Fasting glucose: what it is, what it tells you and why we test it

Fasting glucose is a snapshot of your blood sugar after a night without food. It shows how well your body keeps sugar in check. But it is affected by your last meal, stress or a bad night.

Why we test it

Sugar in your blood is fuel for your cells. Insulin, a hormone made by the pancreas, moves it into the cells and keeps your glucose in a narrow window.

If your body responds less to insulin, the pancreas makes more of it, and for years your glucose stays normal. By the time it starts to rise, the problem has usually been there for a while.

Prediabetes and type 2 diabetes cause no symptoms at first. A blood test picks them up in time.

What it tells you, and what it does not

If it is high, your body is struggling to keep sugar in check. But a single result can come from your last meal, stress or a bad night.

If it is normal, that does not have to mean all is well: insulin can rise for years while glucose stays normal.

And it shows your blood sugar that morning, not over the past months.

That is why it is read together with other tests:

• HbA1c: Your average blood sugar over the past months
• Insulin: How hard your pancreas works to keep it normal
• Triglycerides: With glucose, they show how well your body uses insulin
• HDL cholesterol: It often drops when sugar and triglycerides rise

Normal fasting glucose levels

The lab decides what is normal. Fasting, it is usually 70 to 99 mg/dL, and your own report may print slightly different limits. Glucose rises after a meal: these limits only apply when fasting.

From 100 to 125, fasting sugar is a little high, and it is often called prediabetes. From 126 up, diabetes is diagnosed if a second test confirms it.

In adults the limits do not change with age: they are the same for young and old. In pregnancy, different and lower limits apply: ask your obstetrician.

In the Rosa report you also see your optimal range and what to do to reach it or stay in it.

What your result means

Find the row for your fasting result, in mg/dL. Which values need a doctor? Below 54 and 126 or more always, 200 or more the same day.

• Below 54: Very low. Your blood sugar has dropped a lot. It can cause shaking, sweating and confusion. Talk to a doctor, even if you feel well.
• 54 to under 70: Low. Below your report’s limit. In people who feel well it often comes from a long fast. If you feel well, a repeat test is usually enough. If you shake or sweat between meals, talk to your doctor.
• Within your report’s limits, Usually 70 to 99: Normal. Your fasting sugar is fine. But insulin can rise years before glucose does. In the Rosa report you also see whether you are in your optimal range.
• 100 to 125: Slightly high. Your fasting sugar is a little high. It is often called prediabetes: a signal, not a disease. Read it with your HbA1c. Movement, less sugar and more fibre help.
• 126 to under 200: High. At this level, diabetes is diagnosed if a second test confirms it. Talk to a doctor: they will usually ask you to repeat the test.
• 200 or more: Very high. Your blood sugar is very high, and the cause needs to be found now. Talk to your doctor today, or the out-of-hours doctor if you cannot reach them. Go to the emergency department straight away if you are vomiting, have belly pain, are breathing fast, or feel confused or very drowsy.

If your glucose is high or low

• High blood sugar: It is often temporary, from a meal or stress. If it stays high, your body is usually using insulin less well.
• Low blood sugar: Fasting, it is usually nothing to worry about in people who feel well. Below 54 it should be seen by a doctor.

What causes high blood sugar

A high result once often has a passing cause: a meal, stress, an illness. If it stays high, one of these is usually behind it.

• Insulin resistance: Your body responds less to insulin. For years the pancreas makes up for it by making more, then glucose starts to rise.
• Extra weight, little movement: Belly fat and sitting a lot make your body less sensitive to insulin.
• Sugar and white flour: A stretch of sweets, sugary drinks and white bread can push it up.
• Prediabetes and diabetes: When it stays high on more than one test. HbA1c helps tell.
• Cortisone: Tablets or injections can raise it, sometimes a lot. Do not stop a medicine on your own: talk to your doctor.

Symptoms of high and low blood sugar

With slightly high blood sugar you usually feel nothing. That is why it is often found by chance, on a blood test.

When it is very high, though, you may get:

• Strong thirst and a dry mouth
• Needing to pass urine often, even at night
• Tiredness
• Blurred vision
• Weight loss for no reason

When it is low, the signs come fast: shaking, sweating, sudden hunger and a racing heart. If it drops further, confusion and trouble concentrating can follow.

Low blood sugar: what it means and what to do

On a fasting test, a result just under 70 is usually nothing to worry about in someone without diabetes who feels well. A longer fast than usual is often behind it, or a sample that waited too long before it was analysed.

Below 54 is different: it should be seen by a doctor, even if you feel well. If you suddenly feel shaky, sweaty and hungry:

• Take something sugary straight away: fruit juice, a regular sugary soft drink (not diet or zero) or a few sugar cubes.
• If you do not feel better soon, take some more and ask someone nearby for help.
• Once you feel better, eat something, for example bread or crackers.
• If it happens often, talk to your doctor. If you take medicine for diabetes, follow their advice.

If someone passes out or cannot swallow, put nothing in their mouth: lay them on their side and call 112 straight away.

High blood sugar: what to do and what to eat

What to do right away? From 126 up, talk to your doctor, and at 200 or more do it the same day. If it is only a little high there is no rush: your everyday habits matter most. Where to start:

• Move every day. Even a short walk after meals helps: your muscles use the sugar.
• Less sugar and white flour: sugary drinks, juices, sweets, packaged cakes and biscuits, white bread and pasta.
• More fibre: legumes, barley, oats and vegetables slow down how fast sugar reaches your blood.
• What to eat: fish, olive oil, whole fruit and whole grains, with vegetables at most meals.
• If you carry a few extra kilos, losing even a few helps.
• Sleep enough: poor sleep pushes it up.

No food, herbal tea or supplement brings it down in minutes. If you take medicine for diabetes, do not change it on your own.

What can skew the result

A few things change the number for a while. It helps to know them before you read it.

• You had not fasted: Even a sweetened coffee or a juice raises it. From the evening before, drink only water.
• Stress or poor sleep: A hard stretch or a bad night can raise it for a while.
• Flu or a fever: An illness can raise it until you recover. Best to repeat the test once you are well.
• The time of the draw: Fasting glucose runs a little higher in the morning than in the afternoon. To compare two tests, have them at the same time of day.

When to worry

Talk to a doctor if:

• Your fasting glucose is 126 or more: the doctor will usually ask for a second test to confirm it.
• It is 200 or more: see them the same day.
• It is below 54, even if you feel well.
• It stays high together with a raised HbA1c.
• You are very thirsty, pass urine often or lose weight for no reason: see a doctor the same day. If you also have vomiting, belly pain, fast breathing, confusion or severe drowsiness, call 112 or go to the emergency department.
• It is very high and you are vomiting, have belly pain, are breathing fast, or feel confused or very drowsy: go to the emergency department now.

With Rosa, a glucose of 126 or more or below 54 is always flagged to a doctor, and 200 or more urgently.

Frequently asked questions

Fasting glucose of 100 or 110: should I worry? It is usually not urgent. From 100 to 125, fasting sugar is a little high: it is often called prediabetes, a signal and not a disease. Repeat the test fasting, read it with your HbA1c and work on movement, food and sleep.

What should blood sugar be after meals? It is normal for it to rise after eating. In people without diabetes it usually stays below 140 mg/dL and is back to where it was within two to three hours. If you often see higher numbers, talk to your doctor.

Blood sugar at 300 or more: what to do right away? It is a very high value: call your doctor now, or the out-of-hours doctor if you cannot reach them. Go to the emergency department if you are vomiting, have belly pain, are breathing fast, or feel confused or very drowsy. If you have diabetes, also follow your diabetes team’s plan.

How long do I need to fast for a glucose test? At least eight hours without food, usually from the evening before. You can drink water, but a sweetened coffee or a juice is enough to raise it.

How often should it be checked? For people who feel well there is no fixed rule: Rosa measures it twice a year. If it is high, your doctor decides the timing: from 126 up, they usually ask for a second test soon.

Sources

• American Diabetes Association, Standards of Care in Diabetes 2025: diagnosis and classification. Diabetes Care, 2025. PMID 39651986
• International Hypoglycaemia Study Group, a glucose below 54 mg/dL marks serious hypoglycaemia. Diabetologia, 2017. PMID 27872948
• Ceriello and Colagiuri, the International Diabetes Federation guideline on blood sugar after meals. Diabetic Medicine, 2008. PMID 19046192
• Knowler and colleagues, fewer cases of type 2 diabetes with lifestyle changes or metformin. New England Journal of Medicine, 2002. PMID 11832527
• Cryer and colleagues, evaluation and management of low blood sugar in adults, an Endocrine Society guideline. Journal of Clinical Endocrinology and Metabolism, 2009. PMID 19088155