Before asking “What is the acid–base disorder?”, first ask:

Was this ABG collected, handled and processed correctly?

And then:

Is the ABG internally consistent?

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1. Why do we perform an ABG?

An arterial blood gas can help assess:

Oxygenation
→ PaO₂, SaO₂

Ventilation
→ PaCO₂

Acid–base status
→ pH, PaCO₂

Oxygen-carrying capacity
→ PaO₂, Hb, total Hb and dyshemoglobins

Electrolytes, Hb, Hct, Osmolality, etc.

It can also be used to assess response to interventions such as oxygen therapy and to monitor the severity or progression of disease.

2. Getting the sample right

Before puncture

• Allen’s test as a prerequisite

• Slight wrist extension for radial artery sampling

• Clean the site with 70% alcohol

• Use an appropriate heparinised syringe

• Enter the skin at approximately 45° for radial artery sampling

• Obtain approximately 1 mL of blood

• Do not pull back the syringe plunger

If an air bubble enters the syringe:

Remove it immediately.

Then apply firm pressure to the puncture site for at least 5 minutes.

3. The sample can be wrong even when the puncture was right

Several pre-analytical errors can alter the ABG:

Too much or too little anticoagulant

The source highlights the potential effects of excess/inadequate anticoagulant, including electrolyte binding.

Venous contamination

Mixing venous and arterial blood during puncture can produce misleading values.

Air bubbles

Air exposure can:

↑ PaO₂
↓ PaCO₂

Inadequate mixing

Can cause RBC stacking and compromise the sample.

4. Time matters

Analyse within 5 minutes
Maximum: 15 minutes

If a delay of >30 minutes is expected, it is recommended to keep in a glass syringe with an ice slurry, with iced-sample analysis possible for up to approximately 1 hour.

Practical takeaway:

Don’t let a good arterial puncture become a bad ABG because of poor handling.

5. When should you avoid a particular puncture site?

These cautions:

• Negative Allen’s test

• Surgical shunt/fistula or bypass graft

• Infection at the site

• Limb ischemia/Raynaud’s disease

• Bleeding diathesis — described as a relative contraindication in the post-thrombolysis setting

6. Complications of ABG sampling

Even a routine ABG is an arterial puncture. Potential complications include:

Arterial spasm
Hematoma
Arterial occlusion
Air/thrombus embolism
Local infection
Nerve or vessel injury
Vasovagal response
Needle-stick injury

7. Now comes the part most people skip…

CHECK THE ABG BEFORE INTERPRETING IT

Before jumping to:

❌ “It’s metabolic acidosis.”
❌ “It’s respiratory alkalosis.”
❌ “The patient is hypoxic.”

First ask:

Does the ABG make physiological sense?

The modified Henderson–Hasselbalch relationship

[H⁺] = 24 × PaCO₂ / HCO₃⁻

Use the reported PaCO₂ and HCO₃⁻ to estimate the corresponding hydrogen ion concentration.

Then compare it with the reported pH.

If the pH and calculated H⁺ are inconsistent:

Question the validity of the ABG.

A corresponding pH–H⁺ table as a bedside reference.

The Frontline Takeaway

Don’t interpret the numbers before validating the sample.

Good ABG interpretation begins before the ABG reaches the analyser.

Collect correctly → Handle correctly → Process promptly → Check consistency → Then interpret.

Coming next…

ABG series #2 — Oxygenation

Hypoxemia ≠ Hypoxia

We’ll start with:

PaO₂ → SaO₂ → FiO₂ → P/F ratio → A–a gradient

And then ask the more important question:

Why is this patient hypoxemic?

Follow @humans.of.em for the next part of the ABG series.

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