Nursing care
Arterial blood gas sampling: Allen test, sample errors and preventing haematoma
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated October 2026
Short answer
Before a radial arterial puncture, a modified Allen test checks that the ulnar artery can supply the hand. The sample goes into a heparinised syringe, air bubbles are expelled at once, and it is analysed promptly. Afterwards, firm pressure on the site for several minutes, longer with anticoagulants, helps prevent bleeding and haematoma.
Before the puncture: site and Allen test
The radial artery is the usual site because it is superficial and has collateral supply from the ulnar artery. Whether a nurse may perform arterial puncture depends on local scope and training; respiratory therapists or other clinicians often perform it. The nurse prepares the patient, checks anticoagulant use and records oxygen settings on the request.
For a modified Allen test, the patient clenches the fist while both radial and ulnar arteries are compressed, then opens the blanched hand. The ulnar artery is released while radial pressure continues. Colour returning to the palm promptly suggests adequate collateral flow. Delayed or absent return suggests that site may not be safe; report it and use another site per policy.
Collecting a reliable sample
Use a pre-heparinised blood gas syringe rather than adding liquid heparin, because excess heparin dilutes the sample and alters carbon dioxide and electrolyte values. After a change in oxygen therapy or ventilator settings, allow time for values to stabilise according to protocol before drawing, so the result reflects the current settings.
Once the sample is collected, expel any air bubbles immediately, cap the syringe and roll it gently to mix with the anticoagulant. Label it at the bedside with the time, oxygen delivery, and temperature if required. Venous blood mixed into the sample lowers oxygen values and can mislead interpretation.
Errors that distort the result
Air bubbles exchange gas with the blood: oxygen tension tends to move toward that of room air and carbon dioxide falls, producing a falsely reassuring or confusing result. Even a small bubble can have a meaningful effect, which is why bubbles are removed at once rather than when the sample reaches the laboratory.
Delay is the other major error. Blood cells keep metabolising after collection, consuming oxygen and producing carbon dioxide, so a sample left at room temperature drifts. Analyse it promptly according to laboratory guidance; cooling may be recommended only when a delay is unavoidable and depends on the syringe type. Clotted or haemolysed samples are also unreliable.
After the puncture: pressure and complications
Apply firm direct pressure over the puncture site for about five to ten minutes, and longer if the patient takes anticoagulants or has a bleeding disorder. Do not let the patient press lightly through a bulky dressing. Check the site for bleeding or swelling before leaving, and recheck after the pressure dressing is applied.
Complications include haematoma, bleeding, arterial spasm, reduced blood flow to the hand, nerve irritation, infection and vasovagal fainting. Assess colour, warmth, capillary refill and sensation distal to the site. Report expanding swelling, pallor, numbness or severe pain. Document the site, Allen test result, oxygen settings, pressure time and site condition.
Original practice scenario
In an invented item, a radial sample arrives at the laboratory forty minutes after collection with a visible air bubble, and the result shows an unexpectedly high oxygen level. The options are to treat the result as accurate, adjust oxygen therapy downward immediately, report the handling problem and request a repeat sample, or perform an Allen test now.
Reporting and requesting a repeat is correct, because both the bubble and the delay make the result unreliable. Changing therapy on a flawed result could harm the patient, and the Allen test belongs before the puncture. Interpreting the values themselves is a separate skill once a trustworthy sample has been obtained.
Sources and further reading
Indian Journal of Critical Care Medicine (PMC): Interpretation of arterial blood gas. Expelling air bubbles, effect of small bubbles on oxygen values, analysis timing, limited role of cooling, liquid heparin dilution and venous contamination.
Cleveland Clinic: Arterial blood gas (ABG) test. Radial site, Allen test before sampling, five to ten minutes of pressure or longer on anticoagulants, and risks including haematoma, bleeding and fainting.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our dosage calculation and lab values practice questions are the closest set to what this page covers.
Common questions
What does the modified Allen test check?
It checks that the ulnar artery can supply the hand if the radial artery is damaged or blocked after puncture. Prompt return of palm colour suggests adequate collateral flow.
How do air bubbles affect an ABG result?
Bubbles exchange gas with the sample, shifting oxygen toward room air values and lowering carbon dioxide. Expel bubbles immediately and cap the syringe.
How long should pressure be held after an arterial puncture?
Commonly about five to ten minutes of firm pressure, longer for patients on anticoagulants or with bleeding disorders, then check the site for bleeding or haematoma.
More on dosage calculation and lab values