Lab results explained
Fasting Blood Glucose Explained: What Your Blood Sugar Number Means
A fasting blood glucose test measures the sugar in your blood after at least 8 hours without food. It is one of the main tests used to screen for prediabetes and diabetes. Unlike HbA1c, which reflects the past few months, it is a snapshot of one morning. This guide explains the ranges, what can throw the number off, and what is tested next.
Part of Diabetes blood tests
Typical fasting ranges at a glance
- Normal
- 70 to 99 mg/dL (3.9 to 5.5 mmol/L)
- Prediabetes (impaired fasting glucose)
- 100 to 125 mg/dL (5.6 to 6.9 mmol/L)
- Diabetes range
- 126 mg/dL or higher (7.0 mmol/L or higher), confirmed by a repeat test
- Low (hypoglycemia)
- Below 70 mg/dL (below 3.9 mmol/L)
Some countries start the prediabetes range at 110 mg/dL (6.1 mmol/L). To convert, divide mg/dL by 18 to get mmol/L. Always use the reference range printed on your own report.
What does a fasting glucose test measure?
Glucose is the main sugar your body uses for energy. After you eat, glucose rises and insulin moves it into your cells. After a night without food, your blood sugar should settle back into a steady range. A fasting result above that range suggests your body is not handling glucose as well as it should, often because of insulin resistance.
Normal, prediabetes and diabetes
A fasting glucose of 70 to 99 mg/dL (3.9 to 5.5 mmol/L) is normal. From 100 to 125 mg/dL (5.6 to 6.9 mmol/L) is called impaired fasting glucose, a form of prediabetes that raises the risk of type 2 diabetes. A result of 126 mg/dL (7.0 mmol/L) or higher on two separate days, or with an HbA1c of 6.5% or more, points to diabetes.
What can throw the result off?
Eating or drinking anything but water before the test, a recent illness or infection, stress, poor sleep, and medicines such as steroids can raise glucose. Many people also have a natural early-morning rise in blood sugar. Heavy exercise the day before can lower it. If a result surprises you, ask whether to repeat it.
What is tested next?
A high fasting result is usually repeated, often with an HbA1c, which shows your average over two to three months. Your clinician may also suggest an oral glucose tolerance test, in which glucose is measured before and two hours after a sweet drink. A random glucose of 200 mg/dL (11.1 mmol/L) or more with symptoms such as thirst and frequent urination also suggests diabetes.
Low blood sugar
A level below 70 mg/dL (3.9 mmol/L) is low, and below 54 mg/dL (3.0 mmol/L) is significantly low. Symptoms include shaking, sweating, a fast heartbeat, hunger and confusion. It mainly happens in people taking insulin or some diabetes tablets. Low blood sugar without diabetes is uncommon and worth discussing with a clinician. If someone with low blood sugar is confused, very drowsy or cannot swallow safely, call your local emergency number.
What can lower fasting glucose?
Regular activity, including a short walk after meals, losing weight if you carry extra, fewer sugary drinks and refined carbohydrates, more fibre, enough sleep and not smoking all help your body use insulin better. In prediabetes, losing about 5% to 7% of body weight lowers the chance of developing type 2 diabetes. If medication is needed, that choice is for you and your clinician.
Frequently asked questions
Is a fasting blood sugar of 100 bad?
A fasting glucose of 100 mg/dL (5.6 mmol/L) is the lower edge of the prediabetes range in many countries. It is not diabetes, but it is a useful early warning. A repeat test or an HbA1c helps confirm it, and many people bring it down with changes to activity, diet and weight.
How long do I need to fast before a glucose test?
Usually at least 8 hours, with only water. Most people have the test first thing in the morning. Take your usual medicines unless your clinician tells you otherwise.
Why is my blood sugar higher in the morning?
Hormones released in the early morning, such as cortisol and growth hormone, raise blood sugar before you wake up. This is called the dawn phenomenon. It is common and usually mild, but it can push a fasting result up, especially with diabetes.
Which is better, fasting glucose or HbA1c?
They answer different questions. Fasting glucose is a snapshot of one morning; HbA1c is an average over about three months. Clinicians often use both, and one can confirm the other.
See your blood sugar next to your meals, sleep and activity
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