Fasting before a blood test: when is it needed and when not?

Do you need to fast before your blood test? For some values yes, for many others no. Fasting usually means not eating for 8 to 12 hours, while drinking water is fine. For glucose it is still often common, but for a cholesterol profile a European consensus says fasting is usually no longer needed (Nordestgaard 2016). In our experience that one rule causes a lot of unnecessary empty stomachs.

This article belongs to the overview about your blood values and helps you prepare well.

What does fasting mean?

Fasting means not eating for a period before the draw, usually 8 to 12 hours. Drinking water is normally fine, and even helpful, as it makes the draw easier. Coffee with milk or sugar counts as eating. The reason is simple: food temporarily changes some values in your blood, mainly your blood sugar and, to a lesser degree, your fats.

For which values is fasting useful?

If your draw includes both glucose and a blood count, the strictest requirement decides whether you fast. Often that is the glucose. Read more at complete blood count.

Why is fasting for cholesterol often no longer needed?

For a long time fasting was the standard for a cholesterol profile. Research showed that most lipid values change only slightly after a normal meal, little enough to measure reliably non-fasting too (Nordestgaard 2016). That is welcome, as it makes the draw easier to plan. Still, a specific situation or lab may ask for fasting, so follow the instruction with your test.

How do you prepare well?

• Check beforehand which values are measured and whether fasting is needed
• If you fast, do drink water: it makes the draw easier
• Plan your draw at a set time, so values stay comparable later
• If you use supplements like biotin, discuss pausing them, as they can disturb some measurements

To measure a number of values without a referral, you can do a basic health checkup at Vitalcheck. Your test states whether fasting is needed, and every result is reviewed by a BIG-registered doctor. For the meaning of your values, the overview about your blood values helps.

Frequently asked questions about fasting draws

Can I drink water if I have to fast?

Yes, water is normally fine. It keeps you hydrated and makes the draw easier. Coffee with milk or sugar, sweet tea and soft drinks count as eating, so leave those.

How long do I need to fast?

Usually 8 to 12 hours without food. The exact time is in your test instruction. If unsure, ask the lab that draws your blood.

What if I accidentally ate?

Usually not a disaster, but it can affect your glucose. Mention it at the draw so the result is read in context. Sometimes a fasting value is then redrawn.

References

• Nordestgaard BG, et al. Fasting is not routinely required for determination of a lipid profile. Eur Heart J. 2016. PMID: 27122601.
• Ozarda Y. Reference intervals: current status, recent developments and future considerations. Biochem Med (Zagreb). 2016. PMID: 26981015.
• Thuisarts.nl. About blood testing.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.