Venepuncture and Blood Cultures OSCE Guide

1. Introduction
2. Gather and Prepare Equipment
3. Site Selection and Applying the Tourniquet
4. Cleaning the Skin
5. Performing the Venepuncture
6. Order of Draw and Blood Bottles
7. Labelling and Documentation
8. Blood Cultures: Aseptic Technique
9. Blood Cultures: Filling the Bottles
10. Complications and Managing Difficulty
11. Completing the Procedure
12. Quiz

Wash your hands thoroughly and put on a disposable apron. Hand hygiene before patient contact is the single most effective step in preventing healthcare-associated infection, and it is doubly important here because you are about to breach the skin barrier.

Introduce yourself to the patient and state your grade, e.g. medical student, foundation doctor or registrar.

Confirm the patient's details using 3 points of identification: full name, date of birth and NHS/hospital number. Cross-check these against the request form and the patient's wristband. Correct identification is the foundation of safe sample-taking — a mislabelled sample can lead to a serious transfusion error or the wrong patient being treated.

Explain the procedure and gain consent. Tell the patient they will feel a sharp scratch, roughly how long it will take, and why the blood is being taken.

Ask about relevant history before you start. The key questions are:

• Any needle phobia or history of fainting with blood tests — if so, perform the procedure with the patient lying down.
• A preferred arm, and whether previous samples have been easy or difficult to obtain.
• Any lymphoedema or previous mastectomy with axillary clearance — avoid that arm, as impaired lymphatic drainage increases the risk of infection and swelling.
• An arteriovenous fistula (for dialysis) — never use a fistula arm, as venepuncture risks damaging or infecting it.
• Any anticoagulant or antiplatelet medication or known bleeding tendency, as you will need to apply pressure for longer afterwards.
• Any allergies, particularly to latex, chlorhexidine or plasters.

Position the patient comfortably, ideally seated or lying with the arm supported and extended, and ask about any pain before you touch them.

Collecting all your equipment beforehand keeps the procedure smooth and means you never have to leave a patient mid-procedure or break your technique to fetch a missing item.

You will typically need:

• A clean procedure tray, cleaned with a detergent wipe and allowed to dry
• Non-sterile gloves (and an apron)
• A tourniquet
• 2% chlorhexidine in 70% isopropyl alcohol skin-cleansing wipes (e.g. ChloraPrep)
• A needle and vacuum holder (a closed Vacutainer-style system) or a winged "butterfly" device for smaller or more fragile veins
• The appropriate blood collection bottles for the tests requested
• Gauze and a plaster (check for plaster allergy)
• A sharps bin, taken to the bedside
• Sample labels and the completed request form

Always take the sharps bin to the point of use. Carrying an exposed needle across a room to dispose of it is a leading cause of needlestick injury.

Check the expiry dates on the blood bottles and skin-cleansing wipes, and inspect the packaging for damage before opening.

Image - Venepuncture being performed at the antecubital fossa, the most common site for taking blood

Creative commons source by Harrison Keely [CC BY 4.0 (https://creativecommons.org/licenses/by/4.0)]

The most common site for venepuncture is the antecubital fossa (the front of the elbow), because the superficial veins here are usually large, well-anchored and easy to feel. The median cubital vein is often the first choice as it is typically prominent, relatively fixed in position, and overlies the bicipital aponeurosis, which gives some protection to the underlying brachial artery and median nerve. The cephalic and basilic veins are alternatives, although the basilic vein is used more cautiously because the brachial artery and median nerve lie close by on the medial side.

Apply the tourniquet roughly 4–5 finger-widths (about 7–10 cm) above the intended site. The tourniquet works by being tight enough to occlude venous return while still allowing arterial inflow, so blood pools in the superficial veins and makes them easier to see and feel. You should still be able to palpate a radial pulse — if you cannot, the tourniquet is too tight and is restricting arterial flow.

Ask the patient to clench and unclench their fist to help engorge the veins. A good vein feels soft, bouncy and refills when pressed; you are selecting by feel as much as by sight. Avoid veins that feel hard or cord-like (thrombosed), areas that are bruised, infected or inflamed, and sites near a fistula or on the side of a previous mastectomy.

Do not leave the tourniquet on for more than about one minute before sampling. Prolonged application causes haemoconcentration and stasis, which can falsely raise results such as potassium, calcium and lactate and cause sample haemolysis.

Image - The superficial veins of the upper limb. The median cubital vein in the antecubital fossa is usually the first choice for venepuncture

Creative commons source by OpenStax College [CC BY 3.0 (https://creativecommons.org/licenses/by/3.0)]

Once you have chosen your vein, clean the skin with a 2% chlorhexidine in 70% isopropyl alcohol wipe. Work outwards from the centre of the chosen site in a single sweep, clean for 30 seconds, then allow the skin to dry completely (do not fan, wipe or blow on it). Skin disinfection reduces the patient's own skin flora, which is the commonest source of contamination, and letting it dry is essential because chlorhexidine and alcohol only kill organisms while wet and acting — sampling too soon also stings and can haemolyse the sample.

For routine venepuncture, once the skin is clean you should not re-palpate the vein over the cleaned area. If you really need to feel the vein again, you must re-clean the skin afterwards.

Put on your gloves. Warn the patient of a sharp scratch. Anchor the vein by gently pulling the skin taut distal to the puncture site with your non-dominant thumb — this stops the vein rolling away as the needle approaches.

Insert the needle bevel up at an angle of 30 degrees or less to the skin, in line with the vein. A shallow angle keeps you within the lumen rather than passing straight through the back wall of the vein. As the tip enters the vein you will usually feel a sudden give or loss of resistance and see a flashback of blood; advance the needle a further 1–2 mm to make sure the tip is securely within the lumen before attaching bottles.

Keeping the needle still, attach each blood bottle in turn and allow the vacuum to fill it to the marked line. Underfilling matters: tubes containing additives (such as the citrate in coagulation tubes) rely on a fixed blood-to-additive ratio, and an underfilled coagulation sample will give a falsely prolonged result.

Once the final bottle is filled, release the tourniquet first, then remove the needle. Releasing the tourniquet before withdrawing the needle reduces the venous pressure and the size of any bruise. Apply gauze with firm pressure over the site as you withdraw — but do not press until the needle is fully out, as pressing on the needle is painful and can tear the vein.

Activate the needle's safety guard and dispose of the whole sharp immediately into the sharps bin at the bedside. Ask the patient to keep pressure on the gauze for a minute or two, then apply a plaster once bleeding has stopped.

When you are filling several bottles from a single venepuncture, the order of draw matters. Filling tubes in the wrong order can carry additive from one tube back into the next and produce inaccurate results.

The general principles to remember are:

• Blood cultures first (if being taken), to minimise the chance of contamination.
• Coagulation samples (citrate, often a blue-topped tube) are taken before tubes containing other additives, so that contamination does not interfere with clotting tests.
• Biochemistry before haematology — specifically, serum/biochemistry tubes are taken before EDTA (the additive in the full blood count tube). EDTA contains potassium and binds calcium, so even tiny carry-over into a biochemistry sample can cause spuriously high potassium and low calcium readings.

Bottle colours vary between manufacturers and NHS trusts, so always check your local guidance rather than relying on colour alone. As a rough guide in many UK labs, the blue top holds citrate for coagulation, the gold/yellow top is the serum tube for biochemistry (U&Es, LFTs, CRP, bone profile), the purple top contains EDTA for the full blood count, and the pink top is used for group and save. The important exam point is being able to explain why the order matters: to protect each test from cross-contamination by the additive in another tube.

After filling, gently invert (do not shake) the tubes that contain additives so the blood and additive mix properly — vigorous shaking causes haemolysis, which can render samples unusable and falsely elevate potassium and other intracellular markers.

Image - A closed vacuum (Vacutainer-style) collection system. The vacuum tube draws a fixed volume of blood, which is why filling to the line matters

Creative commons source by Whispyhistory [CC BY 3.0 US (https://creativecommons.org/licenses/by/3.0/us)]