Blood from a vein

Most laboratory blood tests start with a venous sample. A healthcare professional takes a small amount of blood, usually from a vein on the inside of the elbow, using a needle; sometimes blood is taken from the wrist, or from a finger in a finger-prick test. A soft strap called a tourniquet may be put around the arm first — it makes the vein easier to find and the draw easier.

The procedure itself is short: the appointment usually takes about 10 minutes, and the blood collection part is quicker still. What is felt is a scratch as the needle goes in, which may be a bit uncomfortable; afterwards the puncture spot is covered with cotton wool or a plaster. The blood goes into a small tube and is sent to the laboratory.

Preparation depends entirely on which test was ordered. For some tests, for a short time beforehand, it may be necessary not to eat or drink anything other than water — fasting — or to stop taking certain medicines, such as anticoagulants that help prevent blood clots. For many tests no preparation is needed at all. The only reliable rule is to follow the instructions that come with the specific order, and to mention everything being taken, including supplements.

Timing matters for some markers. Hormones such as cortisol follow a daily rhythm, so the laboratory schedules those draws for the morning; other markers are stable all day. If several tests are ordered together, one venous sample is usually enough for all of them.

How the sample is collected

1. Check the instructions for the specific test: whether fasting is required and for how long, and whether any medicine should be paused.
2. Drink water as usual unless told otherwise — being hydrated makes the veins easier to work with.
3. Wear a sleeve that rolls up easily above the elbow.
4. At the appointment, the professional cleans the skin, may apply a tourniquet, and draws blood into a small tube with a needle.
5. Afterwards, press the cotton wool or plaster to the puncture spot for a few minutes.
6. Mention anything unusual — a tendency to faint, a bleeding disorder, blood-thinning medicines — before the draw, not after.

What can spoil the sample

• Eating or drinking before a fasting test — even juice or sweetened coffee counts as breaking the fast.
• Stopping a prescribed medicine on your own; only the provider decides what is paused.
• Forgetting to mention biotin and other supplements, which interfere with several hormone and marker assays.
• Heavy exercise right before the draw — it shifts muscle enzymes and some other markers.
• Scheduling a cortisol or other rhythm-dependent test for the evening when the reference range assumes a morning sample.

Tests that use this sample

Frequently asked questions

Sources