What Happens During a Blood Test? Step by Step

Why Knowing What to Expect Helps

Fear of the unknown is one of the biggest contributors to blood test anxiety. When you know exactly what will happen at each stage, the experience becomes predictable and far less daunting. This step-by-step guide walks you through a typical blood draw, whether it takes place at a GP surgery, a hospital phlebotomy department, or during a home visit from a mobile phlebotomist.

Step 1: Arrival and Identification

The phlebotomist will greet you and confirm your identity. This is a safety-critical step and involves asking you to state your full name and date of birth. In a hospital setting, they may also check your wristband. For a home visit, they will check the details on the blood test request form.

They will confirm which tests have been requested and check the blood test form for any special instructions (e.g. fasting required). This is a good moment to mention any relevant information, such as allergies (particularly to latex or adhesive), medications you are taking, or if you have a history of fainting during blood tests.

Step 2: Consent

In the UK, verbal consent is standard for routine blood tests. The phlebotomist will explain what they are going to do and ask if you are happy to proceed. You have the right to withdraw consent at any point — including after the tourniquet is on but before the needle is inserted.

Step 3: Tourniquet Application

A tourniquet — an elasticated band — is tied around your upper arm, approximately 7–10 centimetres above the intended puncture site. This temporarily restricts blood flow, causing the veins to fill and become more visible and palpable. The tourniquet feels tight but should not be painful. It will be in place for less than a minute in most cases.

The phlebotomist may ask you to make a fist to help the veins stand out. They will then palpate (feel) the area to identify the best vein.

Step 4: Vein Selection

The most commonly used vein is the median cubital vein, located in the inner crease of the elbow (the antecubital fossa). It is large, superficial, and relatively well-anchored, making it the ideal target. If this vein is not accessible, the phlebotomist may use the cephalic vein (on the outer side of the arm) or the basilic vein (on the inner side).

In some cases, veins in the back of the hand or the wrist are used instead. This is more common in patients with difficult venous access, such as those who are dehydrated, elderly, or have had chemotherapy.

Step 5: Cleaning the Site

The phlebotomist cleans the puncture site with an alcohol swab (typically 70% isopropyl alcohol) using a circular motion from the centre outwards. They then allow the area to air-dry for approximately 30 seconds. This step is essential for preventing contamination of the sample and reducing infection risk.

Step 6: Needle Insertion

The phlebotomist anchors the vein by gently stretching the skin below the puncture site, then inserts the needle at an angle of approximately 15–30 degrees. You will feel a brief, sharp sensation — often described as a “scratch” or “pinch” — which lasts about one second. Once the needle is in the vein, most people feel only pressure.

The needle used for blood collection is a small-gauge (typically 21G) vacutainer needle or, for patients with smaller veins, a butterfly needle (23G). Both are designed for single use and are disposed of immediately after the draw.

Step 7: Blood Collection and Tube Filling Order

Blood flows into colour-coded vacutainer tubes, each containing a different additive depending on the test required. Tubes must be filled in a specific order — known as the “order of draw” — to prevent cross-contamination between additives. The standard UK order is:

• Blue top (sodium citrate) — coagulation studies (INR, APTT)
• Gold/yellow top (serum separator) — biochemistry, hormones, antibodies
• Green top (lithium heparin) — urgent biochemistry
• Purple/lavender top (EDTA) — full blood count, HbA1c
• Grey top (fluoride oxalate) — glucose, lactate
• Pink top (EDTA) — blood group, cross-match

Each tube takes a few seconds to fill. The phlebotomist gently inverts each tube several times after filling to mix the blood with the additive — this is essential for accurate results.

Step 8: Needle Removal and Pressure

Once all tubes are filled, the tourniquet is released and the needle is withdrawn in a single, smooth motion. A cotton wool pad or gauze is placed over the puncture site and you are asked to apply firm pressure for at least 5 minutes. Do not bend your arm, as this can cause a haematoma (a pocket of blood under the skin).

After 5 minutes, the phlebotomist will check the site and apply an adhesive dressing. If you are on blood-thinning medication, you may need to apply pressure for 10 minutes or more.

Step 9: Sample Labelling

Every tube is labelled at the bedside or chairside with your name, date of birth, and a unique identifier. This is done immediately after collection to prevent any possibility of mix-up. The phlebotomist may ask you to confirm the details on the label.

Step 10: Dispatch to the Laboratory

The labelled tubes are placed in a specimen bag with the request form and dispatched to the laboratory. In a hospital, porters or pneumatic tube systems move samples. For a home blood test, the mobile phlebotomist transports the samples in a temperature-controlled bag and drops them at the laboratory on the same day.

What to Do After Your Blood Test

Keep the dressing on for at least 30 minutes. Avoid heavy lifting or strenuous exercise with the affected arm for the rest of the day. Drink plenty of water and have a snack if you feel light-headed or if you were fasting. A small bruise around the puncture site is normal and should fade within a few days.

Results are sent to whoever ordered the test — usually your GP or hospital consultant. For guidance on turnaround times, see our article on how long blood test results take.