ABG – Arterial Blood Gas – taking a sample
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An arterial blood gas sample is an important investgiation in the acutely unwell patient, particularly those with respiratory distress. Venous blood gas samples are increasingly used to assess the acutely unwell patient where oxygenation status and carbon dioxide retention are not a concern. has to be analysed within minutes of the blood being taken. In the Emergency Department and Intensive Care, this often happens within the department in an arterial blood gas machine, but on the wards you may have to take the sample directly in person to the lab, and hand it over to a technician in person. You should send it with regular blood samples, or put it in a pod without at least calling the lab to let them know you have sent an ABG (or VBG) sample. Best practice is also to send the sample ‘on ice’.

Thus, before you begin, make sure you have ice ready, tell someone on the ward what you are doing, and make sure somebody is ready to take the sample in person to the lab. You have approximately 15 minutes to get the sample to the lab.

Samples are usually taken from the radial artery, but you can also use the femoral and brachial arteries
When sending off the sample you must record what % of O2 the patient is on (if any).


Introduce / check right patient / explain procedure / check anticoagulation therapy / gain consent. The procedure is used to check levels of oxygen and carbon dioxide in the blood. It is also used to check acid/base balance. Explain to the patient that knowing these levels helps asses what treatment is best. If the patient is on anti-coagulant therapy you won’t change the way you do the procedure, but note that it will probably take a lot longer for them to stop bleeding afterwards!
Wash your hands


Go and get your equipment. You will need – an ABG pack – this has the syringe, needle, a bung, and the syringe cap – ice, swabs, cotton wool bud, bag to put sample in, forms to go with sample, plaster. Don’t forget to clean out your tray before you put all this stuff in it!
Ice is used because it slows down the metabolism of red cells. If the sample is left for a long time (e.g. transferred to another hospital) then the use of oxygen by cells can lead to a falsely low O2 level in the sample.


Open out the ABG pack – you should find a syringe already with heparin in it. You need to put the needle on the syringe (put it on, do a ¼ turn, then pull off the cap and you should be ok). Once you have done this, you can squirt all of the heparin out of the syringe (just do this with the cap half on, it doesn’t really matter where the heparin goes)– this leaves a very small amount in the needle which will help prevent clots.
Wash/alco gel, then put gloves on

Allen’s Test

You need to check the collateral circulation to the hand – a rare complication of blood sampling is that you may end up occluding the radial artery (e.g. as a result of a clot), and thus you should test collateral circulation to the hand to prevent ischaemia of the hand if this event were to occur. Get the patient to make a fist. Find both the radial and ulnar arteries, then press down on them to occlude them for a few seconds. Ask the patient to relax their hand and show their palm. It will appear blanched. Let go of the ulnar artery but keep holding down the radial. The hand should turn red again as it is perfused. As long as this perfusion takes no longer than 10-15 seconds, then there is adequate perfusion of the hand via the ulnar artery, and you are able to continue. If there is not adequate perfusion, you are unable to use the radial artery for this procedure.
Feel the radial artery – you are going to use the radial artery. Have a really good feel of it – you are going to go in where the pulse is strongest – this is a different place on different people! Once you are happy you have got a good place, then swab the area (leave it 30 seconds to dry). Be careful when taking your sample – try not to touch the sterile area (you will still need to palpate the artery as you take the sample)

Collect Sample

Go in! – hold the syringe a bit like a pen. You want it into the skin at 45’. You should go ‘up’ the artery, i.e. towards the elbow. You want to go in at the point where the pulse feels strongest. Usually you would put a finger on the pulse to help orientate yourself, but be careful not to contaminate the exact spot where you will insert the needle. Insert the needle slowly. The syringe should now rise by itself. You only need 1-2 ml. Once you are happy you have got this amount, take the needle out, and apply pressure and a sterile dressing immediately. This will need to be held on (either by the patient or by you, or if you really have to you can tape it down firmly) for at least 2-3 minutes, possibly 10 (or longer if on warfarin).
If you don’t get into the artery, you can try again – as long as you don’t withdraw the needle fully from the skin, you can poke around – but remember this is very painful!
Bung it! – stick the needle into the bung, whilst you deal with the cotton wool. Once you are happy with the cotton wool situation, you can put the needle (and bung) in the sharps bin.
The cap – the ABG kit comes with a cap that allows air but not blood through it. Put the cap on the syringe once you have taken the needle off. Flick the syringe to get all the air to the top, and then you can squeeze the air out. As soon as you get a drop of blood in the cap, stop squeezing! The cap is to allow you to squeeze air out without spilling blood. Once all the air is out, you should mix the sample to make sure the anti-coagulant is thoroughly mixed with the sample.  You should label up the sample and use the ice, and make sure it gets delivered in person and all that.

Taking an arterial blood gas sample
Taking an arterial blood gas sample. Image by iem-student.org is licensed with CC BY-NC-SA 2.0.

Finishing off

Finish off – check the site isn’t still bleeding, and check the patient is comfortable. Once you are happy it’s no longer bleeding, you can stick a plaster on (helps prevent infection).

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Dr Tom Leach

Dr Tom Leach MBChB DCH EMCert(ACEM) FRACGP currently works as a GP and an Emergency Department CMO in Australia. He is also a Clinical Associate Lecturer at the Australian National University, and is studying for a Masters of Sports Medicine at the University of Queensland. After graduating from his medical degree at the University of Manchester in 2011, Tom completed his Foundation Training at Bolton Royal Hospital, before moving to Australia in 2013. He started almostadoctor whilst a third year medical student in 2009. Read full bio

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