A fasting blood sugar test measures the amount 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 prediabetes, diabetes, and gestational diabetes, and it works by catching your blood sugar at its lowest natural point, when no recent meal is influencing the number.
How the Test Works
When you fast overnight, your body doesn’t stop managing blood sugar. Your liver keeps a steady supply of glucose flowing into your bloodstream by breaking down its stored form of sugar (called glycogen) and by building new glucose from amino acids and other raw materials. The pancreas coordinates this process by releasing hormones that tell the liver how much glucose to produce. In a healthy body, this system keeps blood sugar in a tight range even while you sleep. In someone developing diabetes, the system starts losing precision, and fasting glucose drifts higher than it should.
The test itself is straightforward. A technician draws blood from a vein in your arm, and the sample goes to a lab. Sometimes a provider will prick your finger instead and use a portable glucose monitor to get a reading in the office.
How to Prepare
You need to fast for 8 to 12 hours before the test, which is why most people schedule it first thing in the morning. Plain water is fine to drink during the fast, and staying hydrated actually matters for accuracy. Research on people with type 2 diabetes found that even mild dehydration (from three days of reduced water intake) raised fasting glucose readings by about 1 mmol/L compared to when the same people were well-hydrated. The mechanism appears to involve cortisol, a stress hormone that rises when you’re dehydrated and pushes blood sugar up.
Skip coffee, juice, soda, and flavored water, even if they’re sugar-free. These can enter your bloodstream and skew the results. If you take medications, ask your provider whether to take them before the test or wait until after, since several common drug classes can raise fasting glucose on their own. Steroids are the biggest offenders, causing elevated blood sugar in roughly a third of people without pre-existing diabetes. Blood pressure medications like certain beta blockers and thiazide diuretics can nudge fasting glucose up as well, and statins carry a 9% to 12% incidence of new diabetes diagnoses in some analyses.
What Your Results Mean
The American Diabetes Association’s 2025 standards define three ranges for fasting blood sugar:
- 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
A single high reading doesn’t mean you have diabetes. A diabetes diagnosis requires a result of 126 mg/dL or higher on two separate tests. If your first result falls in the prediabetes or diabetes range, your provider will likely repeat the fasting test or order a complementary test to confirm.
Why Morning Timing Can Affect Results
Your body naturally releases a surge of hormones between roughly 4 a.m. and 8 a.m., including cortisol, growth hormone, and glucagon. These hormones increase insulin resistance and signal the liver to produce more glucose, preparing your body for waking activity. In people with diabetes, this “dawn phenomenon” can push fasting blood sugar noticeably higher than it would be at, say, 2 a.m. If your fasting results seem higher than expected, the timing of your blood draw relative to this hormonal surge could be a factor.
Fasting Glucose vs. the A1C Test
Your provider might order an A1C test instead of, or alongside, a fasting glucose test. While fasting glucose captures a single snapshot, the A1C reflects your average blood sugar over the previous two to three months. It doesn’t require fasting, and day-to-day fluctuations from stress, exercise, or a bad night’s sleep don’t affect it. A1C also shows less natural variation between tests: its measurement varies by about 3.6% from test to test, compared to 5.7% for fasting glucose.
That said, fasting glucose has its own advantages. It directly measures blood sugar, which is the actual problem in diabetes, rather than measuring a downstream effect on red blood cells. A1C can be thrown off by conditions that affect red blood cell turnover, including anemia, significant blood loss, pregnancy, and certain hemoglobin traits that are common in some populations. In those situations, fasting glucose gives a more reliable picture. The two tests also don’t always agree. Some people with a normal A1C will have elevated fasting glucose, and vice versa, so providers often use both for a fuller view.
Who Should Get Tested
The U.S. Preventive Services Task Force recommends diabetes screening for adults aged 35 to 70 who are overweight or obese, defined as a BMI of 25 or higher. This applies to people without symptoms. If your initial result comes back normal, repeating the test every three years is a reasonable schedule, though your provider may suggest testing sooner based on other risk factors like family history or a history of gestational diabetes.
What Happens After an Abnormal Result
If your fasting glucose falls in the prediabetes range (100 to 125 mg/dL), that’s a signal your blood sugar regulation is starting to slip, but it’s also the stage where lifestyle changes have the most impact. If results point toward diabetes, your provider will confirm with a second test, either another fasting glucose or an A1C of 6.5% or higher. From there, the conversation shifts to management, which varies widely depending on how elevated your levels are and whether you have other health conditions.