Arterial blood gas and the modified Allen test

An arterial blood gas, or ABG, samples blood from an artery rather than a vein to measure oxygenation and acid–base balance. It is a specialist procedure, performed only by staff with formal training and demonstrated competency, and in most settings it sits outside routine phlebotomy. It is worth understanding all the same — both because the principles appear in nearly every theory syllabus, and because the checks that make it safe show the logic of arterial sampling clearly.

What an ABG is for

A venous sample tells you what the blood looks like on its way back to the heart. An arterial sample tells you what the lungs have just done to it. That is why ABGs are used to:

• assess a patient's oxygenation;
• evaluate acid–base balance;
• monitor a patient on a ventilator.

Site, and why the radial comes first

The first-choice site is the radial artery at the wrist. The brachial and femoral arteries are alternatives, but both carry higher risk. The radial is preferred for one specific reason: the hand has a second arterial supply through the ulnar artery, so if the radial is damaged, the hand is not left without blood. That backup supply is called collateral circulation, and before a radial puncture it is checked rather than assumed.

The modified Allen test

The modified Allen test confirms the ulnar artery can supply the hand on its own before the radial is punctured.

1. Ask the patient to clench the fist.
2. Press on both the radial and the ulnar arteries to block the blood flow through them.
3. Ask the patient to relax the hand — it should look blanched and pale.
4. Release the pressure on the ulnar artery only.
5. The hand should flush back to colour within 5 to 15 seconds. That is a positive test.

Collection technique

The draw itself differs from a venous one in three details that follow from the higher pressure of an artery:

• A pre-heparinised syringe, so the sample does not clot before analysis.
• A needle angle of about 45 degrees, steeper than a venous insertion.
• No pulling on the plunger — arterial pressure fills the syringe on its own.

Afterwards, firm pressure is held and the site is checked after two to three minutes; five minutes or more may be needed for a patient with high blood pressure, a bleeding disorder, or on anticoagulants — in every case longer than a venous draw needs, because an artery bleeds under pressure. The sample is then transported promptly, and on ice where the protocol calls for it. The same bright-red, pulsing flow that confirms a deliberate arterial sample is also the warning sign of an accidental arterial puncture during a venous draw — there, it is a complication to recognise and manage at once.

---

This guide is a free extract from PhlebMastery's phlebotomy theory course, with content aligned to published WHO guidance. The full treatment — arterial sampling alongside blood-culture, paediatric, capillary, and donor collection — is in Module 7: Special Collections & Advanced Techniques. New here? Start with the free Module 1, or see the whole course — full access is a one-time purchase.