Arterial blood gases: the Allen test, drawing the sample and reading it step by step
- Assessment
Check collateral flow, draw the arterial sample without air, press the site, send it quickly, then read pH, PaCO2, HCO3 and PaO2 in order.
What it is
An arterial blood gas measures pH, the partial pressures of oxygen (PaO2) and carbon dioxide (PaCO2), and bicarbonate (HCO3). It shows oxygenation, ventilation and acid–base status, and is used to check the response to treatment, including ventilator changes. The sample comes from a radial artery puncture or from an arterial line. WHO says arterial puncture is done only by staff for whom it is within their legal scope and who have shown proficiency after training; in Colombia this follows the institutional protocol. Venous and capillary samples cannot be used to judge oxygen levels.
Reference values
- pH: 7.38–7.42 (MedlinePlus); WHO describes acidosis as a process that lowers pH below 7.35.
- PaCO2: 38–42 mmHg (MedlinePlus).
- PaO2: 75–100 mmHg breathing room air (MedlinePlus).
- HCO3: 22–28 mmol/L (MedlinePlus).
- SaO2: 94–100% (MedlinePlus). Ranges vary between laboratories.
What you need
- Pre-heparinised blood gas syringe and a needle chosen for the site (WHO lists 20, 23 and 25 gauge).
- A safety device to cover the needle, gauze and a bandage.
- 70% alcohol, gloves, and face protection if splashes are expected.
- Label, and a container with ice if the sample is not analysed at the point of care and the laboratory asks for it.
How to measure it (and why)
- Check the order, identify the patient and note FiO2 and ventilator settings — a PaO2 means little without the oxygen given.
- Keep the patient at rest, settings unchanged for some minutes — breath-holding, crying or recent changes alter results.
- Do the modified Allen test: press radial and ulnar, open the hand, release the ulnar; colour back in 5–15 s means good flow.
- Perform hand hygiene, wear gloves and face protection, disinfect with 70% alcohol and let it dry — prevents infection and exposure.
- Insert the needle at about 45° toward the pulse and let arterial pressure fill the syringe; do not pull the plunger.
- Withdraw and press firmly; check at 2–3 minutes, longer with anticoagulants — arteries bleed more than veins.
- Cover the needle with the safety device, expel air bubbles, cap and roll the syringe — air alters PaO2; mixing prevents clots.
- Label and send at once as the lab protocol says; from an arterial line, clear the flush solution first — delay and dilution alter values.
- Check the site again for bleeding or a haematoma.
- Read pH, PaCO2, HCO3, the compensation and PaO2 with FiO2, and report — see «How to read it».
Primary acid–base disorders (simplified)
| Disorder | pH | PaCO2 | HCO3 |
|---|---|---|---|
| Respiratory acidosis | Low | High (primary change) | Normal, or high if compensating |
| Respiratory alkalosis | High | Low (primary change) | Normal, or low if compensating |
| Metabolic acidosis | Low | Normal, or low if compensating | Low (primary change) |
| Metabolic alkalosis | High | Normal, or high if compensating | High (primary change) |
How to read it
- Step 1, pH: below the range is acidaemia; above it is alkalaemia.
- Step 2, PaCO2: if it moves in the opposite direction to pH, the primary problem is respiratory.
- Step 3, HCO3: if it moves in the same direction as pH, the primary problem is metabolic.
- Step 4, compensation: the other system shifts in the same direction to bring pH back; respiratory compensation is faster than metabolic, and full compensation is rare.
- Step 5, PaO2 with FiO2: the same PaO2 is reassuring on room air and worrying on high oxygen; the PaO2/FiO2 ratio is used in ARDS.
Next step: when to call for help
- pH outside the range, a rising PaCO2 with drowsiness, or a low PaO2 despite oxygen: report to the nurse in charge or the doctor now.
- A sudden change after a ventilator change: report it with the settings.
- A growing haematoma, bleeding that does not stop or numbness of the hand after the puncture: press the site and report.
- A result that does not fit the patient (air in the sample, clot, delay): tell the team and repeat as ordered.
Test yourself
Think of the answer, then tap the question to see it.
How is a negative modified Allen test recognised and what does it mean?
The hand does not flush within 5–15 seconds after releasing the ulnar artery. Ulnar flow is inadequate, so that radial artery is not punctured.
Why must air bubbles be expelled from an arterial blood gas sample?
Air in the syringe alters the gas values, especially PaO2, even in small amounts.
In an ABG, pH is low and PaCO2 is high. What is the primary disorder?
Respiratory acidosis: PaCO2 moves opposite to pH, so the primary problem is respiratory.
Why must FiO2 be recorded with every ABG?
PaO2 can only be interpreted against the oxygen given; the same PaO2 means different things on room air and on high oxygen.
Elsewhere in DurgiPedia
- Reading an arterial blood gas step by step
- Respiratory acidosis and alkalosis
- Metabolic acidosis, the anion gap and compensation
- Allen test
Connections
Educational content for students; it does not replace your institution's protocols, supervised clinical training or a prescriber's order. Dose-calculation examples use made-up numbers: they are educational, not a clinical order.
Sources: WHO guidelines on drawing blood: best practices in phlebotomy (chapter 5 and annex I) (World Health Organization, 2010); Blood gases (MedlinePlus, U.S. National Library of Medicine, 2024); Interpretation of arterial blood gas (Indian Journal of Critical Care Medicine (open access, PMC), 2010); Clinical care for severe acute respiratory infection: toolkit, update 2022 (COVID-19 adaptation), tool 4.3 (World Health Organization, 2022).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.