How to prepare for a blood test: fasting, timing and medication

Having a blood test is quick: the sample itself usually takes just a few minutes. A little preparation beforehand helps make sure your results are as accurate as possible.

1. Drink plenty of water

Being well hydrated makes your veins easier to find and the sample quicker to take. Drink water as normal on the day, and a glass or two before you arrive.

2. Do I need to fast?

Usually not. Most of our tests, including cholesterol, HbA1c, thyroid and vitamin D, don’t need fasting. If your doctor has specifically asked for a fasting test, don’t eat for 10–12 hours beforehand; water is fine.

3. Time hormone tests carefully

Some hormones change through the day or through your cycle:

• Testosterone is highest in the morning, so book a testosterone test before 11am.
• FSH, LH and oestradiol are usually tested on day 2–5 of your cycle.
• Progesterone is tested about 7 days before your next period, to check ovulation.
• AMH can be tested on any day.

4. Pause biotin supplements

Biotin (vitamin B7), found in many hair, skin and nail supplements, can interfere with the laboratory methods used for thyroid and hormone tests, causing falsely high or low results. Stop taking it 48 hours before your test, unless your doctor has told you otherwise.

5. Keep taking your medicines

Carry on taking prescribed medicines as normal unless your doctor has advised otherwise, and tell us what you take. If you take levothyroxine, have your thyroid test before your daily dose.

6. A few test-specific tips

• PSA: avoid ejaculation and vigorous exercise such as cycling for 48 hours before.
• Kidney function: avoid a large meat meal and very intense exercise the day before.
• Liver function: avoid alcohol for 24 hours if you can.

Nervous about needles?

You’re not alone. Tell us when you arrive. We can take your sample while you lie down, talk you through each step and go at your pace.

What happens next?

Most results are ready within 1–3 days and arrive in a clear, easy-to-understand report. Browse all blood tests and prices or call us on 020 3435 5474 for help choosing.