Primus Pathology Laboratory

Fasting Before Sugar (FBS) Test

Last Updated on July 28, 2022 by Dr. Neeraj Gujar


Fasting blood sugar can be used to screen for diabetes, prediabetes, and gestational diabetic (during pregnancy). This is also known as a fasting glucose test.

The fasting blood sugar levels can give important clues as to how someone is managing their blood sugar. After eating, blood sugar levels tend to rise about an hour before dropping.

High fasting blood sugar levels point to insulin resistance. And abnormally low fasting blood sugar could be due to diabetes medications. Knowing when to test and what to look for can help people stay healthy, especially if they have diabetes or are at risk of developing the condition.

Why Test Is Done?

For energy, the body requires glucose. This glucose is obtained from the foods we eat. The body doesn’t use all this energy at once. Insulin allows you to store and release glucose when it is needed.

After eating, blood sugar levels increase and peak around an hour later.

The exact timing and how high blood sugar rises depends on the diet of each person.

Significant rises can be triggered by food-related factors, such as:

  • Sugary foods and drinks
  • Having large meals
  • Eating foods that contain simple carbs like a bread

Insulin is released when blood sugar levels rise. Insulin lowers blood sugar and breaks it down so that the body can either use it for energy or store it for later.

Diabetes sufferers have difficulty with insulin in either of these two ways.

Type 1 diabetic sufferers do not produce enough insulin. Their bodies attack insulin-producing cells.

Type 2 diabetic sufferers do not respond to insulin. They may not be able to make enough insulin either.

Both cases result in high blood sugar and difficulty using glucose.

Three factors affect fasting blood sugar:

  • Size of the last meal
  • Contents of the last meal
  • Body’s ability to produce insulin and respond

The body’s ability to manage sugar can be seen in the blood sugar levels that are measured between meals. The body is unable to lower fasting blood sugar levels, which can be a sign that there was no way for it to do so.

This could indicate insulin resistance or inadequate insulin production. In some cases, it may be both.

Low blood sugar levels could indicate that diabetes medication is not working well.

When To Get Tested?

Many people don’t notice symptoms of gestational, Type 2, or prediabetes. It is possible to have the condition but not be aware. A fasting blood sugar test is usually ordered by healthcare providers in case of:

  • A standard annual physical exam to monitor the person’s blood sugar over time.
  • Pregnant women should ensure that hormones during pregnancy are not causing diabetes.
  • If a person is suffering from diabetes symptoms, they should have a family history of it or other risk factors (such as obesity).
  • If a person’s previous blood glucose level was higher than normal.

People are likely to experience symptoms if their blood sugar levels are too low or too high.

Low blood sugar levels

Blood sugar that is too low can cause symptoms such as:

  • Shaking And Sweating
  • Difficulty Concentrating
  • Feeling Jittery
  • Lack Of Energy
  • Fatigue or Tiredness
  • Weakness
  • Headaches or Muscle Aches
  • Fast Or Irregular Heartbeat
  • Skin That Looks Paler Than Usual
  • Lack Of Coordination

In extreme cases, low blood sugar can trigger seizures, loss of consciousness, confusion, and an inability to drink or eat.

High blood sugar levels

High blood sugar levels, or hyperglycemia, can cause the following symptoms:

  • Increased Hunger or Thirst
  • Blurred Vision
  • Excessive Urination
  • Tiredness
  • Headache

As with low blood sugar, high blood sugar may cause loss of consciousness or seizures if people leave it untreated. Persistent high blood sugar can increase the risk of serious complications that doctors relate to diabetes, such as cardiovascular disease.

How To Get Tested?

Image of medical syringes

Individuals and healthcare professionals can use either a glycosylated hemoglobin test (HbA1c) or a traditional blood sugar test to assess fasting blood sugar levels.

The main test doctors use for managing diabetes is HbA1c. The HbA1c test is used to measure how well the body manages blood sugar over time. It is usually taken from the last 2 months.

This test will be performed in a doctor’s office or laboratory, or you can opt for a home visit. A second test may be required if levels are extremely high. Results are expressed as a percentage.

Before the Test

Fasting glucose tests are best prepared by not eating or drinking anything for at least 8 to 12 hours. You will be told by your healthcare provider how long you should fast.

During the Test

Common blood tests are often used to perform a fasting glucose test. The healthcare provider will:

  • To kill germs, clean the elbow area.
  • Tie a band around the upper arm to help blood flow through your veins.
  • Place a needle in a vein. It will feel like a pinch.
  • Draw blood in a vial.
  • Take off the band, then remove the needle.
  • To stop bleeding, apply pressure to the site of the needle insertion.
  • Apply a bandage to the area.

The healthcare provider will take the blood sample and send it to the lab for testing.

Your healthcare provider may be able, in certain cases, to test your blood sugar using a prick on your finger rather than a needle in your vein. To measure your blood glucose levels in the office, the provider uses a glucose monitor and test strip.

After The Test

A blood test is very safe and common, with virtually no risk of complications.

Test Results

The results of a fasting glucose test can usually be obtained quickly within days or even hours.

The fasting blood sugar test results will be returned as a number

  • 99 mg/dL and lower: This refers to a normal fasting blood glucose level.
  • 100-125mg/dL: A fasting blood sugar reading in this range usually indicates prediabetes. This indicates that your blood sugar levels are elevated above normal, but not high enough for you to be diagnosed with diabetes.
  • 126 mg/dL and above: This is high blood sugar, which is the main sign of diabetes.

Your healthcare provider may order a repeat test if you have high fasting blood sugar levels. If you are diagnosed with prediabetes, your healthcare provider may recommend that you take a fasting sugar test once every two years. These results will let you know if you are on the verge of developing Type 2 diabetes.

Also Read: Important Medical Tests for People Above 50

What To Do in Case of High Levels?

To prevent fasting blood sugar levels from rising excessively, it is important to eat a healthy diet. These strategies include:

  • Limit your sugar and salt intake
  • Ensure you eat plenty of fiber-rich foods
  • Whole grain pasta and bread are better than white bread and pasta.
  • Choose non-starchy vegetables
  • Consume high-protein foods to increase your feeling of fullness

A similar diet should be followed by people who are taking diabetes medication and at high risk for dangerous blood sugar drops. They must also take preventative steps to stop blood sugar from falling.
These include:

  • Regular meals throughout the day
  • Avoiding or limiting alcohol
  • During intense exercise, increase food intake and snack frequency

Final Takeaway

It is essential to maintain healthy fasting sugar levels to manage diabetes. People with prediabetes can reverse their condition by controlling blood sugar levels and avoiding developing diabetes.

Diabetics and prediabetes should regularly test their blood sugar levels and keep track of their results. If they notice any unusual symptoms or changes, they should consult their doctor.


Can we drink water before fasting blood sugar test?

Yes. It is fine to drink water before a blood test unless you are instructed by your doctor to not drink any water.

What should your fasting blood sugar be in the morning?

Fasting blood sugar in the morning before eating should be less than 100 mg/dL. And around 1-2 hours after a meal, it should be less than 140 mg/dL. Whereas, again after 2-3 hours of eating it should come back to less than 100 mg/dL.

Can you brush your teeth before a fasting blood test?

​Yes, brushing your teeth is allowed unless indicated by your doctor otherwise, assuming you do not swallow the lather or use a large amount of toothpaste.


by Dr. Neeraj Gujar

Dr. Neeraj Gujar has done his M.D. in Pathology from Government Medical College. He has worked at various organisations such as Breach Candy Hospital and Tata Memorial Hospital along with many other private organisations. During his working career, he realised the pressing need for exceptional quality diagnostic services. A diagnostic centre that can consistently give accurate results verified by a consultant doctor (M.D. Pathologist) on which your doctor can depend. This has been the founding principle of Primus Pathology and Diagnostics.

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