backup og meta

FBS Test (Fasting Blood Sugar): Why and How is it Done?

Medically reviewed by Regina Victoria Boyles, MD · Pediatrics

Written by Nikita Bhalla · Updated Jul 26, 2022

FBS Test (Fasting Blood Sugar): Why and How is it Done?

A fasting blood sugar test or FBS test measures the glucose levels in the blood. 

The fasting blood sugar test helps the doctor to understand if the person is suffering from prediabetes or diabetes. Also, it helps the doctor to understand if the person is suffering from hyperglycemia (high blood sugar) or hypoglycemia (low blood sugar). 

Sugar is one of the main sources of energy of the human body. A body converts the carbohydrates we consume into glucose to run throughout the day. Diabetes is a health condition that increases blood sugar levels abnormally.

Fasting blood sugar test is usually done to assess the risk of gestational diabetes, type 2 and type 1 diabetes, or to monitor the condition in those already diagnosed with these conditions. 

Sugar levels in the blood are controlled by a hormone called insulin. However, if a person is suffering from diabetes, it is either because of lesser insulin production or the insulin produced does not work properly.

This causes the sugar to build up in the body that keeps on increasing if not treated on time. Increased glucose levels can lead to severe organ damage.

In some cases, health experts suggest fasting blood sugar or FBS test to detect hypoglycemia. It is a condition where the blood sugar levels are very low.

Why is Fasting Blood Test Done?

Doctors may suggest undergoing a fasting blood sugar test to assess the conditions related to hypoglycemia or hyperglycemia.

Your doctor may advise this test if you show the following symptoms.

The symptoms of hypoglycemia may include:

  • Sweating
  • Headaches
  • Confusion
  • Hunger
  • Anxiety
  • Trembling

The symptoms of hyperglycemia may include:

  • Wounds that are slow to heal
  • Numbness or “pins and needles” sensation in the fingers and toes
  • Increased thirst
  • More frequent urination
  • Blurred vision
  • Fatigue

Also, a doctor may suggest an FBS test if a person has certain risk factors for diabetes. These include: 

  • Lack of exercise
  • Heart disease
  • A family history of diabetes
  • High blood pressure
  • Being overweight

If a woman is pregnant, she will likely undergo an FBS test depending on her risk of diabetes. An oral glucose tolerance test (OGTT) is conducted between 24th and 28th week to check for gestational diabetes. Gestational diabetes is a form of diabetes that occurs during pregnancy. It is important to follow medical advice. 

Prerequisites of a Fasting Blood Test

Before undergoing a fasting blood sugar test, a person should fast for at least eight hours, preferably overnight while the body is at rest. For pregnant women, it is important to discuss the best type of blood sugar test recommended by your doctor and follow their advice regarding fasting. 

People who are undergoing a fasting blood sugar test should mention all the medicines she or he is using for numerous reasons to the doctor. A doctor may suggest avoiding a few medicines to prevent any test result abnormalities like a false positive or abnormally high or low sugar levels. Therefore, ensure that a doctor knows every medicine you use for different reasons.

Follow your doctor’s advice on specific instructions regarding diet and medicine.

Understanding the Results

Below is the way you can understand your results.

The normal result for the FBS test is between 70 and 100 milligrams per decilitre (mg/dL). For a random blood sugar test, a normal level is usually under 125 mg/dL. 

However, if a person has abnormal fasting blood sugar test results, it may indicate diabetes or prediabetes.

  • A blood sugar level of 126 mg/dL or higher may indicate a person has diabetes.
  • A blood sugar level of 100-125 mg/dL may indicate a person has prediabetes.

Normally, if a random or non-fasted blood sugar test is done earlier and it shows abnormal results, your doctor may advise a fasting blood sugar test. If the fasting blood sugar test results are abnormal, your doctor will suggest undergoing additional tests to confirm the diagnosis of diabetes or other related conditions.

When should it be repeated?

If your doctor is unsure about the fasting blood sugar test accuracy or wants to be sure of the results, you may be advised to repeat the fasting blood sugar test. It may also be repeated along with other confirmatory blood sugar tests.

For those already receiving treatment for high blood sugar levels, your doctor may advise repeating the test at regular intervals to monitor the progress and effectiveness of the treatment.


The test is very simple and does not take much time. It hardly takes 5 minutes to undergo an FBS test. 

A person is required to visit a pathologist’s lab where the healthcare assistant will first tie an elastic band around the upper arm. This helps the blood to block and bulge out the vein around the elbow joint. 

Once the vein bulges out due to blood blockage, the healthcare assistant will clean the area with an antiseptic. Next, he or she will insert a needle and draw a few drops of blood. Once done, the assistant will remove the needle, clean the area with an antiseptic, and apply a bandage over the puncture site.

The assistant will transfer the blood drawn into a glass tube container and pass it forward for the analysis.

However, there are very low chances that you can experience any problem during or after the blood test. The possible risks are the same as those associated with all blood tests. The risks of fasting blood sugar or FBS test include:

  • Hematoma, or blood pooling underneath the skin
  • Infection
  • Excessive bleeding
  • Lightheadedness or fainting

If you experience these after a blood test, you should immediately visit a doctor and get medical help.

The results of the FBS test are usually available within a few hours. Once received, you are required to visit a doctor’s clinic to get the results interpreted and plan appropriate treatment.


Hello Health Group does not provide medical advice, diagnosis or treatment.

Medically reviewed by

Regina Victoria Boyles, MD


Written by Nikita Bhalla · Updated Jul 26, 2022

advertisement iconadvertisement

Was this article helpful?

advertisement iconadvertisement
advertisement iconadvertisement