An FBS test, or fasting blood sugar test, measures the level of glucose in your blood 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 falls below 100 mg/dL, while anything at 126 mg/dL or higher on two separate tests indicates diabetes.
How the Test Works
The concept is straightforward: by measuring your blood sugar in a fasted state, the test captures your body’s baseline ability to regulate glucose without the influence of a recent meal. When you eat, your blood sugar naturally rises, then your body releases insulin to bring it back down. The fasting test removes that variable entirely, giving a clearer picture of how well your system handles glucose on its own.
In healthy people, blood sugar and insulin levels stay remarkably flat and constant overnight. A small, brief bump in insulin secretion happens just before dawn to keep the liver from releasing too much stored glucose. In people with type 2 diabetes, this pre-dawn regulation breaks down. The liver produces extra glucose in the early morning hours without enough insulin to compensate, which is why morning fasting numbers can run high even if you ate well the night before. This is sometimes called the “dawn phenomenon,” and it can raise your average blood sugar enough to move your A1C up by about 0.4%.
What the Numbers Mean
The diagnostic ranges, based on Mayo Clinic and American Diabetes Association criteria, break down into three categories:
- Normal: Below 100 mg/dL (5.6 mmol/L)
- Prediabetes: 100 to 125 mg/dL (5.6 to 6.9 mmol/L)
- Diabetes: 126 mg/dL (7.0 mmol/L) or higher on two separate tests
That “two separate tests” part matters. A single high reading isn’t enough for a diabetes diagnosis because plenty of temporary factors can push your number up. Your doctor will typically confirm with a second fasting test or a different type of glucose test before making a diagnosis.
How to Prepare
You’ll need to fast for 8 to 12 hours before the test, which is why most people schedule it first thing in the morning. During the fasting window, you can drink plain water, and you should. Staying hydrated makes the blood draw easier. But skip everything else: no flavored water, no juice, no coffee. Even black coffee can affect the results.
Avoid strenuous exercise the day before the test, since intense physical activity can temporarily lower blood sugar and give you a misleadingly low reading. Normal activity is fine. If you take any medications in the morning, check with your provider about whether to take them before or after the blood draw.
What Happens During the Draw
The test itself takes only a few minutes. A small needle goes into a vein in your arm, and a vial of blood is collected. You might feel a brief sting. Results typically come back within a day or two, depending on the lab.
FBS vs. A1C: Two Different Snapshots
Your doctor might order an FBS test, an A1C test, or both. They measure different things. The FBS captures your blood sugar at a single point in time. The A1C reflects your average blood sugar over the past two to three months by measuring how much glucose has attached to your red blood cells.
Research comparing the two tests has found that FBS is generally more reliable for separating people with diabetes from those without. A low A1C is strong evidence against diabetes, but a slightly elevated A1C alone isn’t enough to confirm it. The A1C can be thrown off by anemia, certain medications, and even demographic factors like race. FBS has its own downsides: it requires fasting, it can only be done in the morning, and a single reading can be skewed by stress or a bad night’s sleep. That’s why doctors often use both tests together to get the most complete picture.
Medications That Can Raise Your Results
Several common medications can push fasting blood sugar higher, sometimes enough to mimic prediabetes or diabetes when you don’t actually have it. The biggest offenders are corticosteroids (like prednisone), which cause elevated blood sugar in 40 to 65% of people who take them. Other medications with a notable effect include thiazide diuretics (a type of blood pressure medication), which raise fasting glucose by an average of about 5 mg/dL and increase diabetes incidence by roughly 10%. Beta blockers like metoprolol and atenolol have also been shown to elevate fasting glucose, with diabetes risk increasing by as much as 22% in some studies. Statins, antipsychotics, and certain immunosuppressants round out the list.
If you’re taking any of these and your fasting number comes back higher than expected, that context matters. Make sure your provider knows every medication you’re on before interpreting the result.
What a Prediabetes Result Means in Practice
A result between 100 and 125 mg/dL puts you in the prediabetes range, which affects roughly one in three American adults. This doesn’t mean diabetes is inevitable. It means your body is starting to struggle with insulin regulation, and the trajectory can be changed. Losing 5 to 7% of your body weight and getting about 150 minutes of moderate activity per week has been shown to cut the risk of progressing to type 2 diabetes by more than half.
If your result lands in this range, your doctor will likely recommend retesting in 6 to 12 months to see whether lifestyle changes are moving the number in the right direction. Some people stay in the prediabetes range for years without ever developing diabetes. Others progress quickly. Regular testing is the only way to know which direction you’re heading.