Blood sugar tests are among the most commonly ordered investigations worldwide. Understanding the normal ranges helps patients interpret their results and ask the right questions at their next clinic visit.
Written & Reviewed by
MBBS (Col) · MD (Col) · SLMC No. 27329 · Medical Director, Sineth Hospitals
🩺 Medically Reviewed
📅 Updated: April 2026
🕐 8 min read
🔬 Overview
What Is Blood Sugar?
Blood sugar — also called blood glucose — is the body's main source of energy. After you eat, carbohydrates are broken down into glucose, which enters the bloodstream. The hormone insulin, produced by the pancreas, helps move glucose from the blood into cells where it is used for energy.
When this system breaks down — either because the pancreas produces too little insulin or the body does not respond to it properly — blood glucose levels remain persistently elevated. Over time, this can lead to diabetes and its complications.
Glucose is the body's primary fuel source
Insulin moves glucose from blood into cells
Persistent high sugar may indicate diabetes
Very low sugar can also be dangerous
Two ways blood sugar is reportedDifferent laboratories and countries report blood sugar using different units. mg/dL (milligrams per deciliter) is widely used in Sri Lanka and many other countries. mmol/L (millimoles per litre) is used internationally. Both measure the same thing — the conversion formula is simple and explained later in this article.
When Blood Sugar Persistently High
Persistently elevated blood sugar may indicate one of the following conditions:
- Prediabetes — blood sugar is above normal but not yet at the diabetic threshold; a window for prevention
- Diabetes mellitus — full diabetes, where blood sugar regularly exceeds the diagnostic cut-off values
- Gestational diabetes — high blood sugar that develops during pregnancy, even in women who did not previously have diabetes
🧪 Fasting Test
1. Fasting Blood Sugar (FBS / FPG)
The fasting blood sugar test — also called the Fasting Plasma Glucose (FPG) test — measures blood glucose after you have fasted for at least 8 hours. It is one of the most widely used tests for screening and diagnosing diabetes and prediabetes.
| Result | mg/dL | mmol/L | Interpretation |
|---|---|---|---|
| Normal | Less than 100 | Less than 5.6 | Normal |
| Prediabetes | 100 – 125 | 5.6 – 6.9 | Prediabetes |
| Diabetes | 126 or higher | 7.0 or higher | Diabetes |
Important Notes for FBS
- Plain water is generally allowed during fasting — check with your laboratory
- Tea, coffee, milk, or any sugary drink will affect the result
- Illness, stress, and some medications (e.g. steroids) can temporarily raise blood sugar
- In the absence of diabetes symptoms, a diagnosis requires a repeat abnormal result
🍽️ After-Meal Test
2. Postprandial Blood Sugar (PPBS)
The PPBS (Postprandial Blood Sugar) test measures blood glucose approximately 2 hours after a meal. It shows how effectively your body clears glucose from the bloodstream after eating.
| Result | mg/dL | mmol/L | Interpretation |
|---|---|---|---|
| Normal | Less than 140 | Less than 7.8 | Normal |
| Prediabetes | 140 – 199 | 7.8 – 11.0 | Prediabetes |
| Diabetes | 200 or higher | 11.1 or higher | Diabetes |
Why PPBS mattersSome people have a normal fasting blood sugar but develop high glucose after meals — a pattern called postprandial hyperglycaemia. This can go undetected without a PPBS test and carries its own risk of cardiovascular complications. PPBS is therefore a valuable complement to FBS testing.
🩸 Random Test
3. Random Blood Sugar (RBS)
The random blood sugar (RBS) test can be performed at any time, regardless of when you last ate. It is often used in emergency or symptomatic situations — for example, when a patient presents with symptoms that may suggest diabetes.
| Result | mg/dL | mmol/L | Interpretation |
|---|---|---|---|
| Usually normal | Below 140 | Below 7.8 | Normal |
| Possible diabetes* | 200 or higher | 11.1 or higher | Investigate |
*Especially when associated with typical symptoms of diabetes (see below).
Symptoms That Raise Suspicion of Diabetes
Excessive thirst (polydipsia)
Frequent urination (polyuria)
Unexplained weight loss
Unusual fatigue or tiredness
Blurred vision
Slow-healing wounds
A single abnormal random result may not confirm diabetesA random blood sugar of 200 mg/dL or above in a person with typical diabetes symptoms is strongly suggestive of diabetes. However, in the absence of symptoms, additional testing — such as a repeat fasting blood sugar or HbA1c — is usually required before a diagnosis is made.
🧪 Glucose Challenge Test
4. Oral Glucose Tolerance Test (OGTT)
The OGTT evaluates how the body handles a standardised glucose load. The patient fasts overnight, then drinks a glucose solution, and blood sugar is measured 2 hours later. It is more sensitive than the FBS for detecting early abnormalities in glucose metabolism.
How OGTT Is Performed
- 1Fast for at least 8–10 hours overnight (water is allowed)
- 2Drink a standardised glucose solution (75 g of glucose in adults)
- 3Rest quietly — avoid food, smoking, or strenuous activity
- 4Blood sugar is measured at 2 hours (and sometimes at 1 hour)
OGTT Reference Values (2-hour result)
| Result | mg/dL | mmol/L | Interpretation |
|---|---|---|---|
| Normal | Less than 140 | Less than 7.8 | Normal |
| Impaired Glucose Tolerance | 140 – 199 | 7.8 – 11.0 | Prediabetes |
| Diabetes | 200 or higher | 11.1 or higher | Diabetes |
When OGTT Is Commonly Used
Gestational diabetes screening during pregnancy
Borderline diabetes cases requiring clarification
Early detection in high-risk individuals
Confirmatory test when FBS is inconclusive
🧬 3-Month Average
5. HbA1c (Glycated Haemoglobin)
The HbA1c test measures the percentage of haemoglobin in red blood cells that has glucose attached to it. Because red blood cells live for approximately 2–3 months, HbA1c reflects your average blood sugar over that entire period — not just at a single point in time.
Importantly, no fasting is required for the HbA1c test — it can be done at any time of day.
HbA1c Reference Values
| Result | HbA1c (%) | Interpretation |
|---|---|---|
| Normal | Below 5.7% | Normal |
| Prediabetes | 5.7% – 6.4% | Prediabetes |
| Diabetes | 6.5% or higher | Diabetes |
HbA1c Target for People Living With Diabetes
For most adults with diabetes, the general target is HbA1c below 7%. However, individual targets may differ based on:
- Age and life expectancy
- Pregnancy status
- Risk of low blood sugar (hypoglycaemia)
- Presence of other medical conditions
HbA1c to Estimated Average Glucose
| HbA1c (%) | Estimated Average Glucose (mg/dL) | Estimated Average Glucose (mmol/L) |
|---|---|---|
| 5% | 97 | 5.4 |
| 6% | 126 | 7.0 |
| 7% | 154 | 8.6 |
| 8% | 183 | 10.2 |
| 9% | 212 | 11.8 |
🤰 Pregnancy
Blood Sugar Levels During Pregnancy
Pregnancy requires different blood sugar targets because elevated glucose can affect both the mother and the developing baby. The thresholds used during pregnancy are stricter than general diabetes cut-offs.
| Test Timing | Target (mg/dL) | Target (mmol/L) |
|---|---|---|
| Fasting | Less than 95 | Less than 5.3 |
| 1 hour after meal | Less than 140 | Less than 7.8 |
| 2 hours after meal | Less than 120 | Less than 6.7 |
Always follow your obstetric team's advicePregnancy glucose targets can vary depending on your specific risk factors, gestational age, and whether gestational diabetes has been diagnosed. The values above are general reference ranges. Your doctor or midwife will advise the targets that apply to you personally.
🔢 Unit Conversion
How to Convert Between mg/dL and mmol/L
Both units measure the same thing — glucose concentration in the blood — but on different scales. The conversion factor is 18.
mg/dL ÷ 18 = mmol/L
mmol/L × 18 = mg/dL
Quick examples
A fasting sugar of 90 mg/dL = 90 ÷ 18 = 5.0 mmol/L (normal).
A fasting sugar of 126 mg/dL = 126 ÷ 18 = 7.0 mmol/L (diabetes threshold).
An HbA1c of 7% corresponds to an estimated average glucose of 154 mg/dL or 8.6 mmol/L.
A fasting sugar of 126 mg/dL = 126 ÷ 18 = 7.0 mmol/L (diabetes threshold).
An HbA1c of 7% corresponds to an estimated average glucose of 154 mg/dL or 8.6 mmol/L.
📋 Causes
What Causes High or Low Blood Sugar?
Common Causes of High Blood Sugar (Hyperglycaemia)
- Diabetes mellitus (Type 1 or Type 2)
- Obesity and physical inactivity
- Family history of diabetes
- Stress and infections
- Steroid medications (e.g. prednisolone, dexamethasone)
- Gestational diabetes during pregnancy
- Polycystic ovarian syndrome (PCOS)
- Certain pancreatic diseases
Common Causes of Low Blood Sugar (Hypoglycaemia)
- Skipping meals or prolonged fasting
- Excess insulin or diabetes medication
- Heavy exercise without adequate food intake
- Alcohol consumption
- Certain rare tumours (e.g. insulinoma)
- Liver or kidney disease
Symptoms of Low Blood Sugar
Sweating
Trembling or shakiness
Sudden hunger
Dizziness or lightheadedness
Confusion or difficulty concentrating
Fainting in severe cases
Severe hypoglycaemia requires urgent medical attentionIf blood sugar falls very low and the person cannot be treated with glucose by mouth — particularly if they are unconscious, confused, or fitting — this is a medical emergency. Call for emergency help immediately.
🏥 When to Seek Help
When Should You See a Doctor?
Seek medical advice if you:
- 1Have repeatedly high or low blood sugar readings on a home glucometer or lab report
- 2Develop symptoms of diabetes — excessive thirst, frequent urination, unexplained weight loss, or blurred vision
- 3Feel dizzy, faint, or confused — which may indicate dangerously low blood sugar
- 4Are pregnant and have received abnormal blood sugar results
- 5Have a strong family history of diabetes, are overweight, or are physically inactive
Early diagnosis saves livesIdentifying prediabetes or diabetes early allows lifestyle changes and treatment to begin before complications develop. Long-term high blood sugar silently damages the heart, kidneys, nerves, and eyes. Regular screening in at-risk groups is one of the most effective preventive health measures available.
📌 Key Takeaways
- Fasting blood sugar below 100 mg/dL (5.6 mmol/L) is generally normal
- PPBS below 140 mg/dL (7.8 mmol/L) at 2 hours after a meal is normal
- HbA1c below 5.7% is normal; 6.5% or above indicates diabetes
- A random blood sugar of 200 mg/dL or above with symptoms is a red flag
- Pregnancy targets are stricter than general cut-offs
- To convert mg/dL to mmol/L: divide by 18
- Persistently elevated blood sugar requires medical evaluation — early treatment prevents complications
❓ Frequently Asked Questions
Frequently Asked Questions
No. A fasting blood sugar of 110 mg/dL (6.1 mmol/L) falls within the prediabetes range (100–125 mg/dL). A normal fasting result is below 100 mg/dL (5.6 mmol/L). You should discuss this result with your doctor, who may recommend lifestyle changes, repeat testing, or further investigations.
It depends on the type and timing of the test. A PPBS (2 hours after a meal) below 140 mg/dL (7.8 mmol/L) is normal. A fasting blood sugar of 140 mg/dL (7.8 mmol/L) is in the diabetic range and requires medical evaluation. Always interpret your results in context with your doctor.
Yes. Stress hormones — particularly cortisol and adrenaline — can temporarily raise blood glucose levels. This is called stress hyperglycaemia. It is more pronounced during severe illness, surgery, or major emotional stress. A single elevated reading during a stressful period may not necessarily indicate diabetes — repeat testing in a stable state is important.
They provide different and complementary information. The HbA1c reflects your average blood sugar over the past 2–3 months and does not require fasting. Fasting blood sugar measures your glucose level at one specific moment. For diagnosing diabetes and monitoring long-term control, HbA1c is generally preferred. For day-to-day management, fasting and postprandial values are also important. Your doctor will choose the most appropriate combination for your situation.
Usually yes — plain water does not affect blood glucose and is generally permitted during fasting. However, unless your doctor or laboratory specifically advises otherwise, avoid tea, coffee, fruit juice, milk, or any other drink before the test.
Yes. Diabetes can occur in people of any body weight. Type 1 diabetes typically affects lean individuals. Some forms of Type 2 diabetes also develop in people without obesity — particularly in South Asian populations, who can develop insulin resistance at lower BMI levels than Western populations. Do not assume a normal weight protects against diabetes.
Not always. In the absence of clear symptoms of diabetes, the diagnosis typically requires confirmation with a second test — either a repeat fasting blood sugar, a random blood sugar, an OGTT, or an HbA1c. However, if someone has unambiguous symptoms (excessive thirst, frequent urination, unexplained weight loss) combined with a clearly elevated random blood sugar (200 mg/dL or above), a single result may be sufficient. Your doctor will advise on the appropriate approach.
Very high blood sugar — particularly above 300 mg/dL (16.7 mmol/L) — may indicate a serious situation requiring urgent medical assessment. Levels above 600 mg/dL can lead to a life-threatening condition called Hyperosmolar Hyperglycaemic State (HHS). If you or someone you know has a very high blood sugar reading with symptoms such as confusion, severe dehydration, or drowsiness, seek emergency medical care immediately.