By Dr. Marcus Judge

OSCE

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During any practical procedure in an OSCE it is important to understand the reasoning behind each step, not simply to perform the actions in the correct order. This article explains how to perform venepuncture (taking a venous blood sample) and how to collect blood cultures. It shows what each step achieves and why it matters for patient safety and accurate results.

Venepuncture is one of the most common invasive procedures (procedures that enter the body) in healthcare. Blood cultures are a closely related skill that uses the same access but needs a stricter aseptic approach (one that keeps germs out). Mastering both, and being able to justify every action, is a core expectation for medical students and junior doctors.

Contents

  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

Introduction

Wash your hands thoroughly and put on a disposable apron. Hand hygiene before patient contact is one of the most important steps in preventing infections that patients catch in healthcare, and it matters even more here because you are about to break the skin barrier.

Introduce yourself to the patient and state your grade, e.g. medical student, foundation doctor (a doctor in their first two years after qualifying) or registrar (a more senior trainee doctor).

Confirm the patient's details using 3 points of identification: full name, date of birth and NHS/hospital number. Ask the patient to say their full name and date of birth themselves, rather than just agreeing when you read them out. Then 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. This helps you choose the best site first time and avoid extra attempts.
  • Any lymphoedema (long-term swelling caused by poor drainage of lymph fluid) or previous mastectomy with axillary clearance (breast removal together with removal of the lymph nodes in the armpit). Avoid that arm where possible, as impaired lymphatic drainage increases the risk of infection and swelling.
  • An arteriovenous fistula (a surgically made link between an artery and a vein, used for access during dialysis, the treatment that does the work of failed kidneys). Never use a fistula arm, as venepuncture risks damaging or infecting it.
  • Any anticoagulant or antiplatelet medication (medicines that make the blood clot less easily, such as warfarin or aspirin) or known bleeding tendency, as you will need to apply pressure for longer afterwards.
  • Any allergies, particularly to latex (a rubber used in some gloves), chlorhexidine (an antiseptic used in some skin wipes) or plasters. If the patient has one, use a different product, such as latex-free gloves.

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

Gather and Prepare Equipment

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 system in which a vacuum tube fills with blood) 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 risks a needlestick injury (an accidental stab from a used needle).

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

Blood being drawn from a vein in the antecubital fossa SimpleMed

Image - Venepuncture being performed at the antecubital fossa (the front of the elbow), the most common site for taking blood

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

Site Selection and Applying the Tourniquet

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 (the vein that crosses the elbow and joins the cephalic and basilic veins) is often the first choice. It is typically prominent and relatively fixed in position. It also overlies the bicipital aponeurosis, a flat sheet of fibrous tissue that gives some protection to the underlying brachial artery and median nerve. The cephalic vein (on the thumb side of the arm) and the basilic vein (on the little finger side) are alternatives. The basilic vein is used more cautiously because the brachial artery and median nerve lie close by on the medial (inner) side.

Apply the tourniquet roughly 4–5 finger-widths (about 7–10 cm) above the intended site. The tourniquet is tight enough to block venous return (the flow of blood back towards the heart in the veins) but still allows arterial inflow. Blood therefore pools in the superficial veins, which makes them easier to see and feel. You should still be able to feel a radial pulse at the wrist. If you cannot, the tourniquet is too tight and is restricting arterial flow.

Clean your hands and put on your gloves before you touch the arm to feel for a vein. Ask the patient to clench and unclench their fist to help the veins fill with blood and stand out. 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 stasis (blood pooling in the veins) and haemoconcentration (the blood becoming more concentrated as fluid leaves the vessels). This can falsely raise results such as potassium, calcium and lactate. It also makes sample haemolysis (breakdown of red blood cells) more likely.

Superficial veins of the upper limb including cephalic, basilic and median cubital veins SimpleMed

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)]

Cleaning the Skin

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 using firm back-and-forth strokes for 30 seconds. Then allow the skin to dry completely, which takes at least 30 seconds (do not fan, wipe or blow on it). Skin disinfection reduces the patient's own skin flora (the bacteria that normally live on the skin), the commonest source of contamination. Letting the skin dry is essential because the antiseptic needs enough contact time to kill organisms. Sampling too soon also stings and can cause haemolysis.

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.

Performing the Venepuncture

Warn the patient of a sharp scratch. Anchor the vein by gently pulling the skin taut distal to the puncture site (on the hand side of it) with your non-dominant thumb. This stops the vein rolling away as the needle approaches.

Insert the needle bevel up (with the slanted opening of the needle tip facing upwards) at an angle of 30 degrees or less to the skin, in line with the vein. A shallow angle keeps you within the lumen (the hollow inside the vein) 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 in the tube. Then 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, which stops the blood clotting in the tube) 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. Hold a piece of gauze over the site as you withdraw the needle, but do not press until the needle is fully out, as pressing on the needle is painful and can tear the vein. Once it is out, apply firm pressure with the gauze.

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.

Order of Draw and Blood Bottles

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 (ethylenediaminetetraacetic acid, 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 tests. The gold/yellow top is the serum tube for biochemistry. These tests include urea and electrolytes (U&Es), liver function tests (LFTs), C-reactive protein (CRP, a marker of inflammation) and the bone profile (calcium, phosphate and related tests). The purple top contains EDTA for the full blood count. The pink top is used for group and save (finding the patient's blood group and keeping a sample in case blood is needed for a transfusion). 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 (breakdown of red blood cells), which can render samples unusable and falsely elevate potassium and other intracellular markers.

Vacutainer needle holder with a blood collection tube SimpleMed

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)]

Labelling and Documentation

Label the bottles at the bedside, immediately, before you leave the patient. Never pre-label bottles before taking the sample and never label away from the patient. Both practices risk a sample being attributed to the wrong person, which is a recognised cause of serious patient harm.

Each label must carry the patient's full name, date of birth, NHS/hospital number, and the date and time of collection, plus your identity as the person who took the sample. Samples for group and save / crossmatch (tests that find the patient's blood group and check that donor blood is compatible) have particularly strict, often handwritten, labelling requirements because a labelling error here can cause a fatal transfusion reaction.

Place the labelled samples in the appropriate bags with the completed request form and arrange prompt transport to the laboratory. Some samples (such as blood cultures and certain biochemistry tests) deteriorate if delayed.

Blood Cultures: Aseptic Technique

Blood cultures are taken when bacteraemia (bacteria in the blood) or sepsis (a life-threatening reaction to infection) is suspected, to identify the causative organism and guide antibiotic therapy. Because the sample is incubated to grow any organisms present, even a few contaminating skin bacteria can grow and produce a false-positive result. This can lead to unnecessary antibiotics, extra investigations and longer hospital stays. For this reason blood cultures demand a stricter approach than routine venepuncture, using an Aseptic Non-Touch Technique (ANTT).

Key principles of ANTT for blood cultures:

  • Ideally take blood cultures before antibiotics are given, so the organism is not suppressed. In suspected sepsis, do not delay antibiotics to take cultures: if taking cultures would cause a delay, give the antibiotics first.
  • Perform meticulous hand hygiene and prepare your equipment on a clean tray, opening packs without touching the parts that will contact the blood (these are called the key parts).
  • Clean the patient's skin with 2% chlorhexidine in 70% isopropyl alcohol and allow it to dry fully. Do not re-touch the cleaned site afterwards, not even with a gloved finger.
  • Clean the top of each culture bottle: remove the plastic cap and wipe the rubber top of the bottle (the septum) with a separate cleaning wipe for 30 seconds. Allow it to dry before you fill (inoculate) the bottle. The rubber top is not sterile, even under the plastic cap, so cleaning it stops you pushing germs from its surface into the broth.

If you need to take cultures and routine bloods from the same venepuncture, fill the culture bottles first.

Blood Cultures: Filling the Bottles

A blood culture set usually consists of an aerobic bottle and an anaerobic bottle, which between them allow growth of organisms that do or do not require oxygen.

The order in which you fill them depends on the system:

  • If you are using a needle and syringe (where a little air is drawn up with the blood), fill the anaerobic bottle first so that the trapped air does not enter it. Then fill the aerobic bottle.
  • If you are using a winged collection set into a vacuum holder, air in the tubing would enter the first bottle. Fill the aerobic bottle first, so that any air goes into it rather than into the anaerobic bottle.

In adults, each bottle should be filled with around 8–10 ml of blood (check the fill line on the bottle). Children need smaller volumes that depend on their weight, so follow local paediatric guidance. Adequate volume directly improves the chance of detecting an organism, because the number of bacteria in the blood may be very low. Underfilled bottles substantially reduce the sensitivity of the test.

Do not exceed the recommended volume either, as overfilling alters the blood-to-broth ratio and can also affect detection. After filling, label the bottles at the bedside and send them to the laboratory promptly. Ideally note the collection time and site, so that a true infection can be distinguished from contamination.

Where more than one set is requested (for example to help interpret possible contaminants, or when endocarditis, an infection of the heart valves, is suspected), take each set from a separate venepuncture site, each with a fresh aseptic preparation. Your local policy will say how many sets are needed.

When endocarditis is suspected, the 2023 European Society of Cardiology guideline recommends at least three sets of blood cultures before antibiotics are started.

Blood culture bottle SimpleMed

Image - A blood culture bottle. The broth is incubated to grow any organisms present, which is why strict aseptic technique is essential to avoid false positives

Creative commons source by Ajay Kumar Chaurasiya [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Complications and Managing Difficulty

Even with good technique, venepuncture can be difficult or have complications. Knowing how to recognise and respond to these is an important part of the skill.

If the veins are hard to find, a few simple measures often help. Make sure the patient is warm and well hydrated. Lower the arm below the level of the heart for a minute to encourage venous filling. Apply a warm compress to the site, and gently tap over the vein (firm tapping can cause the vein to constrict). For fragile or small veins, a winged "butterfly" device with a small syringe generates gentler suction and is less likely to collapse the vein.

Limit yourself to no more than two attempts. If you are still unsuccessful, stop and ask a more experienced colleague rather than causing the patient repeated discomfort and bruising.

  • No flashback / missed vein: withdraw slightly and gently redirect; if unsuccessful, remove the needle, apply pressure, and try a new site with fresh equipment. Do not "dig around", which is painful and damages tissue.
  • Haematoma (bruising): caused by blood leaking from the vein into the surrounding tissue. It is often due to the tourniquet still being on when the needle is removed, or too little pressure afterwards. Apply firm pressure to manage it.
  • Vasovagal episode (fainting): if the patient feels faint, stop, lie them flat and raise their legs.
  • Arterial puncture: suspected if the blood is bright red and pulsatile or the patient reports severe pain. Remove the needle and apply firm pressure for at least 5 minutes.
  • Nerve irritation: sharp, shooting or electric-shock pain radiating down the arm suggests the needle is near a nerve. Withdraw immediately.
  • Needlestick injury (to you): encourage the wound to bleed gently, wash it with soap and running water without scrubbing or sucking it, and cover it. Then seek urgent medical advice, for example from occupational health, because medicines to help prevent infection are available. Report and document the injury following the local inoculation-injury policy.

Completing the Procedure

Ensure the puncture site has stopped bleeding and apply a plaster or dressing. Check the patient is comfortable and has no further concerns.

Dispose of all sharps and clinical waste correctly, and clean the tray. Remove your gloves and apron and wash your hands.

Confirm the bottles are correctly labelled and send them, with the request form, promptly to the laboratory.

Thank the patient.

To complete the procedure, you should document in the notes what was done, the site used, who took the sample and the date and time, and any complications. State that you would follow up the results and act on them appropriately. For example, in suspected sepsis (a life-threatening reaction to infection), make sure blood cultures are taken as part of the Sepsis Six. This is a bundle of urgent actions to complete within an hour for high-risk patients, including blood cultures, intravenous antibiotics and intravenous fluids. NICE advises that adults at high risk of severe illness or death from sepsis should have antibiotics within 1 hour of their NEWS2 score (an early warning score based on vital signs) being calculated.

Quiz

Preview the Venepuncture and Blood Cultures OSCE Guide quiz

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