This is most conveniently performed via the radial artery. Many patients will have given a venous blood sample, or have donated blood. They won’t necessarily understand the difference between arterial and venous sampling. Explain to the patient that this is a different experience, as the needle has to penetrate deeper into their arm. Done well, it doesn’t hurt, so try to do it well. Learn from someone who has a reputation for taking painless ABGs.

Gather the required equipment:
- A heparinised 2 mL syringe and a 23G (blue) needle
- A cap for the syringe once the sample has been taken
- Alcohol gel or swab to sterilise the site
- Swab or cotton wool to prevent bleeding afterwards
- Sharps bin for disposal of the needle
- Gloves

Explain the need for the test to the patient and obtain consent

Perform an Allen’s test:
• Occlude both radial and ulnar arteries and wait for the hand to appear blanched from lack of blood flow.
• Release pressure over the ulnar artery. If there is sufficient collateral flow to the hand in this artery, colour should return to the hand in a few seconds.
• Choose the best side. One might be better than the other.
• If there is not sufficient collateral flow, sampling should be performed at another site (e.g. femoral artery).

(Optional) Infiltrate the area with 1% lidocaine. This is said by many to reduce the amount of pain caused by the procedure.

Have the arm held by an assistant, or supported with a pillow, with the hand towards you.

Take your time feeling for where the radial artery pulse seems strongest. This varies from person to person and is the ideal site for taking the sample. (Top tip: mark the position by pressing into the skin with a fingernail, in case you get distracted.)

Sterilise the field.

Palpate the artery above where you intend to sample with the hand not holding the syringe, to be sure of your target.

Insert the needle at a 45° angle, in one swift movement. This is the bit that hurts, so do it well, once.

The syringe may or may not fill itself from the arterial pressure, depending on the make.

Press a swab or cotton wool against the site after withdrawing the needle for at LEAST 5 minutes.

Expel any bubbles and cap the syringe. Keep on ice if there will be any delay in performing the analysis.