The list that needs no fasting
A complete blood count needs none: cell counts are not moved by a meal. A thyroid panel needs none, because TSH follows a daily rhythm rather than your meals, which is why the useful instruction is to prefer the morning rather than to fast. A liver panel needs none in ordinary circumstances, a kidney panel needs none, and HbA1c needs none at all because it reflects roughly three months and a breakfast is invisible to it.
Vitamin and hormone levels are generally taken without fasting too, with timing mattering more than food for several of them.
That already covers most of what appears on a routine prescription.
The short list that still does
Fasting glucose is the one clear case. It is the test the instruction exists for: digestion pushes glucose into the blood within minutes, so a fasting glucose measured after a meal measures the meal.
Iron studies are usually asked for in the morning and fasting, less because of food than because serum iron varies through the day and a supplement taken that morning distorts it.
Beyond those, a specific request may carry its own instruction, which is always the one to follow.
The lipid panel changed sides
For decades a cholesterol test meant an overnight fast. Since 2016 the European Atherosclerosis Society and the European Federation of Clinical Chemistry and Laboratory Medicine have jointly recommended the opposite: measure lipids without fasting as the default.
The reasoning is that the differences are small. After an ordinary meal the maximal average changes are about +26 mg/dL for triglycerides and −8 mg/dL for LDL cholesterol, while HDL cholesterol, apolipoprotein B and lipoprotein(a) are not affected at all, and non-fasting values predict cardiovascular disease just as well. Fasting is reconsidered in one situation: when non-fasting triglycerides come back above 5 mmol/L, or 440 mg/dL.
Some laboratories still ask. Follow your own prescription, which is the instruction that governs your sample, but know that the requirement is no longer the international default.
Why the confusion persists
Because the instruction is usually given for the whole prescription rather than line by line. One fasting glucose on a sheet of fifteen tests makes the entire appointment a fasting one, which leaves the impression that all fifteen required it.
The practical version: read the instruction on your prescription rather than a table on the internet, this one included. And if you have eaten when the instruction said otherwise, say so at reception rather than letting the sample through.