Fasting Blood Sugar (FBS)

What is Fasting Blood Sugar (FBS)?

Fasting Blood Sugar, or FBS, measures the amount of glucose in your blood after you have gone without food or drink (other than water) for several hours. Glucose is the body's main source of energy, and it comes from the carbohydrates you eat as well as stores released by the liver. Testing it in a fasted state removes the temporary rise that happens right after a meal, giving a clearer baseline reading of how your body manages blood sugar.

FBS is one of the most commonly ordered blood tests in India, used both as a routine health check and as a first step in screening for diabetes.

Why is this test done?

●       To screen for diabetes and prediabetes, including in people with no symptoms

●       To monitor blood sugar control in someone already diagnosed with diabetes

●       As part of a routine annual health checkup or pre-employment screening

●       To check for gestational diabetes during pregnancy

●       To follow up on a previous blood sugar reading that was higher than expected

Who should get this test?

Talk to your doctor about this test if you notice symptoms such as unusual thirst, urinating more often than normal, unexplained fatigue, or blurred vision. These can be associated with high blood sugar, though they also overlap with several other conditions, so a test is the only way to know what's actually happening.

It is also worth discussing this test with your doctor if any of the following apply to you:

●       A parent or sibling with diabetes

●       Being overweight, especially with a sedentary routine

●       A personal history of gestational diabetes or polycystic ovary syndrome (PCOS)

●       High blood pressure or high cholesterol

●       Age over 35, particularly alongside any of the factors above

How is the test done?

FBS uses a standard blood draw from a vein in your arm, and takes only a few minutes. You will be asked to fast, meaning no food or drink other than plain water, for 8 to 12 hours before the sample is taken. Most people schedule this first thing in the morning after an overnight fast. Routine medications can usually be taken as prescribed unless your doctor tells you otherwise, and results are typically available the same day.

Normal Range

Reference ranges can vary slightly between labs depending on their equipment and method. Always check the range printed on your own report.

Sources vary somewhat on the exact cutoff for a normal fasting reading in non-pregnant adults, with some citing below 100 mg/dL and others citing 70 to 99 mg/dL as the normal band. The 126 mg/dL diabetes threshold, however, is consistent and is the cutoff endorsed by the World Health Organization. For pregnant women, the World Health Organization's diagnostic threshold for gestational diabetes is a fasting level of 92 mg/dL or above, which is lower than the general adult cutoff because pregnancy changes how the body handles glucose.

What does a High result mean?

A fasting blood sugar above the normal range is associated with prediabetes or diabetes, depending on how high the number is and whether it's confirmed on a repeat test. A single high reading is not enough on its own to diagnose diabetes; doctors typically want to see the pattern confirmed, sometimes alongside an HbA1c or oral glucose tolerance test.

High readings aren't always about diabetes. They can also be associated with acute illness or infection, hormonal conditions such as thyroid disorders or Cushing's syndrome, pancreatitis, and certain medications, including corticosteroids. If you're taking any medication and notice a high reading, discuss it with your doctor rather than adjusting the dose yourself.

What does a Low result mean?

A fasting blood sugar below the normal range is called hypoglycemia. In people with diabetes, this is often linked to insulin or certain oral diabetes medications, particularly if a dose was taken without enough food afterward. Always consult your doctor before changing the timing or dose of any diabetes medication.

In people without diabetes, low readings can be associated with prolonged fasting, heavy alcohol intake, some liver conditions, or hormonal issues involving the adrenal or pituitary glands. Symptoms like shakiness, sweating, or lightheadedness alongside a low reading are worth mentioning to your doctor.

Factors that can affect results

●       Not fasting for the full 8 to 12 hours, or forgetting and having a snack

●       Acute stress, illness, or infection at the time of the test

●       Certain medications, including steroids and some diuretics

●       Dehydration

●       Intense exercise shortly before the blood draw

●       Poor sleep the night before the test

How to Keep It in the Normal Range

Blood sugar control is shaped by everyday patterns more than any single food or habit. A few general principles that support a healthy fasting glucose level:

●       Eat balanced meals built around whole grains, vegetables, and protein rather than heavily processed or sugary foods

●       Stay physically active with regular movement through the week

●       Drink enough water through the day

●       Aim for consistent, adequate sleep

●       Keep up with routine health checkups so trends over time are visible, not just one number

Questions You May Be Wondering

Is a fasting blood sugar of 100 mg/dL bad?

A reading of 100 mg/dL falls at the start of the range some sources classify as prediabetes. It isn't a diagnosis on its own, and many people with a single borderline reading go on to have normal follow-up tests. Your doctor is best placed to interpret it alongside your history and any repeat testing.

Can stress affect fasting blood sugar?

Yes. Stress hormones like cortisol can raise blood sugar temporarily, even in people without diabetes. This is one reason doctors often ask for a repeat test before drawing conclusions from a single high result.

Do I really need to fast before this test?

Yes, for an accurate fasting reading. Eating or drinking anything other than water within the fasting window can raise your glucose level and make the result unreliable.

What is the difference between FBS and HbA1c?

FBS shows your blood sugar at a single point in time. HbA1c reflects your average blood sugar over roughly the past two to three months. Doctors often use the two together for a fuller picture.

Can I drink water during the fasting period?

Yes, plain water is fine and is actually encouraged, since dehydration can affect your reading. It's food, sugary drinks, tea, and coffee that need to be avoided during the fasting window.