The FBS test measures blood glucose after 8–10 hours of fasting. A small blood sample is taken from a vein, usually in the morning.
Written & Reviewed by
MBBS (Col) · MD (Col) · SLMC No. 27329 · Medical Director, Sineth Hospitals
🩺 Medically Reviewed
📅 Updated March 2026
🕐 6 min read
🔬 Overview
What is the FBS Test?
FBS (Fasting Blood Sugar) is a blood test that measures the level of glucose (sugar) in your blood after you have fasted for 8–10 hours. It is one of the most commonly ordered blood tests in clinical practice.
Because you have not eaten for several hours before the test, fasting glucose reflects how well your body manages blood sugar in its baseline state — without the influence of a recent meal. This makes it a reliable tool for detecting problems with blood sugar regulation.
Diagnose diabetes
Detect pre-diabetes
Monitor blood sugar control
Screen high-risk individuals
FBS and other namesYou may see this test written as FBS, FBG (Fasting Blood Glucose), or FPG (Fasting Plasma Glucose) on your report — they all mean the same thing.
📋 Indications
When Should You Do an FBS Test?
1. If You Have Symptoms of Diabetes
Do not wait — get tested as soon as possible if you notice any of the following:
Frequent urination
Excessive thirst
Unexplained weight loss
Unusual fatigue
2. Screening — Even Without Symptoms
Diabetes and pre-diabetes often cause no symptoms in the early stages. Screening is recommended based on age and risk factors, following guidance from the American Diabetes Association (ADA) and World Health Organization (WHO).
| Who Should Be Screened | When to Start |
|---|---|
| All adults (general population) | From age 35 years |
| Overweight or obese + any other risk factor | At any age |
| Family history of diabetes in a first-degree relative (parent, sibling, or child) diagnosed before age 50–60 | Significant risk — start screening earlier, from young adulthood. Repeat every 1–3 years |
| Family history of diabetes only in elderly relatives (diagnosed in old age) | Lower risk — standard screening from age 35 is usually sufficient |
| High blood pressure | At diagnosis and periodically |
| History of gestational diabetes | 6–12 weeks after delivery, then every 1–3 years |
| Sedentary lifestyle | Earlier screening, especially if overweight |
3. To Confirm a Diabetes Diagnosis
If your FBS result is ≥126 mg/dL (7.0 mmol/L), this is in the diabetic range. However, a single result is usually not enough to make a definitive diagnosis on its own. It must be confirmed by:
Confirmation is required
Repeat the FBS on a separate day — or — confirm with another test (HbA1c or OGTT), unless you have clear symptoms of diabetes, in which case a single value may be sufficient.
4. To Monitor Existing Diabetes
If you already have diabetes, FBS is used regularly to assess your fasting blood sugar control and to help adjust your treatment. However, it is important to know its limitations:
FBS alone is not enough for long-term monitoringFBS shows your blood sugar only at one point in time. HbA1c is needed alongside FBS to assess your average blood sugar control over the past 2–3 months.
🍽️ Preparation
How to Prepare for the FBS Test
Correct preparation is essential for an accurate result. An improperly performed fast is one of the most common reasons for a misleading FBS reading.
Fast for 8–10 hours
Plain water is allowed
No tea or coffee
No smoking during the fast
Best time to do the testPlan your test for the morning. Fast from the night before (after dinner), sleep through most of the fast, then go for the blood test first thing in the morning. This is the easiest and most reliable way to complete the fast without discomfort.
Avoid during the fastTea, coffee, fruit juice, soft drinks, milk, sweets, chewing gum, smoking, and any food or caloric drink. These will raise your blood sugar and give a falsely high result.
💊 Medicines
What to Do With Your Medicines During Fasting
This is one of the most important — and most commonly misunderstood — parts of preparing for an FBS test. The answer is different depending on the type of medicine.
Diabetes Medications (Tablets & Insulin)
Do NOT take your morning diabetes tablets or insulin before the blood sample is taken.
Take them after the test, once you have eaten.
Why? Taking diabetes medication during the fast lowers your blood sugar artificially. This means:
• Your result will be falsely low — missing a true diagnosis or making your control look better than it is.
• More dangerously, it can cause hypoglycaemia (low blood sugar) while you are still fasting.
Always follow any specific instructions your doctor has given you — this general advice applies in most cases.
Take them after the test, once you have eaten.
Why? Taking diabetes medication during the fast lowers your blood sugar artificially. This means:
• Your result will be falsely low — missing a true diagnosis or making your control look better than it is.
• More dangerously, it can cause hypoglycaemia (low blood sugar) while you are still fasting.
Always follow any specific instructions your doctor has given you — this general advice applies in most cases.
Blood Pressure & Cholesterol Medications
These can generally be taken as usual with a small amount of plain water during the fast.
Blood pressure medications (e.g. amlodipine, losartan, atenolol) and cholesterol medications (e.g. statins) do not significantly affect blood glucose levels and are safe to continue.
Blood pressure medications (e.g. amlodipine, losartan, atenolol) and cholesterol medications (e.g. statins) do not significantly affect blood glucose levels and are safe to continue.
Other Medications
Most medicines can be continued as usual during the fast. If you are unsure about a specific medication, ask your doctor beforehand. Do not skip any essential medication without medical advice.
When in doubt, ask your doctorIf you have been given specific instructions by your treating doctor about your medicines on the day of the test, always follow those instructions over general guidance.
📄 Results
The Sample & Understanding Your Result
The Blood Sample
A small blood sample is taken from a vein — usually in the arm. The sample is collected in a tube and sent to the laboratory. Results are usually available within a few hours to the same day.
How the Report Looks
Your report will show the result in one of two units:
Example report entries
FBS: 95 mg/dL | or | FBS: 5.3 mmol/L
Both express the same measurement — just in different units. In Sri Lanka, mg/dL is most commonly used.
Both express the same measurement — just in different units. In Sri Lanka, mg/dL is most commonly used.
Reference Values
| Category | mg/dL | mmol/L |
|---|---|---|
| Normal | <100 | <5.6 |
| Pre-diabetes | 100 – 125 | 5.6 – 6.9 |
| Diabetes | ≥126 | ≥7.0 |
If You Do NOT Have Diabetes
| Result | What it Means | Action |
|---|---|---|
| <100 mg/dL | Normal | Continue routine screening |
| 100 – 125 mg/dL | Pre-diabetes (at risk) | Lifestyle changes + regular monitoring |
| ≥126 mg/dL | Possible diabetes | Confirm with repeat test or HbA1c / OGTT |
If You Already Have Diabetes
| Result | Interpretation |
|---|---|
| 80 – 130 mg/dL (4.4 – 7.2 mmol/L) | ✅ Within target — good fasting control |
| Above target range | ⚠️ May need treatment adjustment — discuss with your doctor |
| <70 mg/dL (<3.9 mmol/L) | 🚨 Risk of hypoglycaemia — seek medical advice |
Do not interpret results in isolationAlways discuss your results with your doctor. A number alone does not tell the full story — your doctor will interpret it in the context of your symptoms, other tests, and medical history.
🔁 Frequency
How Often Should You Repeat the Test?
| Situation | Recommended Frequency |
|---|---|
| Normal FBS, no risk factors | Every 1–3 years from age 35 |
| Normal FBS with risk factors present | Every 1–2 years or as advised |
| Pre-diabetes | Every 6–12 months |
| Diabetes — well controlled | Every 1–3 months as advised |
| Diabetes — poor control or recent treatment change | More frequent — as directed by your doctor |
| HbA1c (for all diabetes patients) | Every 3 months |
Pre-diabetes is reversibleIf your FBS is in the pre-diabetic range, regular follow-up and lifestyle changes (healthy diet, physical activity, weight management) can significantly reduce the risk of progressing to full diabetes. Do not ignore a borderline result.
❌ Common Mistakes
Common Mistakes to Avoid
-
Not fasting properly — Having tea, coffee, a biscuit, or smoking during the fasting period invalidates the result. Even small amounts of food or sugary drinks will raise your blood sugar. The test must be repeated.
-
Taking diabetes medication before the test — Tablets or insulin taken before the blood sample can lower the result artificially, hide a true diagnosis, and cause dangerous low blood sugar during the fast.
-
Ignoring a borderline result — A fasting blood sugar in the pre-diabetes range (100–125 mg/dL) is a serious warning. It should not be dismissed as "almost normal". Without action, many people with pre-diabetes develop diabetes within a few years.
🔬 Key Takeaways — FBS Test
- FBS is a key test for diagnosing and monitoring diabetes and pre-diabetes
- Fast for 8–10 hours before the test — plain water is allowed
- Do NOT take diabetes tablets or insulin before the blood sample is collected
- Blood pressure and cholesterol medicines can generally be taken as usual
- A result ≥126 mg/dL must be confirmed before a diabetes diagnosis is made
- Screening should start at age 35, or earlier if risk factors are present
- Pre-diabetes is a warning sign — lifestyle changes can prevent progression
- Always interpret your result together with your doctor
❓ FAQs
Frequently Asked Questions
No. Do not take your morning diabetes tablets or insulin before the FBS blood sample is taken. Take them after the test, once you have eaten. Taking diabetes medication during the fast can cause low blood sugar (hypoglycaemia) and will also affect the accuracy of the result. Always follow any specific instructions your doctor has given you.
Yes, blood pressure and cholesterol medications can generally be taken as usual with a small amount of plain water during the fast. They do not significantly affect blood sugar results. If you are unsure about a specific medicine, ask your doctor or pharmacist before the test.
No, not usually. A single elevated FBS result (≥126 mg/dL) must be either repeated on a separate day or confirmed with another test such as HbA1c or OGTT. However, if you have clear symptoms of diabetes (excessive thirst, frequent urination, unexplained weight loss), your doctor may make a diagnosis based on a single result.
If your FBS is normal and you have no risk factors, testing every 1–3 years from age 35 is generally recommended. If you have risk factors (overweight, family history of diabetes, high blood pressure), start testing earlier and repeat every 1–2 years or as your doctor advises.
Yes. You can and should drink plain water during the fasting period. Plain water does not affect blood glucose levels. However, avoid tea, coffee, fruit juice, soft drinks and any sweetened or flavoured drinks, as these will affect the result.
FBS measures your blood glucose at a single point in time — on the morning you take the test after fasting. HbA1c reflects your average blood sugar level over the past 2–3 months. FBS is used mainly for screening and diagnosis; HbA1c is the best test for monitoring long-term diabetes control. Both are important and are often used together.