Why Venepuncture and Cannulation Are Core OSCE Skills
Venepuncture and IV cannulation are among the most frequently performed procedures in clinical medicine and appear in almost every OSCE circuit as procedural stations. Examiners mark on two things equally: correct sterile technique (ANTT) and clear, confident communication with the patient throughout. A technically perfect venepuncture that ignored the patient will not score full marks.
Core Principle: ANTT — Aseptic Non-Touch Technique
🧠 Mnemonic
ANTT — the four principles to state and demonstrate
A — Aseptic field: maintain a clean working area; do not contaminate your key parts
N — Non-touch: never touch key parts (needle tip, cannula hub, syringe tip, open port)
T — Touch only non-key parts: hub of needle can be held; never touch the tip
T — Technique: wash hands, gloves, correct disposal into sharps bin immediately
State "I am using ANTT throughout this procedure" out loud — examiners are listening for this.
Venepuncture — Step by Step
Before You Start
- Wash hands, don gloves
- Introduce yourself, confirm patient name and DOB
- Explain the procedure and gain consent: "I need to take a small blood sample from your arm. Is that okay?"
- Check for allergies (latex, plasters, chlorhexidine)
- Ask which arm is preferred; avoid the side of a mastectomy, fistula, or IV line
Choosing the Vein
- 1Apply tourniquet 5–10 cm above the intended site
- 2Ask patient to open and close their fist (pumps blood into veins)
- 3Inspect and palpate — do not just look
Best sites (in order of preference):
- Antecubital fossa: median cubital vein (largest, most accessible)
- Dorsum of hand (more painful, smaller veins — use for difficult cases)
- Avoid sites over joints, bruised areas, oedematous limbs, lymphoedema arm
A good vein feels: bouncy when pressed, refills quickly, is straight, and rolls less than smaller veins.
The Procedure
- 1Clean the site with a 2% chlorhexidine swab for 30 seconds — let it dry completely (at least 30 seconds)
- 2Do not re-palpate after cleaning (breaks asepsis — if you must, re-clean)
- 3Anchor the vein: stretch the skin distal to the site with your non-dominant thumb
- 4Warn the patient: "You'll feel a sharp scratch now"
- 5Insert needle bevel-up at 15–30° angle, advance until flashback appears in the chamber
- 6Attach bottles — first bottle immediately; subsequent bottles while holding needle steady
- 7Release tourniquet before removing the last bottle
- 8Place cotton wool over site, withdraw needle smoothly, apply pressure immediately
- 9Dispose of needle directly into sharps bin — do not resheath
- 10Label tubes at the bedside — never pre-label or label elsewhere
⚠️ Red Flag
Never resheath a used needle. If you must recap, use a one-handed technique. Needle-stick injuries are most common during resheathing. State this in the OSCE: "I am disposing directly into the sharps bin without resheathing."
Blood Tube Order — Colour Coding
🧠 Mnemonic
Tube order: Gold, Green, Purple, Blue, Grey
(Some trusts vary — always check local protocol, but this is a common sequence)
| Colour | Additive | Tests |
|---|---|---|
| Gold/Yellow (SST) | Clot activator + gel | U&E, LFT, TFT, CRP, bone profile, glucose |
| Green | Heparin | Urgent bloods (results faster), ammonia, some hormones |
| Purple/Lavender | EDTA | FBC, HbA1c, blood film, Group and Save |
| Blue (citrate) | Sodium citrate | Clotting (PT, APTT, fibrinogen) — must be filled to exact line |
| Grey (fluoride oxalate) | Fluoride | Glucose (fasting), lactate |
💎 Clinical Pearl
The blue citrate tube must be filled to the line — under-filling changes the blood-to-anticoagulant ratio and gives a falsely prolonged clotting time. If you underfill it, discard and use a fresh tube. Examiners know this.
IV Cannulation — Additional Steps
Cannulation follows the same setup and ANTT principles, with these differences:
Choosing a cannula size:
- Green (18G): standard adult, most uses
- Pink (20G): fluids, antibiotics, elderly patients with fragile veins
- Grey (16G) / Orange (14G): trauma, rapid fluid resuscitation, blood transfusion
Technique differences:
- 1Insert cannula bevel-up at 15–30°, advance until flashback appears
- 2Lower the angle, advance the cannula (plastic) a further 2–3 mm into the vein
- 3While holding the needle still, advance the plastic cannula fully off the needle into the vein
- 4Apply pressure proximal to the tip, withdraw the needle, connect the bung or giving set
- 5Flush with 5–10 ml normal saline — confirm patency and absence of swelling (extravasation)
- 6Secure with a transparent dressing and date/time label
- 7Document: date, time, size, site, and your name
💡 Tip
"Advance the plastic before withdrawing the needle" is the step most students fail on. The needle provides stiffness — if you withdraw the needle first, the floppy cannula kinks and fails to thread.
Complications to Know
| Complication | How to recognise | What to say in OSCE |
|---|---|---|
| Haematoma | Bruising or swelling at site | "I would apply firm pressure for at least 2 minutes" |
| Extravasation | Swelling around cannula, pain on flushing | "Remove cannula, elevate limb, re-site proximally" |
| Phlebitis | Redness, warmth, tenderness along vein | "Remove cannula, apply warm compress, re-site" |
| Nerve injury | Sharp pain radiating down the arm | "Withdraw needle immediately, document" |
| Arterial puncture | Bright red, pulsatile blood | "Remove, apply firm pressure for 5–10 minutes, do not proceed" |
Frequently Asked Questions
"What is ANTT and why does it matter in an OSCE?"
Aseptic Non-Touch Technique is the standard framework for all invasive clinical procedures. The key principle is protecting key parts — the needle tip, cannula hub, and syringe tip — from contamination at all times. In an OSCE, state "I'm using ANTT throughout" and demonstrate: washing hands, gloves, cleaning the site, not re-touching after cleaning, and immediate sharps disposal. Examiners mark this explicitly.
"What is the correct order of blood tubes?"
A common sequence is gold or yellow (biochemistry), green (heparin), purple or lavender (haematology and FBC), blue (clotting — fill to the line), and grey (glucose and lactate). The order matters because anticoagulant carry-over between tubes can affect results. Always check your local trust protocol as sequences vary.
"How do I choose the right cannula size in an OSCE?"
Pink (20G) for routine use, elderly patients, and fragile veins; green (18G) for standard adult IV fluids and medications; grey (16G) or orange (14G) for trauma, rapid transfusion, or surgery. Larger bore = faster flow rate but more discomfort and vein trauma. State your choice and reasoning out loud.
"What do I do if the vein blows during cannulation?"
Remove the cannula immediately and apply firm pressure with gauze. Apologise to the patient. Apply pressure for at least 2 minutes to prevent haematoma. Re-site the cannula proximal to the failed attempt on the same vein, or use a different vein. Document the failed attempt, the site, and the reason.
"What should I say to the patient throughout the procedure in an OSCE?"
Before: introduce, explain the procedure, gain consent, check allergies, ask which arm they prefer. During: warn before the sharp scratch, narrate key steps briefly ("I'm just applying some pressure now"), check for pain. After: confirm they are comfortable, apply pressure, give aftercare advice ("Keep this covered for an hour — if it bruises or swells, let us know").
Related guides: [Prescribing Safety OSCE](/blog/prescribing-safety-osce) · [IV Fluids Prescribing OSCE](/blog/iv-fluids-prescribing-osce) · [A&E Assessment OSCE](/blog/ae-assessment-osce)