Some tests come with an instruction attached, and many reports print whether it was followed. The reason is mechanical: for a handful of markers, what you ate is part of what gets measured.
What fasting means here
MedlinePlus describes it as taking nothing "except plain water, for several hours before your test", and lists gum, smoking and exercise as excluded alongside food. The NHS phrases the same instruction as "not eat or drink anything other than water (fast)".
The usual window is "8 to 12 hours". For a cholesterol test specifically, MedlinePlus gives "9 to 12 hours before your blood cholesterol test", and adds the practical consequence: "That's why the tests are often done in the morning."
Why it changes the number
Because digestion puts the measured substances into your blood. MedlinePlus: "When you eat and drink, your bloodstream absorbs sugar, fat, protein, and other nutrients in those foods and beverages. These nutrients can then change the levels of various substances in your blood and therefore affect the accuracy of your test results."
That is the entire mechanism, and it explains which tests carry the instruction: those measuring something food delivers directly. Blood glucose and lipid panels are the common ones.
Why the report tells you
A fasting glucose and a glucose measured two hours after eating are different tests with different reference intervals, not one test performed under different conditions. Many labs print the fasting state on the report — "F", "fasting", or a separate row entirely — because without it the reference interval beside the number is ambiguous.
Some labs report a non-fasting sample with a note rather than refusing it. That note is the most important thing on the row: it says which interval applies.
If you didn't fast
MedlinePlus is direct about the consequence for the result: "Inaccurate results give your provider an inaccurate view of your health and may lead to a wrong diagnosis," and if you have eaten, "you may need to reschedule the test for another time when you can fast to make sure you get accurate results."
Why it matters for a history
If you are comparing a marker across years, fasting state is one of the conditions that has to match for the comparison to mean anything — alongside the laboratory and the units. A fasting value and a non-fasting value plotted on the same line are not two points on one trend. This is worth checking on old reports before reading anything into a change.