FPG stands for fasting plasma glucose, a blood test that measures your blood sugar level after you haven’t eaten for at least 8 hours. It’s one of the simplest and most widely used tools for diagnosing diabetes and prediabetes. A normal FPG is below 100 mg/dL, while a reading of 126 mg/dL or higher on two separate tests means diabetes.
How the Test Works
The FPG test requires a blood draw after 8 to 12 hours of fasting. During that window, you can drink water but nothing else. The test is typically done first thing in the morning, since most of the fasting period happens while you sleep.
What the test actually measures is how well your body manages blood sugar on its own, without any help from food. When you fast overnight, your liver takes over the job of keeping your blood sugar stable. It does this two ways: by breaking down stored glycogen (a starchy reserve it keeps on hand) and by building brand-new glucose molecules from raw materials like amino acids and lactate. About 80% of the glucose circulating in your blood during a fast comes from the liver, with the kidneys contributing the rest.
Insulin and glucagon, two hormones from the pancreas, work in opposition to keep this system balanced. Insulin tells the liver to stop releasing glucose and store it instead. Glucagon does the opposite, signaling the liver to ramp up production when blood sugar dips. In a healthy person, this back-and-forth keeps fasting blood sugar in a tight range. When the system breaks down, fasting glucose creeps up, and the FPG test catches it.
What Your FPG Number Means
The American Diabetes Association’s 2026 Standards of Care define three ranges:
- Normal: below 100 mg/dL (5.6 mmol/L)
- Prediabetes (impaired fasting glucose): 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 isn’t enough for a diagnosis. Unless you have obvious symptoms of high blood sugar along with a random glucose above 200 mg/dL, guidelines call for a confirmatory test on a different day. Repeating the same FPG test is preferred, though an A1C test can also serve as confirmation.
FPG vs. A1C Testing
The A1C test measures your average blood sugar over the past two to three months by looking at how much sugar has attached to your red blood cells. It doesn’t require fasting, which makes it more convenient. FPG, on the other hand, gives you a snapshot of a single moment.
Each test has trade-offs. A1C is less affected by day-to-day swings from stress, a bad night of sleep, or what you ate the day before. But FPG has an edge in diagnostic precision. A large systematic review comparing both tests against the gold-standard oral glucose tolerance test found that FPG at 126 mg/dL had the highest specificity of any single test (98%), meaning it very rarely tells someone they have diabetes when they don’t. It also ranked first for overall diagnostic accuracy. The same analysis found that combining both tests improved sensitivity, catching more true cases than either test alone.
Cost matters too. A1C requires specialized lab equipment and standardized methods that aren’t always available in lower-resource settings. FPG is cheaper and more accessible, which is one reason it remains the recommended first-line test in many parts of the world.
Why a Prediabetes Reading Matters
A fasting glucose between 100 and 125 mg/dL, called impaired fasting glucose, is more than a warning label. Even at this stage, before blood sugar reaches the diabetes threshold, damage to blood vessels can begin. A large Korean cohort study found that people with impaired fasting glucose had an 18% higher relative risk of cardiovascular disease compared to people with normal glucose levels. The progression of artery hardening in prediabetic individuals is measurably faster than in people with normal blood sugar, raising the risk of heart disease and stroke.
The same study tracked rates of kidney and eye complications in the prediabetic group. While the absolute numbers were lower than in people with full diabetes, the risk of chronic kidney disease was notably elevated, especially in those who also had high blood pressure, a family history of diabetes, or a high BMI. Retinal disease risk increased in the higher end of the prediabetes range. These findings reinforce that a prediabetes FPG result is a meaningful window for lifestyle changes, not just a number to recheck in a year.
What Can Throw Off Your Results
Several common medications can raise fasting glucose levels and potentially push a reading into an abnormal range. Corticosteroids (like prednisone) are the most well-known culprits, though they tend to raise post-meal glucose more than fasting levels. Thiazide diuretics, often prescribed for blood pressure, raise fasting glucose by an average of about 5 mg/dL. Statins, certain antipsychotics, and beta blockers can also nudge numbers upward. In many cases, the elevation resolves when the medication is stopped.
There’s also a biological quirk called the dawn phenomenon that can affect your reading. In the early morning hours, your body naturally ramps up glucose production and becomes slightly more resistant to insulin. This pre-waking surge can add roughly 12 mg/dL to your fasting glucose compared to your lowest overnight level. For someone with type 2 diabetes, this bump contributes about 0.4 percentage points to their A1C over time. If your FPG seems unexpectedly high, the timing of your blood draw relative to this early-morning hormonal shift could be part of the explanation.
Acute stress, illness, and poor sleep can also temporarily raise fasting glucose. If your result comes back borderline, your doctor will typically retest rather than diagnose based on a single draw.
How to Prepare for the Test
Preparation is straightforward. Stop eating and drinking everything except water at least 8 hours before your blood draw. Black coffee, tea, and sugar-free drinks are not permitted during the fast, since caffeine and other compounds can influence glucose metabolism. Schedule the test for the morning so most of your fasting time overlaps with sleep. If you take medications that could affect blood sugar, mention them when discussing your results, but don’t stop taking them without guidance.