Arterial blood gas


Wash your hands, introduce yourself with your name and role, and confirm the patient's full name and date of birth. Explain the procedure, check the patient's understanding and obtain consent.

Identify a strong radial pulse. This will be the wrist you will be sampling from. Ensure that it is adequately exposed.

Ask the patient if they have any allergies, specifically to latex. Also ask whether they are taking any medications, specifically anticoagulants, and if they have had any surgery recently that may affect the arm you take blood from.

Sampling arterial blood is fairly invasive and can be a lot more painful than venepuncture. As a result, it is important to clearly communicate the details of the procedure and its necessity. For example “I need to take a sample of blood to see how acidic it is and what your oxygen levels are like. I will need to take the sample from an artery in your wrist, rather than from a vein, and it will involve a small needle with a sharp scratch. Some patients find that this can be quite painful, so if you wish I can give you something to numb the pain. I will of course do my best to be quick and will talk through what I am doing as I go along.”


Ask the patient if they are in any pain and are comfortable. Check for any use of accessory breathing muscles, breathing symmetry and lung hyperexpanstion, as this may give clues to the patient’s oxygen and CO2 status. Carefully note the colour of the patient. They may show signs of bronchitis (cyanosis, blue lips) or carbon monoxide poisoning (cherry red lips). Assess the age of the patient and note any paraphernalia around the bedside, looking for clues such as cigarettes, oxygen, inhalers, nebulisers, sputum pots or ventilators.

If the patient is on any oxygen therapy, check how much this is and for how long they have been on it, as this will impact interpretation of your results.

A history and examination are naturally crucial to interpretation of an arterial blood gas. In a stable patient, you will already have performed these. However, in scenarios where there isn’t time (for example in resuscitation or an acutely deteriorating patient), or if you feel patient that the patient’s situation has changed since their last examination, a quick inspection is essential to properly interpret your findings.


Collect a plastic tray, and clean both the inside and outside of the box thoroughly with an alcohol surface disinfectant wipe. Determine whether or not you think you will need local anaesthesia for the procedure. You will then need to collect a number of items. Ensure that they are all in date.

For arterial blood sampling

Collect the following equipment:

For anaesthesia

If you choose to give local anaesthesia, then in addition to the above, collect:

Note that in most hospitals, the syringe, needle and bung may come pre-packaged in an arterial blood gas collection set.

Modified Allen's test

Before attempting to sample the radial artery a modified Allen’s test should be carried out to determine the competency of the ulnar artery. This is performed as follows:

  1. 1. Ask the patient to elevate their hand above their heart and form a tight fist.
  2. 2. Locate the radial and ulnar arteries and compress them using the index and middle finger of each of your hands.
  3. 3. Ask the patient to release their fist, keeping their hand elevated. The palm should appear white and pale due to the now obstructed blood supply.
  4. 4. Release the hand blocking the ulnar artery, allowing blood to flow through it once more.

If colour appears back into the hand within 5-15s, the ulnar artery has sufficient enough blood flow to compensate should the radial supply be compromised. This indicates that it is okay to take an arterial blood sample from the radial artery of this hand.

If colour does not appear back into the hand after 15s, the ulnar circulation to the hand is inadequate and the radial artery should not be compromised. If this is the case, then you must either consider taking a sample from the other hand (after conducting a separate Allen’s test) or another site (such as the femoral artery).



Put on your apron and remove all equipment from its packaging so it is ready to use. Position yourself and the patient so that you are both sitting comfortably, and the patient’s wrist is exposed. Ask the patient to slightly extend their wrist. Expose the patient’s wrist adequately.

Ideally you should rest the patient’s arm on a pillow with the forearm supinated and the palm facing upwards.


Locate the radial artery by palpating the radial pulse. Track the pulsation towards its strongest point whilst trying to visualise the course of the radial artery. Remember this location for later.


Local anaesthetic

Before injecting the anaesthetic, confirm with the patient that they have had no previous reactions to local anaesthetic. Using proper technique, inject up to 0.5mL of lidocaine solution subcutaneously at the site, remembering to draw back initially for blood.

Needle insertion

If not already, attach your needle to the pre-heparinised syringe. Flush the heparin through the needle.

Palpate the radial artery with your non-dominant hand. This time start from further away and stop just before the area where you previously felt the strongest pulsation. This is the insertion site, and you want to avoid touching it as much as possible.

Stretch the skin slightly with your non-dominant hand and inform the patient that they may feel a sharp scratch (or a pushing sensation if they have received anaesthetic).

Holding the syringe needle like a dart using your thumb, index and middle fingers. At a 30-45o angle, pierce the skin at the insertion site. Once you have entered the radial artery you will feel a slight give, observe some bubbling or see flash back.

Note that it is easy to miss the artery. If you do this, you will need to withdraw your needle back slightly to see if you can readjust.

Sample collection

The arterial pressure should cause your syringe to fill up quickly, if not start to draw the syringe back slightly.

Once you are satisfied with your sample, carefully withdraw the needle and apply firm pressure to the area with some gauze or cotton wool to stop the bleeding. Immediately cover the needle with the plastic bung. Apply tape to keep the gauze in place.

Discard the needle straight away into a sharps bin. Cap the syringe, push out any air and remove your gloves.

Send your sample for analysis as soon as possible. If you foresee a delay, you may wish to keep it in ice.

If they are able to, ask the patient to take over applying the pressure to the area such that you are able to focus on covering the needle and disposing of it in the sharps bin. The pressure will need to be maintained for up to 5 minutes before the bleeding stops.


Thank the patient and ensure that they are comfortable. Record the time that you took the arterial blood sample in the notes, whether or not the patient was on oxygen, and whether or not they had a fever.

Ideally, prepare any paper work before you start the procedure as you will need to take your sample for analysis and may forget crucial details.

List form

When assessing each other, please click on each list item as you go along. Doing so will turn the list item green. Make careful note of any steps missed at the end.
We recommend completing any examination or procedure in under 10 minutes, but you can adjust the timer to suit your needs.

  1. Introduction: “Hello. I’m SimpleOSCE and I’m a medical student. Could I confirm your full name and DOB?”
  2. Explain procedure and obtain consent: “I need to take a sample of blood to see how acidic it is and what your oxygen levels are like. I will need to take the sample from an artery in your wrist, rather than from a vein, and it will involve a small needle with a sharp scratch. Some patients find that this can be quite painful, so if you wish I can give you something to numb the pain. I will of course do my best to be quick and will talk through what I am doing as I go along.”
  3. Wash your hands.
  4. Identify a hand with a strong radial pulse.
  5. Enquire about allergies (latex), medication (anticoagulants) and whether the patient has had any surgery recently.
  6. Perform a brief inspection and check if the patient is on oxygen therapy.
  7. Optional: Determine whether or not the patient needs local anaesthesia.
  8. Assemble the correct equipment in to a cleaned tray.
  9. Check the expiry dates of the equipment.
  10. Perform the modified Allen’s test.
  11. Position the patient’s hand appropriately, ideally on a pillow with the wrist slightly extended.
  12. Palpate the radial pulse and identify the point of strongest pulsation.
  13. Wash your hands again and put on a pair of non-sterile gloves.
  14. Clean the area with an alcohol swab and wait for 30s.
  15. Optional: Inject local anaesthetic subcutaneously at the site.
  16. Flush the heparin through the needle.
  17. Palpate the radial pulse just distal to the point of strongest pulsation.
  18. Warn the patient about a sharp scratch.
  19. Insert the needle at a 30-45o angle and observe flashback of blood.
  20. Allow syringe to fill and then withdraw the needle.
  21. Apple gauze to the site and apply firm pressure.
  22. Immediately cover the needle with the plastic bung and then dispose of it in the sharps bin.
  23. Cap the syringe and remove any air.
  24. Send the blood for sample analysis.
  25. Instruct the patient to apply firm pressure to the gauze for at least 5 minutes.
  26. Record in the notes the time, whether the patient was on oxygen and whether they had a fever.
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