How to prepare for a blood test

Fasting means 8 to 12 hours after your last meal. Dinner at 20:00 and a sample at 08:00 makes 12 hours. Going without food for more than 14 hours is not good either: glucose and bilirubin change.

Water is allowed and helps: plain, still water. No tea, coffee, juice or chewing gum; even tea without sugar starts digestion. Eight hours of fasting are needed for glucose, ALT and ferritin. The lipid profile needs 12. For a complete blood count, 4 hours after a light breakfast is enough.

Skip alcohol and fatty food: triglycerides and liver enzymes can still be high the next day. Hard exercise, such as a gym session or a long run, raises creatinine and white cells. Keep the workout for after the test.

Do not stop medicines your doctor prescribed; just tell the nurse. Take thyroid pills that morning after the sample. Biotin (vitamin B7) changes the results of some hormone tests: stop it, and any multivitamin that contains it, 2 days before.

Some hormones change through the day. Cortisol is taken before 09:00, TSH and iron before 10:00. For female hormones the cycle day matters: FSH, LH and oestradiol on day 3 to 5, progesterone on day 19 to 21 of a 28-⁠day cycle. If your cycle is shorter or longer, check the day with your doctor.

Sit for 10 to 15 minutes when you arrive, so your pulse and blood pressure settle after the stairs. Taking blood takes 3 to 5 minutes. If you tend to feel faint, tell the nurse first: we will take it with you lying down.

Queues are longest from 07:30 to 10:00. When you choose your tests, book a branch time in 10-⁠minute steps, or call a nurse home. If the sample is taken by 12:00, glucose and the blood count are ready at 18:00 the same day. Each test’s preparation is on its page and in the preparation guide.