose creeping up within the normal range and insulin resistance is explained in our article on insulin resistance and the blood test, which also presents the major prevention trials.

The best-proven levers are well known. The ADA recommends a diabetes prevention program for people at high risk, aiming to lose at least 5 to 7% of body weight and to get at least 150 minutes of moderate activity per week (see what 150 minutes of activity change in your blood test). Cutting back on sugary drinks and refined carbohydrates fits within the same changes.

Low blood sugar and pregnancy: special cases

A low fasting glucose (below 70 mg/dL, 3.9 mmol/L) in someone without diabetes is sometimes harmless: MedlinePlus notes that it may be normal in some thin, young people, and a tube processed late can produce it. The Endocrine Society recommends evaluating true hypoglycemia only when three things come together: symptoms (sweating, trembling, feeling faint), a low glucose measured at the same time, and improvement once glucose is raised. If such episodes recur, talk to your doctor.

During pregnancy, cutoffs are lower. In the US, the USPSTF recommends screening for gestational diabetes at 24 weeks or later (2021, grade B). In the ADA one-step strategy, a fasting glucose of 92 mg/dL (5.1 mmol/L) or higher during a 75 g OGTT at 24 to 28 weeks is one of the diagnostic values. Before 15 weeks, the ADA treats a fasting glucose of 110 to 125 mg/dL (6.1 to 6.9 mmol/L) as abnormal glucose metabolism. The "normal" value of a standard blood test therefore does not apply.

Frequently asked questions

What is a normal fasting blood sugar level?

Below 100 mg/dL (5.6 mmol/L) according to the American Diabetes Association. The WHO uses a higher limit of 110 mg/dL (6.1 mmol/L), and the range printed on your lab report is the one your lab applies.

Do you need to fast for an A1C test?

No, HbA1c can be measured without fasting, at any time of day. It is one of the tests the ADA and the USPSTF accept for screening, alongside fasting glucose and the oral glucose tolerance test.

My fasting glucose is 108 mg/dL: do I have prediabetes?

According to the ADA, 108 mg/dL (6.0 mmol/L) falls within the prediabetes range. According to the WHO, it stays just below the limit of 110 mg/dL (6.1 mmol/L). The most useful step is to look at the trend in your previous tests and to add HbA1c if your doctor thinks it worthwhile.

Does black coffee break a fast before a blood test?

It provides almost no calories, but caffeine may slightly alter blood glucose in some people. To be safe, stick to water before the test, unless your lab says otherwise.

Can you have diabetes with a normal fasting glucose?

Yes, it is possible: some people mainly have high blood glucose after meals. This is why HbA1c or an OGTT are sometimes ordered in addition.

*This article is for information only and does not replace medical advice. Do not start or change any treatment without talking to your doctor.*

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Sources and methodology

International

United States

United Kingdom

*BioSense methodology: our content is based on guidance from US authorities and medical societies (such as the USPSTF, CDC, NIH, FDA, ACC/AHA and ADA), UK bodies (NICE, NHS), European societies and the WHO. Sources are listed by country, and differences between countries are stated when they matter.*