Diabetes Test

Blood Sugar (Glucose) Test

Q: Do I need to fast for a blood sugar test?

It depends on the test: a fasting glucose needs 8–12 hours without food, while a random glucose and HbA1c do not require fasting.

Q: What is a normal fasting blood sugar?

Roughly 4.0–5.4 mmol/L is typical when fasting; 5.5–6.9 suggests pre-diabetes, and 7.0 or above on repeat testing indicates diabetes.

Q: Can stress raise blood sugar?

Yes. Acute stress and illness can temporarily raise glucose, which is one reason borderline results are confirmed with a repeat or HbA1c.

Q: What is the difference between glucose and HbA1c?

Glucose is your level at a single moment, while HbA1c reflects your average over two to three months: the two are complementary.

Blood sugar tests measure how much glucose is in your blood. They are the key tests to diagnose diabetes and prediabetes: and to monitor them once diagnosed.

Written and clinically reviewed by Suman Konda, PharmD, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: 01 August 2026 · How we check our content

Types of blood sugar tests

There are three common blood sugar tests: Fasting Blood Sugar (FBS), measured after 8–12 hours of no food; Postprandial Blood Sugar (PPBS or PP), measured exactly 2 hours after a meal; Random Blood Sugar (RBS), measured at any time without fasting. Each has different normal ranges. Laboratories in the US and India report in mg/dL; UK, Australian and most European labs report in mmol/L.

Blood Sugar Normal Ranges

Test Type	Normal	Prediabetes	Diabetes
Fasting (FBS)	< 100 mg/dL	100 – 125 mg/dL	≥ 126 mg/dL
Post-meal 2hr (PPBS)	< 140 mg/dL	140 – 199 mg/dL	≥ 200 mg/dL
Random (RBS)	< 140 mg/dL	140 – 199 mg/dL	≥ 200 mg/dL (with symptoms)
HbA1c	< 5.7%	5.7 – 6.4%	≥ 6.5%

What does HIGH fasting blood sugar mean?

HIGH Fasting sugar ≥ 126 mg/dL on two occasions: Diabetes

A fasting blood sugar of 126 mg/dL or above on two separate tests confirms diabetes. Common symptoms of diabetes: frequent urination (especially at night), excessive thirst, unexplained weight loss, blurred vision, fatigue, slow-healing wounds, frequent infections. Type 2 diabetes is common worldwide and frequently silent in its early years, which is why screening matters. Type 2 diabetes is preventable with diet, exercise and weight management at the prediabetes stage.

What is Prediabetes?

BORDERLINE Fasting sugar 100–125 mg/dL: Prediabetes

Prediabetes means your blood sugar is higher than normal but not yet at diabetes levels. Without action, 15–30% of people with prediabetes develop Type 2 diabetes within 5 years. The good news: prediabetes is almost always reversible with lifestyle changes, losing 5–7% of body weight, exercising 150 minutes per week, and reducing refined carbohydrates. No medication is needed at this stage in most cases. Recheck every 6–12 months.

What does LOW blood sugar mean?

LOW Blood sugar below 70 mg/dL: Hypoglycaemia

Low blood sugar (hypoglycaemia) causes shakiness, sweating, dizziness, confusion, palpitations and hunger. In people on diabetes medication (insulin, sulfonylureas), hypoglycaemia is a common and potentially dangerous side effect. In people not on medication, persistent low blood sugar (especially fasting) needs investigation for insulinoma or other causes. Eat something with quick sugar (glucose tablets, fruit juice) to treat an episode immediately.

Questions to ask your doctor

Is a single high reading enough to diagnose diabetes or do I need a repeat test?
Should I also check HbA1c for a 3-month average?
Do I need medication or can I manage this with diet changes?
What blood sugar targets should I aim for?

The different ways blood sugar is measured

Blood glucose can be tested in several ways, each answering a different question. A fasting glucose measures your level after an overnight fast; a random glucose is taken at any time; and a glucose tolerance test measures how your body handles a sugary drink over two hours. HbA1c, meanwhile, shows your average glucose over the past two to three months.

Together these detect diabetes and pre-diabetes, and help distinguish them: a fasting glucose of 7.0 mmol/L or above, or an HbA1c of 48 mmol/mol or above, indicates diabetes when confirmed.

What high and low readings mean

Persistently high glucose (hyperglycaemia) is the hallmark of diabetes and, over time, damages blood vessels and nerves. Very high levels with symptoms such as extreme thirst, frequent urination and weight loss need prompt assessment.

Low glucose (hypoglycaemia) is less common but important, usually occurring in people treated with insulin or certain diabetes tablets, causing shakiness, sweating and confusion that need quick treatment with fast-acting carbohydrate.

Getting an accurate result

Preparation matters for some glucose tests: a fasting sample requires 8–12 hours without food (water is allowed), and recent illness, stress or a high-carbohydrate meal can affect a random reading. Following the specific instructions for your test ensures the result reflects your true baseline.

Because a single reading is a snapshot, borderline results are usually confirmed with a repeat test or an HbA1c.

Frequently Asked Questions

Do I need to fast for a blood sugar test?