A fasting plasma glucose test measures the glucose in your blood after at least eight hours without food or caloric drinks. Below 100 mg/dL (5.6 mmol/L) is normal; 100–125 mg/dL (5.6–6.9 mmol/L) is prediabetes; 126 mg/dL (7.0 mmol/L) or above on two occasions is diabetes. The result mainly reflects how much glucose your liver releases overnight, and it can vary by 10–15 mg/dL from one day to the next.
What the test measures
Glucose in the blood comes from two sources: food, and glucose released by the liver from its stores. After eight or more hours without eating, the food contribution has passed, and what remains is the liver's steady overnight output balanced against the body's use of it. The fasting plasma glucose (FPG) test measures that balance.
In a person with normal insulin sensitivity, a small amount of overnight insulin is enough to keep the liver's glucose release in check, and fasting glucose stays in the 70s to 90s mg/dL. In insulin resistance, the liver responds weakly to that signal and releases more than it should, and fasting glucose drifts upward. Raised fasting glucose is therefore a fairly specific sign that the liver's glucose handling is impaired.
The ranges
| Fasting plasma glucose | Interpretation |
|---|---|
| Below 100 mg/dL (5.6 mmol/L) | Normal |
| 100 – 125 mg/dL (5.6 – 6.9 mmol/L) | Prediabetes (impaired fasting glucose) |
| 126 mg/dL (7.0 mmol/L) or higher | Diabetes, if confirmed on a second test |
The World Health Organization uses a slightly higher lower threshold for impaired fasting glucose (110 mg/dL, 6.1 mmol/L), which is why definitions can differ between countries.
Some clinicians pay attention to results in the upper-normal range. A fasting glucose that has climbed from the 80s to the mid-90s over several years, while still "normal", suggests the process behind prediabetes may be under way.
How the test is done
You are asked not to eat or drink anything other than water for at least eight hours, which is why the test is usually done first thing in the morning. A blood sample is taken from a vein and analysed in a laboratory. Finger-prick meters can give an approximate fasting value at home but are not accurate enough to diagnose.
The "plasma" in the name matters: laboratory tests measure glucose in plasma, which reads about 10–15% higher than glucose in whole blood. Most modern home meters are calibrated to display plasma-equivalent values, but older ones are not.
Why the number varies
Fasting glucose is a single point in time, and several things move it:
- Sleep. A short or broken night raises next-morning glucose.
- The dawn phenomenon. Hormones released in the early morning raise glucose in everyone; the effect is larger in insulin resistance.
- Illness, pain and stress, all of which raise cortisol and adrenaline.
- The previous evening. A late, large or high-carbohydrate meal can still be influencing the liver in the morning.
- Medications, including corticosteroids, some diuretics and antipsychotics.
- Laboratory handling. If a sample sits at room temperature before analysis, red cells consume glucose and the result reads low; laboratories use preservative tubes to prevent this.
Because of this variability, a diagnosis is never made on one result. Two tests on different days, or one fasting glucose plus an A1C, are required.
Fasting glucose compared with A1C
The two tests answer different questions and disagree often enough that doctors use both.
| Fasting glucose | A1C | |
|---|---|---|
| Reflects | One morning's baseline | Average over 2–3 months |
| Preparation | 8-hour fast | None |
| Day-to-day variation | High | Low |
| Most sensitive to | Liver glucose output | Overall exposure, including after meals |
| Misled by | Poor sleep, stress, illness | Anemia, blood loss, haemoglobin variants |
A person can have a normal fasting glucose and a prediabetes-range A1C if their after-meal glucose runs high; or a raised fasting glucose with a normal A1C if the morning value is high but the rest of the day is controlled. Neither result is wrong; together they give a fuller picture. When they conflict, an oral glucose tolerance test often settles it.
What to do with your result
A result of 100–125 mg/dL is a reason to repeat the test, add an A1C, and act on weight, activity and sleep as described in the prediabetes guide. A result of 126 or higher is repeated to confirm; if confirmed, your doctor will discuss treatment for type 2 diabetes. A very high result with symptoms — thirst, frequent urination, weight loss — may not need a repeat test before treatment begins.
If you already have diabetes, your target fasting range is set with your care team; a common range is 80–130 mg/dL (4.4–7.2 mmol/L).
Frequently asked questions
Can I drink coffee before a fasting glucose test?
Water is fine. Black coffee has little effect on glucose for most people but can raise it slightly in some; most laboratories ask you to avoid it. Anything with milk, sugar or calories breaks the fast.
Why is my fasting glucose higher than my readings later in the day?
This is common and usually reflects the "dawn phenomenon": a natural early-morning rise in cortisol and growth hormone prompts the liver to release glucose, and in insulin resistance that release is not switched off efficiently. After-meal readings can be lower if muscle is clearing glucose reasonably well.
Is a fasting glucose of 100 a problem?
100 mg/dL is the lower edge of the prediabetes range. On its own it is a prompt to repeat the test, check A1C, and look at weight, activity and sleep — not a diagnosis.
How is a fasting glucose different from a glucose tolerance test?
The fasting test measures baseline glucose. The oral glucose tolerance test measures how well the body clears a standard 75-gram glucose drink over two hours, which detects after-meal problems that a fasting test can miss.
References
- NIDDK. Diabetes Tests & Diagnosis (2023). https://www.niddk.nih.gov/health-information/diabetes/overview/tests-diagnosis
- NIDDK. Insulin Resistance & Prediabetes (2023). https://www.niddk.nih.gov/health-information/diabetes/overview/what-is-diabetes/prediabetes-insulin-resistance
- American Diabetes Association, Diabetes Care 49(Suppl 1):S27–S49. 2. Diagnosis and Classification of Diabetes: Standards of Care in Diabetes—2026 (2026). PubMed PMID 41358893. https://diabetesjournals.org/care/article/49/Supplement_1/S27/163926/2-Diagnosis-and-Classification-of-Diabetes
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