A Q and A on Having a Blood Test

A sample taken from a vein, usually in the arm, for one of several hundred different tests.

Key takeaways

• How long it takes: usually under five minutes
• Fasting: needed only for some tests; follow the instruction you were given
• Most useful single change: Drink water beforehand unless told otherwise, which makes veins easier to find.
• Commonest mistake: Fasting for a test that does not need it, particularly if you have diabetes.
• See a doctor if any of the signs under "When to speak to a doctor" below apply to you.

What is Having a Blood Test?

A sample taken from a vein, usually in the arm, for one of several hundred different tests.

Why does it matter?

Most of the preparation that matters is decided by which test it is, so the instructions you are given override any general advice.

How long it takes for Having a Blood Test?

Usually under five minutes.

What is the single most useful thing to do?

Drink water beforehand unless told otherwise, which makes veins easier to find.

What is the most common mistake?

Fasting for a test that does not need it, particularly if you have diabetes.

When should I see a doctor?

See a doctor if the puncture site becomes hot, swollen, increasingly painful or oozes over the following days. Tell your GP before the test if you take anticoagulants or have a bleeding disorder.

Where to go next

• Having a Blood Test: Common Myths and What Guidelines Say
• The First Steps With Having a Blood Test
• Having a Blood Test — the reference entry
• The Evidence on NHS Referrals and Waiting Times, Without the Hype
• The Evidence on Asking for a Second Opinion, Without the Hype

When to speak to a doctor

See a doctor if the puncture site becomes hot, swollen, increasingly painful or oozes over the following days. Tell your GP before the test if you take anticoagulants or have a bleeding disorder.

Where this comes from

• NHS — Blood tests
• Lab Tests Online UK — Understanding your tests

How this page was put together

Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarised or generalised beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.

Medical disclaimer

This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to a GP, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 999.