Do I need to fast before a blood test?

Tests that don’t need fasting

None of our standard tests need fasting. HbA1c reflects your blood sugar over months, not hours. Cholesterol tests are now usually done non-fasting, because non-fasting results are accurate enough to assess heart risk. Thyroid, vitamin D, hormone, infection and most other tests aren’t affected by food.

When fasting is still used

If your doctor has specifically asked for a fasting cholesterol or triglyceride level, don’t eat for 10 to 12 hours beforehand. Water is fine, and keep taking your usual medicines unless you’ve been told otherwise. Book an early appointment so you’re not without food for too long.

Always drink water

Being well hydrated makes your veins easier to find and the sample quicker to take. Have a couple of glasses of water in the hour or two before your appointment.

Other things that can affect results

Biotin (vitamin B7), common in hair, skin and nail supplements, can upset hormone and thyroid results, so stop it 48 hours before. Hard exercise the day before can raise muscle and kidney markers, and alcohol can affect liver results. Some hormone tests need to be taken on a certain day of your cycle or in the morning; each test page explains what to do.

Tell us about your medicines

Keep taking your usual medicines unless your doctor has told you otherwise, and let us know what you take. A few, such as thyroid tablets, are best taken after your blood test.