Fasting blood sugar is the level of glucose in your blood after you haven’t eaten for at least 8 hours. A normal reading falls below 100 mg/dL (5.6 mmol/L), while 126 mg/dL (7 mmol/L) or higher on two separate tests indicates diabetes. This simple blood draw is one of the most common tools for screening metabolic health, and understanding what the numbers mean puts you in a much better position to act on them.
What the Numbers Mean
Fasting blood sugar results fall 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 mmol/L) or higher on two separate tests
That middle range, prediabetes, is where a lot of people land without realizing it. It means your body is already struggling to manage glucose efficiently, but the process hasn’t progressed to full diabetes. This matters more than many people assume. In a large Australian study tracking cardiovascular outcomes, people with impaired fasting glucose had roughly 2.5 times the risk of dying from heart disease compared to those with normal levels. That’s nearly as high as the risk seen in people with diagnosed diabetes (2.6 times). Overall, 65% of all cardiovascular deaths in the study occurred in people who had some form of abnormal glucose metabolism at baseline.
A single elevated reading doesn’t automatically mean you have prediabetes or diabetes. That’s why a diabetes diagnosis requires at least two separate tests showing elevated results. Temporary spikes can happen for several reasons.
How the Test Works
The test itself is straightforward: a blood draw, typically first thing in the morning. You need to fast for 8 to 12 hours beforehand, which is why morning appointments make the process easiest. A few sips of water the morning of the test are fine and won’t affect your results. Food, coffee, juice, or anything with calories will.
The American Diabetes Association recommends that all adults begin screening by age 35, with repeat testing at least every three years if results come back normal. If your results are borderline, or if you gain weight or develop new risk factors, more frequent testing is reasonable.
Why Morning Readings Can Run High
If you’ve ever tested your blood sugar first thing in the morning and been surprised by a higher-than-expected number, your hormones are likely the explanation. Between roughly 3 a.m. and 8 a.m., your body releases a surge of cortisol and growth hormone. These signal your liver to push out extra glucose, giving you the energy to wake up and start moving. This is called the dawn phenomenon, and it’s completely normal.
For people without diabetes, insulin kicks in and brings that glucose back down quickly. But if you have insulin resistance, prediabetes, or diabetes, the extra glucose lingers. That’s why some people see their highest readings of the day before they’ve eaten anything at all. If your fasting number consistently comes in higher than your pre-meal readings later in the day, the dawn phenomenon is the most likely culprit.
Factors That Skew Your Results
Fasting blood sugar isn’t just a reflection of what you ate yesterday. Several things can temporarily push a reading higher than your true baseline, and being aware of them helps you avoid a misleading result.
Stress is one of the biggest. When you’re under physical or emotional stress, your body releases cortisol and adrenaline, both of which raise blood sugar. A rough week at work, an argument the night before, or even anxiety about the blood draw itself can nudge your numbers up. Illness works the same way: an infection, a cold, or even something like a sunburn or muscle strain triggers a stress response that increases glucose output from the liver.
Poor sleep is another common factor. Even a single night of significantly disrupted sleep can reduce your body’s sensitivity to insulin, leading to a higher fasting reading the next morning. If you pulled an unusually bad night before your test, it’s worth mentioning to your provider.
Certain medications also interfere. Corticosteroids like prednisone and dexamethasone are well known for raising blood sugar by blocking insulin’s action. If you’re taking a steroid for inflammation, allergies, or an autoimmune condition, your fasting glucose may read significantly higher than it would otherwise. Other drug classes, including some blood pressure medications and certain psychiatric medications, can have similar effects.
Fasting Glucose vs. HbA1c
Your doctor might order an HbA1c test alongside or instead of a fasting glucose. These two tests measure different things. Fasting glucose captures a single snapshot of your blood sugar at one moment in time. HbA1c reflects your average blood sugar over the previous two to three months by measuring how much glucose has attached to your red blood cells.
Each test has trade-offs. Fasting glucose is sensitive but has poor reproducibility, meaning the same person can get noticeably different results on different days depending on stress, sleep, or other variables. HbA1c avoids that day-to-day fluctuation, giving a more stable picture. It also doesn’t require fasting, which makes it more convenient.
On the other hand, fasting glucose can miss cases that HbA1c catches, and vice versa. In one large study, fasting glucose identified 6.3% of participants as diabetic while HbA1c flagged 9.7%, suggesting fasting glucose alone may underestimate the true number of people with undiagnosed diabetes. The biggest disagreement between the two tests showed up in the prediabetes range: fasting glucose classified 12.1% of participants as prediabetic, while HbA1c classified 34.6%. That’s a striking gap, and it’s one reason many clinicians now use both tests together rather than relying on either alone.
What Prediabetes Looks Like in Practice
A fasting glucose between 100 and 125 mg/dL puts you in the prediabetes category, but it doesn’t mean diabetes is inevitable. Prediabetes is the stage where lifestyle changes have the most impact. Losing 5 to 7% of your body weight (roughly 10 to 14 pounds for someone who weighs 200) 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.
Most people with prediabetes have no symptoms. You feel fine. That’s part of what makes the fasting blood sugar test so valuable: it catches a problem that’s otherwise invisible until it’s advanced enough to cause damage to blood vessels, nerves, and organs. Given the cardiovascular risk data, a reading in the prediabetes range is worth taking seriously even if you feel completely healthy.
Getting an Accurate Result
To get the most reliable fasting glucose reading, a few practical steps help. Fast for at least 8 hours, sticking to water only. Try to get a normal night’s sleep beforehand. Schedule the test during a relatively low-stress period if possible. Let your provider know about any medications you’re taking, especially corticosteroids, since those can raise your reading independently of your metabolic health. And if a result comes back unexpectedly high, don’t panic. A single elevated reading warrants a retest, not a diagnosis.