How to take an Arterial Blood Gas (ABG) – OSCE Guide

Arterial blood gas (ABG) sampling is a commonly performed procedure which allows healthcare professionals to quickly obtain information on a patient’s respiratory status (blood oxygen and carbon dioxide levels), as well as the patient’s acid-base balance.  Taking an arterial blood gas (ABG) involves using a needle and syringe to directly sample blood from an artery (usually the radial artery). Below is a step by step guide explaining how to take an arterial blood gas sample.

Check out the ABG OSCE mark scheme here.

If you’d like to learn more about interpreting an ABG then check out our guide here or test yourself with our ABG quiz.

If you’re unsure how to document the results of an ABG in the notes then check out this guide.


Wash hands

Introduce yourself

Confirm patient details:

  • Name / date of birth
  • Take note of whether the patient is requiring oxygen and record how much (e.g. FiO2 concentration or flow rate)


Check for any contraindications to ABG sampling:

  • Absolute   poor collateral circulation / peripheral vascular disease in the limb / cellulitis surrounding the site / arteriovenous fistula
  • Relative impaired coagulation (e.g. anticoagulation therapy / liver disease / low platelets <50)


Explain procedure:

“I need to take a sample of blood from an artery in the wrist to assess the oxygen levels in your circulation. It will be a little painful, but should hopefully only take a short amount of time. The procedure does involve some risks which include bleeding, bruising, infection and permanent damage to the artery.”


Gain consent:

“Do you understand everything I’ve said so far? Are you happy for me to go ahead?”

Modified Allen’s test

This test involves the assessment of the arterial supply to the hand.

1Ask the patient to clench their fist

2. Apply pressure over both the radial and ulnar artery to obstruct blood supply to the hand

3. Ask the patient to open their hand, which should now appear blanched (if not you have not completely occluded the arteries with your fingers)

4. Remove pressure from the ulnar artery whilst maintaining pressure over the radial artery

5. If there is adequate blood supply from the ulnar artery, colour should return to the entire hand within 5-15 seconds

It should be noted that there is no evidence performing this test reduces the rate of ischaemic complications of arterial sampling.

  • Ask patient to make a clenched fist

Gather equipment

  • Arterial blood gas syringe
  • Needle (23G)
  • Alcohol wipe –  70% isopropyl 
  • Gauze
  • Tape
  • Lidocaine – with small needle/syringe for administration
  • Sharps container
  • Gloves
  • Apron
  • Gather equipment

Local anaesthetic

The sample is routinely obtained from the radial artery and it is recognised that that the procedure causes significant pain for the patient and that this can be markedly reduced by the use of subcutaneous local anaesthetic.  The British Thoracic Society recommends the routine use of local anaesthetic for obtaining ABG samples except in emergencies, or in unconscious or anaesthetised patients.


1. Position the patient’s arm preferably on a pillow for comfort with the wrist extended (20-30°) 

2. Prepare all the equipment in the equipment tray using an aseptic non touch technique

3. Palpate the radial artery on the patient’s non-dominant hand (most pulsatile over the lateral anterior aspect of the wrist)

4. Clean the site with an alcohol wipe for 30 seconds and allow to dry before proceeding 

5. Wash hands again

6. Don gloves and apron

7. Prepare and administer lidocaine subcutaneously over the planned puncture site (aspirate to ensure you are not in a blood vessel before injecting the local anaesthetic).

8. Allow at least 60 seconds for the local anesthetic to work 

9. Attach the needle to the ABG syringe, expel the heparin and pull the syringe plunger to the required fill level (check with your local laboratory)

  • Palpate radial artery

Taking the sample

1. Palpate the radial artery with your non-dominant hand’s index finger around 1cm proximal to the planned puncture site (avoiding directly touching the planned puncture site that you have just cleaned)

2. Warn the patient you are going to insert the needle

3. Holding the ABG syringe like a dart insert the ABG needle through the skin at an angle of 45° over the point of maximal radial artery pulsation (which you identified during palpation)

4. Advance the needle into the radial artery until you observe blood flashback into the ABG syringe

5. The syringe should then begin to self-fill in a pulsatile manner (do not pull back the syringe plunger)

6. Once the required amount of blood has been collected remove the needle and apply immediate firm pressure over the puncture site with some gauze

7. Engage the needle safety guard 

8. Remove the ABG needle from the syringe and discard safely into a sharps bin

9. Place a cap onto the ABG syringe and label the sample

10. Yourself or a colleague should continue to apply firm pressure for 3-5 minutes to reduce the risk of haematoma formation

  • Insert needle at 45°

To complete the procedure

Dress the puncture site

Thank the patient

Dispose gloves and equipment into an appropriate clinical waste bin

Wash hands

Take the ABG sample to be analysed as soon as possible after being taken as delays longer than 10 minutes can affect the accuracy of results


1. WHO guidelines on drawing blood: best practices in phlebotomy. Published 2010. [LINK]


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