Fasting glucose: what the number means and how it is measured

Updated 13 August 2026 · 5 min read

Fasting plasma glucose is the oldest and simplest of the standard tests. You do not eat for a period, blood is drawn, and the concentration of glucose in it is measured. Most people have had it done without anyone explaining what it is actually capturing.

What "fasting" means here

The standard definition is no caloric intake for at least eight hours. Water is fine. Coffee with milk or sugar is not — and neither, strictly, is black coffee if you want the cleanest result, though its effect is small compared with anything containing calories.

In practice most people fast overnight and have blood drawn in the morning, which is why the test is often called a morning fasting glucose.

What it is actually measuring

After eight or more hours without food, essentially none of the glucose in your blood came from a recent meal. What you are measuring is the balance between two things:

How much glucose your liver is releasing into circulation
How effectively insulin is telling it to stop, and telling your tissues to take glucose up

A raised fasting number therefore points at that balance, not at last night's dinner. It is a measure of the baseline your body is holding when nothing is coming in.

The standard ranges

Fasting plasma glucose	Category
Below 100 mg/dL (5.6 mmol/L)	Normal
100 – 125 mg/dL (5.6 – 6.9 mmol/L)	Impaired fasting glucose (prediabetes)
126 mg/dL (7.0 mmol/L) or above	Diabetes range

Note the units. The United States generally reports mg/dL; most of the rest of the world uses mmol/L. Comparing a number from one system against a threshold from the other is a common and avoidable confusion.

One reading is not a diagnosis. A result in the diabetes range normally needs to be confirmed — either by repeating the test on a different day, or by a second, different test such as A1C or an oral glucose tolerance test. Guidelines are explicit about this.

Things that move the number and have nothing to do with diabetes

Fasting glucose is more variable day to day than most people expect. Among the factors that can shift a single reading:

Acute illness or infection. Stress hormones raise glucose; this is why testing during an illness can be misleading.
Poor or short sleep the night before.
Significant stress in the hours before the draw.
Some medications — corticosteroids are the clearest example, but others contribute.
The dawn phenomenon, the normal overnight hormone-driven glucose release, which affects the timing of when the sample is taken.
Intense exercise the previous evening, which can push the reading either way.

This variability is a large part of why guidelines require confirmation rather than acting on one result.

Home meters and lab draws are not the same test

A laboratory measures glucose in plasma. A home meter measures it in whole capillary blood and applies a conversion to report a plasma-equivalent figure. Home meters are permitted a margin of error against a reference method, so a difference of some percent between your meter and a lab result is expected rather than alarming.

Home meters are designed for tracking patterns over time. Diagnosis is made on laboratory testing.

The takeaway

Fasting glucose tells you what your body holds as a baseline with nothing coming in. It is simple, useful, genuinely variable from day to day, and not intended to be interpreted from a single result. The pattern across several readings is worth far more than any one of them.

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