Diagnosis
A fasting result of 126 mg/dL or higher, confirmed on a second test, helps diagnose diabetes. Results in the 100–125 mg/dL range point to prediabetes. It's a core test in our diabetes diagnosis guide.
The FBS test (fasting blood sugar test) measures the glucose in your blood after an overnight fast of 8–12 hours. It's one of the simplest and most common tests used to screen for, diagnose, and monitor diabetes. A fasting result of 126 mg/dL or higher, confirmed by your doctor, indicates diabetes. This guide explains the normal range, how to prepare, and what your result means.
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The FBS test, or fasting blood sugar test, measures the amount of glucose in your blood after you've gone without food for 8–12 hours. Because you haven't eaten, it shows your body's baseline blood sugar — a key indicator of how well your body manages glucose. It's also called the fasting blood glucose or fasting plasma glucose (FPG) test.
When you eat, your blood sugar rises and your body releases insulin to bring it back down. After several hours without food, a healthy body keeps blood sugar in a steady, low range. If fasting blood sugar stays high, it can be an early sign that this system isn't working well — which is what the FBS test is designed to detect.
The FBS test is one of four main blood tests used to identify diabetes. To see how it fits alongside the others, read our complete guide to how diabetes is diagnosed.
Glucose (sugar) in the blood after fasting — your baseline level.
In the morning, after an overnight fast of 8–12 hours.
Fasting blood glucose, fasting plasma glucose (FPG), fasting sugar test.
So why might your doctor order one? Next: why and when the FBS test is done.
The FBS test is done for three main reasons: to help diagnose diabetes or prediabetes, to screen people at higher risk who have no symptoms, and to monitor blood sugar control in people already living with diabetes. Your doctor may order it if you have symptoms like excessive thirst or frequent urination, or simply as part of a routine health check.
Because it's simple, quick and inexpensive, the fasting blood sugar test is one of the most widely used blood tests in everyday medicine. It serves three distinct purposes.
A fasting result of 126 mg/dL or higher, confirmed on a second test, helps diagnose diabetes. Results in the 100–125 mg/dL range point to prediabetes. It's a core test in our diabetes diagnosis guide.
It's used to check people at higher risk who feel well — catching diabetes or prediabetes early, often before any symptoms appear. Early detection gives the best chance to act.
For people who already have diabetes, periodic fasting tests help track how well diet, lifestyle and medication are keeping blood sugar in range over time.
Not sure if you should be tested? Our diabetes risk calculator gives a quick estimate, and you can read who should be screened in the diagnosis guide.
What counts as a normal result — and what's too high? Next: the FBS normal range.
A normal fasting blood sugar level is below 100 mg/dL (5.6 mmol/L). A fasting result between 100 and 125 mg/dL (5.6–6.9 mmol/L) indicates prediabetes, and 126 mg/dL (7.0 mmol/L) or higher on two tests indicates diabetes. These cut-offs are set by the WHO and the American Diabetes Association, and your doctor confirms the diagnosis.
The table below shows the fasting blood sugar ranges in both mg/dL (the unit used in India) and mmol/L (the SI unit used in some countries). Use it to understand where a result sits — but remember a diagnosis needs a doctor and usually a second test.
| Category | FBS (mg/dL) | FBS (mmol/L) | What it means |
|---|---|---|---|
| Normal | Below 100 | Below 5.6 | Healthy fasting blood sugar |
| Prediabetes | 100 – 125 | 5.6 – 6.9 | Higher than normal; raised risk — often reversible |
| Diabetes | 126 or higher | 7.0 or higher | Diabetes range (confirmed on a second test) |
Prediabetes in the fasting range is sometimes called impaired fasting glucose (IFG). Conversion: mmol/L = mg/dL ÷ 18.
mg/dL vs mmol/L: India and the US report blood sugar in mg/dL; the UK and many other countries use mmol/L. To convert, divide mg/dL by 18 (e.g. 126 ÷ 18 = 7.0 mmol/L). Both describe the same level. See our blood sugar levels chart for all tests and units.
A single high reading isn't a diagnosis. Illness, stress, certain medicines or not fasting properly can raise a result. Diabetes is confirmed only when a doctor repeats or combines tests — so don't panic over one number. The ranges above are general adult guidance; targets can differ in pregnancy and for some conditions.
Accurate results start with correct fasting. Next: how to prepare for the FBS test.
To prepare for an FBS test, fast for 8–12 hours beforehand — usually overnight, with the test in the morning. Drink only water during the fast: no food, tea, coffee, juice or sugary drinks. Keep taking your regular medicines unless your doctor advises otherwise, and tell the lab about everything you take. Proper fasting is what makes the result accurate.
The FBS test is simple, but a few small things affect how accurate it is. The most important is fasting correctly — even a sugary drink can raise your reading and skew the result.
If you take diabetes medicines or insulin, fasting can risk low blood sugar — ask your doctor how to prepare safely, including whether to adjust the timing or dose of any medicine on the test day. Never skip or change a prescribed medicine on your own.
With fasting done, what actually happens at the lab? Next: the FBS test procedure.
The FBS test is quick and simple. A technician cleans a spot on your arm, draws a small blood sample from a vein, and sends it to the lab. The blood draw itself takes only a minute or two, and most people feel just a brief pinch. Results are usually available the same day or within 24 hours.
There's nothing complicated about giving a fasting blood sample. Knowing the steps just helps you feel at ease — especially if you're not a fan of needles.
You confirm how long you've fasted and mention any medicines you take. The sample is best taken in the morning.
A technician ties a band around your upper arm, cleans the skin, and draws a small sample from a vein, usually at the inner elbow.
You may feel a brief sting as the needle goes in. The whole draw takes a minute or two, then a cotton pad is pressed on the spot.
You can eat straight after. Results typically come the same day or within 24 hours, depending on the lab.
The risks are minimal. A blood draw is very safe. You might get slight bruising, brief soreness or light-headedness — these settle quickly. If you feel faint, tell the technician and they'll have you lie down. Long fasting can cause low blood sugar in some people, so let staff know if you feel unwell.
Once your report is ready, what do the numbers mean? Next: understanding your FBS results.
Your FBS result is read against set ranges: below 100 mg/dL is normal, 100–125 mg/dL is prediabetes, and 126 mg/dL or higher (confirmed) is diabetes. A result that's too high is called hyperglycaemia; one that's too low is hypoglycaemia. A single reading rarely tells the whole story, so your doctor interprets it alongside your symptoms and other tests.
It's natural to want to make sense of your number straight away. The block below summarises what each result range means — but the right next step always depends on your doctor's view, not the number alone.
Lower than normal. May cause shakiness, sweating or dizziness. Can happen with too much diabetes medication or very long fasting. A very low reading needs prompt attention.
Your fasting blood sugar is well controlled. Keep up healthy habits and routine checks based on your age and risk.
Higher than normal but not diabetes. Often reversible with diet, activity and weight changes. Your doctor will advise on follow-up.
Meets the diabetes threshold, usually confirmed with a second test. Your doctor plans treatment and checks the type.
A high fasting result means more sugar in your blood than expected. It may point to prediabetes or diabetes — but a one-off high can also follow illness, stress, poor sleep, certain medicines, or not fasting properly. That's why a doctor confirms it with a repeat or another test before diagnosing. If confirmed, early action makes a real difference. You can compare your number on our blood sugar levels chart.
A low fasting result is less common but matters too. In people on diabetes medication or insulin, it can mean the dose is too high or a meal was missed — and should be discussed with a doctor. Symptoms can include shakiness, sweating, hunger, confusion or dizziness.
Seek urgent help for severe low blood sugar — confusion, seizures, or fainting. Very low blood sugar is a medical emergency. If you take diabetes medication, ask your doctor how to recognise and prevent lows safely.
How does the FBS test compare to other diabetes tests? Next: FBS vs HbA1c, PPBS and OGTT.
The FBS test measures fasting blood sugar at one moment, while HbA1c shows your average over 2–3 months, PPBS measures sugar after a meal, and the OGTT measures the response to a sugar drink. FBS is simple and low-cost but needs fasting and reflects only that moment. Doctors often combine tests for a fuller, more reliable picture.
Each test answers a slightly different question. Seeing them side by side makes it clear why the FBS is a great everyday test — and why it's often paired with others.
| Feature | FBS | HbA1c | PPBS | OGTT |
|---|---|---|---|---|
| What it measures | Fasting blood sugar | 3-month average | Sugar 2h after a meal | Response to sugar drink |
| Fasting needed | Yes (8–12h) | No | No (eat first) | Yes (then drink) |
| Diabetes cut-off | ≥126 mg/dL | ≥6.5% | ≥200 mg/dL* | ≥200 mg/dL (2h) |
| Time taken | A few minutes | A few minutes | ~2 hours | ~2 hours |
| Best for | Simple, low-cost check | Long-term control | After-meal spikes | Early/borderline & pregnancy |
* A random/PPBS value of ≥200 mg/dL suggests diabetes when symptoms are present; the 2-hour OGTT is the standardised version.
So where does the FBS test shine, and where does it fall short? Next: the advantages and limitations of the FBS test.
The FBS test is simple, fast, low-cost and widely available, making it an excellent first-line check for diabetes and prediabetes. Its main limitations are that it requires fasting, captures blood sugar at only one moment, and can be affected by illness or stress — so a single result usually isn't enough to diagnose diabetes on its own.
No single test is perfect. Knowing what the FBS test does well — and where it falls short — helps explain why your doctor may pair it with an HbA1c or another test.
Blood sugar naturally varies, and a single fasting test only captures one moment. A temporary illness, stress, broken fast or even a lab variation can push a result up. That's why guidelines call for diabetes to be confirmed with a repeat test or a second type of test — protecting you from a wrong diagnosis based on a one-off reading. Your doctor decides what to repeat and when.
Still have questions about the FBS test? The FAQ below answers the most common ones.
Clear, evidence-based answers to common questions about the fasting blood sugar test.
A normal fasting blood sugar (FBS) level is below 100 mg/dL (5.6 mmol/L). A result of 100 to 125 mg/dL indicates prediabetes, and 126 mg/dL or higher, confirmed on a second test, indicates diabetes.
An FBS test usually requires fasting for 8 to 12 hours beforehand. This is most easily done overnight, with the test taken in the morning. Only water is allowed during the fast — no food, tea, coffee, juice or sugary drinks.
Yes. Plain water is allowed and even encouraged before an FBS test, as it keeps you hydrated and makes the blood draw easier. Avoid tea, coffee, juice, milk and all sugary or flavoured drinks, as these can raise your blood sugar and affect the result.
A fasting blood sugar of 110 mg/dL is higher than the normal range and falls within the prediabetes range of 100 to 125 mg/dL. It isn't diabetes, but it's a signal to act early through diet, activity and weight management. Discuss the result and follow-up testing with your doctor.
Usually not on its own. A fasting result of 126 mg/dL or higher suggests diabetes, but it's normally confirmed with a repeat FBS or a second type of test, unless clear symptoms accompany a very high reading. This protects against a one-off high result leading to a wrong diagnosis.
The FBS test measures your blood sugar at a single fasting moment, while the HbA1c test reflects your average blood sugar over the past 2 to 3 months. FBS needs fasting and is low-cost; HbA1c needs no fasting and shows long-term control. Doctors often use them together — compare them in our HbA1c test guide.
Yes. Tea and coffee — even without sugar or milk — can affect blood sugar and should be avoided during the fasting window before an FBS test. Only plain water is allowed. Drinking tea or coffee can make your result inaccurate.
A fasting result can be temporarily raised by not fasting long enough, recent illness or infection, stress, poor sleep, and some medicines such as steroids. Because of this, a single high reading is confirmed with a repeat or second test before diabetes is diagnosed.
Fasting blood sugar thresholds (normal <100 mg/dL, prediabetes 100–125 mg/dL, diabetes ≥126 mg/dL) follow the WHO and ADA criteria above. They are general adult guidance and not a diagnosis. Confirm against the latest published guidance before publishing.
This content is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Only a qualified doctor can interpret your test results and diagnose diabetes. Read our full Medical Disclaimer and Editorial Policy.