Arterial blood gas sampling and the Allen test

What it is

An arterial blood gas (ABG) measures the pH, the partial pressures of oxygen and carbon dioxide, bicarbonate and, on most analysers, lactate, electrolytes and haemoglobin derivatives. It is the quickest way to judge oxygenation, ventilation and acid–base status in an acutely ill patient. The radial artery at the wrist is the first choice: it is superficial, lies over bone so it can be compressed afterwards, and the hand has a second supply through the ulnar artery and the palmar arches. The artery is felt just proximal to the wrist crease, lateral to the tendon of flexor carpi radialis. Before puncture many units check collateral flow with the modified Allen test, although its ability to predict ischaemia is limited and a normal result does not remove the need for care. The brachial artery (medial to the biceps tendon at the elbow) and the femoral artery (below the inguinal ligament, midway between the anterior superior iliac spine and the pubic symphysis) are alternatives with higher risk. A good sample is arterial, free of air bubbles, anticoagulated, labelled with the inspired oxygen and analysed promptly.

Indications

• Respiratory failure or suspected hypoxaemia or hypercapnia, to classify it and guide oxygen or ventilation.
• Suspected acid–base disorders: diabetic ketoacidosis, sepsis with raised lactate, poisoning, renal failure.
• Monitoring a patient on non-invasive or invasive ventilation, or after a change in settings.
• Shock and cardiac arrest, where a rapid lactate, potassium and haemoglobin also help.
• Carbon monoxide or methaemoglobin suspicion, measured by co-oximetry on the same sample.

Contraindications and precautions

• Abnormal Allen test or known poor collateral flow on that side: choose the other wrist or another site.
• Infection, burn, recent surgery or an arteriovenous fistula or graft on that limb: use another site.
• Severe coagulopathy, low platelets or anticoagulation: not absolute, but use the most compressible site and press longer.
• Peripheral vascular disease, Raynaud phenomenon or previous arterial cannulation on that side raise the risk of ischaemia.
• Femoral puncture is a last resort: it is hard to compress and close to the femoral vein and nerve.

Equipment

• Pre-heparinised blood gas syringe with a cap, and a fine needle (22 to 25 G).
• Gloves, antiseptic swab, gauze and tape.
• Local anaesthetic with a fine needle, if the unit uses it for comfort.
• A rolled towel to extend the wrist, and a sharps container.
• Label and request form with the inspired oxygen and time; ice if the sample cannot be analysed promptly.

Step by step (and why)

1. Identify the patient, explain and get consent; note the inspired oxygen — the result is read against it.
2. Perform the modified Allen test and record it — it checks that the ulnar artery can supply the hand.
3. Extend the wrist slightly over a towel and feel the pulse with two fingers — this fixes the artery and its course.
4. Clean the skin and let it dry; consider local anaesthetic — the puncture is painful and pain causes spasm.
5. Enter at 30 to 45 degrees, bevel up, towards the pulse — a shallow angle follows the artery and limits through-and-through puncture.
6. Let the syringe fill by arterial pressure — a pulsatile flash and self-filling confirm an arterial, not venous, sample.
7. Withdraw and press firmly at once for at least 5 minutes — compression prevents a haematoma and false aneurysm.
8. Expel any air, cap the syringe and roll it gently — air alters the gases; mixing prevents clots.
9. Label and send for analysis promptly — ongoing cell metabolism lowers the oxygen and raises the carbon dioxide.

Sampling errors and what they do to the result

Complications

• Haematoma and bruising, the most common problems, mostly from too little compression.
• Arterial spasm or thrombosis with ischaemia of the hand: check colour and capillary refill afterwards.
• False aneurysm (pseudoaneurysm) or arteriovenous fistula after repeated or poorly compressed punctures.
• Nerve injury: the median nerve at the brachial site, the femoral nerve in the groin.
• Venous sample by mistake: low oxygen tension that does not fit the pulse oximetry.

Next step

• Interpret the result systematically (pH, primary process, compensation, anion gap, oxygenation): see the worked cases.
• Compare the oxygen tension with the pulse oximetry: a mismatch suggests a venous sample or an error.
• Recheck the hand before you leave: pain, pallor or slow capillary refill need a senior review.
• If repeated samples are needed (ventilated or unstable patients), discuss an arterial line.
• Call a senior after two failed attempts at one site, rather than repeating punctures.

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Elsewhere in DurgiPedia

• Allen test
• Reading an arterial blood gas step by step
• Oxygenation: SpO₂, PaO₂, cyanosis and oxygen therapy

Connections

← All clinical skills

Educational content; it does not replace supervised hands-on training, your institution's protocols or a prescriber's order. A procedure is learnt with a teacher and done only once you are trained to do it.

Sources: Arterial Blood Gas Analysis: Fundamentals, Interpretation, and Clinical Utility (StatPearls (NCBI Bookshelf), 2026); Allen Test (StatPearls (NCBI Bookshelf), 2024); Anatomy, Shoulder and Upper Limb, Forearm Radial Artery (StatPearls (NCBI Bookshelf), 2025).

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Representative model, for educational purposes only.