Fasting glucose: what your fasting blood sugar result means

A normal fasting glucose is below 5.6 mmol/L (100 mg/dL) by US standards. The American Diabetes Association calls 5.6–6.9 mmol/L (100–125 mg/dL) prediabetes, while the WHO and UK labs start the prediabetes range at 6.1 mmol/L (110 mg/dL). A fasting glucose of 7.0 mmol/L (126 mg/dL) or more on two occasions is consistent with diabetes.

Updated: 2026-10-26

Reference ranges

Category	Reference range
Normal	below 5.6 (below 100); UK/WHO: below 6.1 (below 110) mmol/L (mg/dL in brackets)
Prediabetes, ADA (US)	5.6–6.9 (100–125) mmol/L (mg/dL in brackets)
Impaired fasting glucose, WHO/UK	6.1–6.9 (110–125) mmol/L (mg/dL in brackets)
Diabetes range (confirmed on repeat)	7.0 or more (126 or more) mmol/L (mg/dL in brackets)

To convert, mmol/L × 18 = mg/dL. Lab ranges may differ slightly; use the range printed on your report.

Fasting blood sugar levels: what counts as normal

Fasting means no food or drinks other than water for at least 8 hours, usually overnight; most labs ask for 8 to 12 hours. In healthy adults, fasting glucose generally sits around 4.0 to 5.4 mmol/L (72 to 99 mg/dL).

The UK and the US draw the upper line differently. US reports often flag 100 mg/dL and above, while many UK reports flag only from 6.1 mmol/L (110 mg/dL). Neither is wrong: they reflect different guideline choices about how early to label raised risk. In the UK, HbA1c is now used more often than fasting glucose to check for diabetes.

Fasting glucose can be measured from a vein in a lab or from a fingertip with a home meter. Meters are useful for day-to-day tracking but are less precise, so a diagnosis is based on a lab sample. If your home readings and lab result disagree, bring both to your appointment.

Fasting glucose 100 mg/dL or 5.6 mmol/L: is that prediabetes?

Under ADA criteria, yes: 100 to 125 mg/dL is "impaired fasting glucose", a form of prediabetes. Under WHO criteria, used in the UK, 5.6 to 6.0 mmol/L is still within normal limits. Either way, a result just over the line is a reason to look at the bigger picture rather than a diagnosis on its own.

Glucose can be slightly raised by a poor night's sleep, a recent illness, stress, steroid medicines or a sample taken later than planned. Doctors usually repeat it or check HbA1c, which reflects the past two to three months, before drawing conclusions.

What raises fasting glucose

A single morning result reflects both long-term metabolism and what happened in the previous day or two. Common reasons for a higher reading:

Insulin resistance, linked to extra weight around the waist, low physical activity and family history of type 2 diabetes.
Not fasting long enough, including milky coffee or juice in the morning.
The dawn effect: hormones released in the early morning raise glucose a little in some people.
Illness, injury and stress, which raise stress hormones.
Medicines such as steroids and some others; your doctor can say whether yours have this effect.
Pregnancy, where lower targets apply and gestational diabetes is checked with a separate test.

What happens if your result is high

One high fasting glucose without symptoms is not enough to diagnose diabetes. It is confirmed with a second test: a repeat fasting glucose, an HbA1c or, sometimes, an oral glucose tolerance test. If you are in the prediabetes range, your doctor may talk about your overall risk, weight, activity and blood pressure, and in the UK you may be offered the NHS Diabetes Prevention Programme. Most people with prediabetes are rechecked once a year.

When to see a doctor

Discuss any result above your lab's range with your GP or doctor. Get urgent medical help (NHS 111, urgent care, A&E or the emergency room) if you have high glucose and:

a result of 11.1 mmol/L (200 mg/dL) or more together with intense thirst, passing a lot of urine, weight loss or blurred vision;
vomiting, abdominal pain, fast deep breathing, fruity-smelling breath, drowsiness or confusion, which can signal diabetic ketoacidosis;
you are pregnant and a glucose result is flagged as high.

Sources

Written using public information from:

American Diabetes Association: Standards of Care in Diabetes, diagnosis and classification
World Health Organization (WHO): Definition and diagnosis of diabetes mellitus and intermediate hyperglycaemia
NHS (National Health Service, UK): Diabetes; NHS Diabetes Prevention Programme
Diabetes UK
NIDDK (US National Institute of Diabetes and Digestive and Kidney Diseases)

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Frequently asked questions

What is a normal fasting glucose level?

Below 5.6 mmol/L (100 mg/dL) by US standards, and below 6.1 mmol/L (110 mg/dL) by WHO and UK standards. Most healthy adults are around 4 to 5.4 mmol/L.

Is a fasting glucose of 100 bad?

It is at the lower edge of the ADA prediabetes range. It is a signal to check HbA1c and look at risk factors, not a diagnosis of diabetes. Many people at this level never develop diabetes.

How many hours should I fast for a blood sugar test?

At least 8 hours, usually overnight. Water is allowed; coffee, tea with milk, juice and chewing gum are not.

Is fasting glucose of 5.6 mmol/L normal in the UK?

By WHO criteria used in the UK, yes, since the impaired range starts at 6.1. By US criteria it is the start of prediabetes. Your doctor may check HbA1c to clarify.

Why is my fasting glucose higher than after eating?

The dawn effect, a short night or a stressful morning can raise fasting levels. Occasional readings like this are common; a persistent pattern is worth discussing with your doctor.

This material is for general information only. It is not a medical opinion and does not replace a consultation with a doctor. Reference ranges depend on the laboratory and the method — rely on the range printed on your own report. Your doctor makes the final interpretation.