How to prepare for blood work: fasting, drinks and medicines

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.

Fasting: a shorter list than you were told

Where fasting is required, the usual instruction is eight to twelve hours with nothing but plain water. It protects one test above all: fasting glucose, which digestion pushes up within minutes. A blood count, a thyroid panel, a liver or kidney panel and an HbA1c do not need it.

The lipid panel changed column in 2016. The European Atherosclerosis Society and the European Federation of Clinical Chemistry and Laboratory Medicine now recommend measuring it without fasting by default: after an ordinary meal, triglycerides rise by about 26 mg/dL at most and LDL cholesterol falls by about 8 mg/dL, HDL cholesterol does not move, and non-fasting values predict cardiovascular disease just as well. Fasting is reconsidered only when non-fasting triglycerides exceed 5 mmol/L, or 440 mg/dL.

What fasting means, and what breaks it

Most people were never told. When researchers questioned 150 people arriving at a laboratory, only 39% could define fasting correctly, 46% thought the time since the last meal did not matter, and more than half had received no instructions at all.

Plain water is allowed throughout the fast, including on the morning of the draw. Black coffee is the most common mistake. In a crossover trial, healthy volunteers given caffeinated coffee before a meal showed a glucose response 147% to 216% larger than when they were given decaffeinated coffee.

Alcohol, tobacco and medicines leave traces of their own

Alcohol works on two clocks. One evening moves triglycerides and some liver values for a day or two; regular drinking shifts GGT and the average size of red cells over weeks to months, which a dry evening before the draw does not undo. Neither line is an alcohol test: a review of alcohol biomarkers stresses that liver enzymes lack specificity for ethanol.

Regular smokers show higher white cell counts, hemoglobin and hematocrit than non-smokers, and the gap grows with the number of cigarettes, so a flagged white cell count in a smoker is often accurate without signaling an infection. For medicines, take your usual treatment unless the prescriber says otherwise in writing. Levothyroxine is usually taken after the draw, and high-dose biotin, sold in many over-the-counter supplements, distorted 9 of the 23 biotin-based assays tested in healthy volunteers taking 10 mg a day for a week.

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.