How to prepare for a blood test

Blood markers can shift with what you did in the hours before the draw. A little preparation makes your results more accurate and, just as importantly, more comparable the next time you test. Always follow the specific instructions from whoever ordered your test - the notes below are general.

Fasting

Some markers - notably glucose, insulin and triglycerides - are measured fasting, usually meaning 8 to 12 hours with only water. If your panel includes those, fasting gives a cleaner, more standard reading. Water is fine and helps (see below).

Timing

Several hormones follow a daily rhythm. Cortisol and testosterone, for example, are typically highest in the morning, so morning draws are standard and make results comparable between tests. Try to test at a similar time each round.

The day before and of

Being well hydrated makes the draw easier and can steady some readings. Intense exercise and alcohol can temporarily move markers such as liver enzymes and hs-CRP, so many people avoid both for a day or so beforehand. If you take supplements (for example biotin or high-dose vitamins), mention them, as some can interfere with certain assays.

The bottom line

Consistency is the real win: same conditions each time means the changes you see reflect your body, not your morning. This is general information - follow the guidance from your test provider or clinician for anything specific to you.