What to eat (and skip) before your blood test

Preparation changes results more than most people expect. None of this is complicated, but getting it wrong is the most common reason a panel has to be repeated.

The night before

If your panel includes lipids or fasting insulin, stop eating ten hours before your draw. Water is fine and encouraged — dehydration concentrates your blood and skews several markers. Skip alcohol for 24 hours; it moves triglycerides and liver enzymes sharply.

The morning of

Black coffee is not neutral: it raises cortisol and can nudge glucose, so leave it until after. Hold biotin supplements for 48 hours, since they interfere with thyroid and hormone assays. Skip hard training that morning — it raises CK, and can make a healthy liver panel look alarming.

Everything else stays normal. Take your prescribed medication as usual unless we have told you otherwise, and tell us what you are taking so we read the numbers in the right context.