What preparation actually changes
Very little, and that is the useful finding. A meal moves glucose and triglycerides for a few hours. Coffee disturbs the same two. An iron supplement distorts a serum iron. Almost everything else on a routine panel is indifferent to what you did that morning.
The pages here take one variable each and say what it moves, by how much, and for how long. Where the evidence has changed, they say so: the lipid panel no longer requires fasting by default, and that is the single biggest shift in this area in a decade.
Where to start
If you were told to fast and want to know whether it applies to your tests, start with the pillar. If your question is narrower, go straight to it: water, coffee, alcohol, smoking or medicines.
One rule outranks every page here, including the pillar: the instruction printed on your own prescription is the one that governs your sample.