FBS stands for fasting blood sugar, a blood test that measures your glucose level after you haven’t eaten for at least 8 hours. It’s one of the most common screening tools for diabetes and prediabetes. A normal result is below 100 mg/dL (5.6 mmol/L), while 126 mg/dL (7.0 mmol/L) or higher on two separate tests indicates diabetes.
What the Test Actually Measures
When you eat, your body breaks down carbohydrates into glucose, which enters your bloodstream. Insulin then helps move that glucose into your cells for energy. The fasting blood sugar test skips the complication of a recent meal and instead measures what your body does with glucose on its own, overnight, with no food coming in.
During a fast, your liver takes over the job of keeping blood sugar stable. It does this two ways: breaking down stored glycogen (a starch-like reserve) into glucose, and manufacturing new glucose from scratch using available raw materials like amino acids. Insulin levels drop during fasting, and another hormone called glucagon rises to trigger this process. Your FBS result reflects how well this system is working. If your fasting glucose is consistently high, it means your body is either producing too much glucose, not responding properly to insulin, or both.
How to Read Your Results
FBS results fall into three categories:
- Normal: Below 100 mg/dL (5.6 mmol/L). Your blood sugar regulation is working as expected.
- Prediabetes: 100 to 125 mg/dL (5.6 to 6.9 mmol/L). Your fasting glucose is elevated but not yet in the diabetic range. This is sometimes called “impaired fasting glucose.”
- Diabetes: 126 mg/dL (7.0 mmol/L) or higher on two separate tests. A single high reading isn’t enough for diagnosis on its own.
The low end of the normal range sits around 70 mg/dL (3.9 mmol/L). If your result falls below that, your doctor may investigate causes of low blood sugar, such as medication effects or other metabolic issues.
How to Prepare for the Test
You need to fast for 8 to 12 hours before the blood draw. Most people schedule the test first thing in the morning so the fasting window overlaps with sleep. You can drink plain water during this time, but avoid flavored water, sparkling water with added sweeteners, coffee, tea, or juice. Even small amounts of sugar or artificial sweeteners can affect results.
If you take prescription medications in the morning, ask your doctor beforehand whether you should take them as usual or wait until after the test. Don’t stop any medication on your own. Some drugs can influence blood sugar readings, and your provider needs to know what you’re taking to interpret the results correctly.
What Can Throw Off Your Results
A high FBS reading doesn’t always mean diabetes. Several temporary factors can push your numbers up.
Stress is one of the biggest. When you’re under physical or emotional stress, your body releases cortisol and adrenaline, hormones designed to flood your bloodstream with quick energy. This raises blood sugar and simultaneously makes your cells less responsive to insulin, so more glucose stays in your blood. If you were especially anxious, sleep-deprived, or fighting off an illness the night before your test, your result may be higher than your true baseline.
Poor sleep has a similar effect. Interrupted or insufficient sleep changes how your body uses insulin and can make blood sugar less predictable. Even one bad night can nudge your fasting number upward. This is one reason doctors confirm a diabetes diagnosis with two separate tests rather than relying on a single result.
FBS Compared to the A1C Test
Your doctor may also order an A1C test (sometimes called hemoglobin A1c or HbA1c), which works differently. While FBS captures a snapshot of your glucose level right now, the A1C reflects your average blood sugar over the previous two to three months. The A1C is convenient because it doesn’t require fasting and can be done at any time of day.
However, the A1C isn’t perfect for everyone. Some people have A1C readings that run higher or lower than their actual glucose levels would suggest, due to variations in how their red blood cells process sugar. This mismatch can lead to overdiagnosis or underdiagnosis. In people already being treated for diabetes, a misleadingly high A1C could prompt unnecessarily aggressive treatment, while a misleadingly low one might result in too little treatment. Research from the VA health system found that using glucose tests alongside A1C, rather than A1C alone, leads to more accurate diagnoses. Most experts now recommend combining both types of testing.
What Happens After an Abnormal Result
If your fasting blood sugar comes back in the prediabetes or diabetes range, your doctor will typically want to confirm it. That usually means repeating the FBS test on a different day, ordering an A1C, or running an oral glucose tolerance test, which measures how your body handles a sugary drink over a two-hour window. A single elevated reading from a pharmacy screening or health fair isn’t enough to make a diagnosis, but it is a reason to follow up with your doctor for formal testing.
A prediabetes result is genuinely useful information, not just an early warning. Fasting glucose in the 100 to 125 mg/dL range means your blood sugar regulation is under strain but hasn’t failed. At this stage, lifestyle changes like increased physical activity, modest weight loss, and dietary adjustments can often bring numbers back into the normal range and delay or prevent progression to type 2 diabetes.
If the result confirms diabetes, your doctor will discuss next steps based on how high your levels are, your A1C, and your overall health picture. Early-stage type 2 diabetes is often managed initially with lifestyle changes and a single oral medication, with treatment adjusted over time depending on how well your blood sugar responds.