Fasting Glucose
Fasting Blood Glucose
What this test measures
This test measures the amount of sugar in your blood after you have not eaten for at least 8 hours. It helps to check if your body is controlling blood sugar properly and can help diagnose diabetes.
Understanding the numbers
Typical range: 4.0 to 5.9 mmol/L
6.0-6.9 mmol/L suggests pre-diabetes. 7.0 mmol/L or higher on two tests suggests diabetes.
- Low: 2–3.9 mmol/L
- Normal: 4–5.9 mmol/L
- Impaired fasting glucose: 6–6.9 mmol/L
- Diabetes: 7–20 mmol/L
Key numbers
- 6.0 mmol/L — Impaired fasting glucose starts
- 7.0 mmol/L — Diabetes threshold
- 11.1 mmol/L — Very high — urgent GP or NHS 111
Thresholds 6.0 and 7.0 mmol/L are from WHO/NICE CKS diabetes diagnostic criteria. Diagnosis requires either two fasting results or one result with symptoms. The 11.1 mmol/L marker is the WHO random-glucose diagnostic level and also a clinician guide for urgent review.
Ranges can vary slightly between laboratories — your report may show slightly different numbers. This page does not read or interpret your result; your GP looks at it alongside your history and other tests.
What a higher-than-normal result can mean
- Your body may not be controlling blood sugar well
- You may have pre-diabetes or diabetes
- You might need to make lifestyle changes
- Your GP may want to do more tests
When not to wait
If your blood sugar is very high or very low and you feel unwell, don't wait.
- Very low sugar (below 4) with confusion, drowsiness, or fits
- Very high sugar (above 15) with vomiting, tummy pain, or fast breathing
- Fruity-smelling breath (like pear drops)
- Unable to keep fluids down
- Feeling drowsy or hard to wake
- Known diabetes plus any new, severe illness
Call NHS 111 for urgent advice, or 999 if someone is seriously unwell.
Questions you could ask your GP
- Do I need an HbA1c test to get a clearer picture of my blood sugar control?
- What lifestyle changes would you recommend based on my result?
- How often should I have my blood sugar checked?
- Are there any symptoms I should watch out for?
Example stories
These are made-up examples to show how results are talked about in real appointments. They are not real patients.
Pre-pregnancy check
Sarah had a fasting glucose as part of her pre-pregnancy health check. It came back at 4.8 mmol/L — comfortably normal.
What the GP said: “Your blood sugar control is fine. No action needed. We'll screen for gestational diabetes in pregnancy at the usual time if needed.”
A single fasting glucose in the normal range is reassuring. It doesn't replace HbA1c for long-term sugar control, but it's a useful snapshot for one-off checks like fitness-to-fly or pre-pregnancy.
Borderline result on NHS health check
Tariq's fasting glucose was 6.3 mmol/L on his NHS health check. His HbA1c was 44 mmol/mol. Both pointed to pre-diabetes.
What the GP said: “This isn't diabetes, but you are heading that way. Your fasting sugar is slightly high and your HbA1c confirms it. I'd like you to join the Diabetes Prevention Programme, and we'll check again in a year.”
What happened next: Tariq lost 5 kg over nine months, swapped sugary drinks for water, and started cycling to work. His repeat fasting glucose was 5.4 mmol/L and his HbA1c came down to 39.
Impaired fasting glucose is a warning light, not a diagnosis. Most people who change lifestyle at this stage can return to normal — the NHS Diabetes Prevention Programme (free, nine months) is designed for exactly this.
Thirsty and tired for weeks
Elizabeth had been drinking bottles of water and getting up twice a night. Her fasting glucose was 9.2 mmol/L and her HbA1c was 62 mmol/mol. Both confirmed a new diagnosis of Type 2 diabetes.
What the GP said: “This is Type 2 diabetes. Your symptoms fit and both tests agree, so I don't need to repeat. We'll start metformin, book you in with the diabetes nurse, and check your eyes, feet, kidneys, and blood pressure.”
What happened next: Elizabeth started metformin, attended DESMOND, and her symptoms settled within two weeks. At three months her HbA1c was 52 and her fasting glucose 6.8.
A fasting glucose of 7 or higher with classic symptoms (thirst, weeing a lot, tiredness) usually confirms diabetes without a second test. The diagnosis sounds big but the treatment pathway is well-worn and most people feel better quickly.
On gliclazide — felt shaky before breakfast
Mahesh has Type 2 diabetes on gliclazide. He felt sweaty and shaky before breakfast and tested his blood sugar — it was 3.1 mmol/L. He took a sugary drink and called the surgery.
What the GP said: “That's hypoglycaemia — low blood sugar. Gliclazide can do this, especially if a meal is delayed or skipped. We'll reduce your dose and I'd like to see you.”
What happened next: The practice nurse checked his technique, reduced the gliclazide, and showed him what to keep in his pocket for low sugar. He hasn't had another episode in six months.
Low fasting glucose in someone on diabetes medication is a sign the dose is too strong for the meals. Always treat a 'hypo' (below 4) with fast sugar first, then tell your GP — the medication usually needs adjusting.
Before you act on anything here
This page is general information, written to help you understand — it is not personal medical advice and it cannot tell you what is right for you. Talk to your own GP, pharmacist or nurse about your own health.
- Need help now but it's not an emergency? Call NHS 111 or use 111.nhs.uk.
- Emergency? Call 999 or go to A&E.