Arterial blood gas
Feel the radial pulse, go in at 45 degrees towards it, let the syringe fill by itself, press for 5 minutes, and run the sample within 15 minutes.
Your senior asks for an ABG on a breathless 70-year-old with COPD on 28% oxygen. Result: pH 7.29, PaCO₂ 70 mmHg (9.3 kPa), PaO₂ 58 mmHg (7.7 kPa), bicarbonate 33 mmol/L, base excess +6, lactate 1.2.
An hour later, a 22-year-old with type 1 diabetes: pH 7.12, PaCO₂ 22 mmHg (2.9 kPa), bicarbonate 7 mmol/L, sodium 136, chloride 100.
How do you take the sample safely, and what does each gas show?
- Indication and consentNote the oxygen the patient is on (FiO₂).
- Check the collateral circulationModified Allen test if in doubt, avoid arms with fistulas.
- PositionWrist extended over a rolled towel, feel the radial pulse 1–2 cm above the wrist crease.
- Local anaestheticA small bleb of lidocaine makes it much kinder.
- Needle at 30–45°, bevel up, toward the pulseArterial blood fills a pre-heparinised syringe by itself.
- Press for 5 minutes10 minutes or more if anticoagulated.
- Expel air, cap, analyse within 10–15 minutesWrite the FiO₂ and temperature on the request.
- pH below 7.2
- Acidotic with high CO₂
- Lactate above 4
- PaO₂ below 60 mmHg (8 kPa) on oxygen
- Hand pale, cold or painful after puncture
- Venous sample mistaken for arterial
- Air bubbles (falsely high PaO₂)
- Delay before analysis
- Not recording the oxygen given
- Mixed acid–base disorders
| Normal pH | 7.35–7.45 |
| Normal PaCO₂ | 35–45 mmHg (4.7–6.0 kPa) |
| Normal PaO₂ on air | 80–100 mmHg (10.7–13.3 kPa) |
| Normal bicarbonate | 22–26 mmol/L |
| Base excess | −2 to +2 |
| Anion gap | Na − (Cl + HCO₃), normal about 8–12 mmol/L |
| Lactate | Under 2 mmol/L (18 mg/dL) |
| Expected PaO₂ | Roughly FiO₂ (%) minus 10 in kPa, or FiO₂ (%) × 5 in mmHg, in healthy lungs |
Check every dose against a current drug reference and your hospital protocol.
On this page
Indications
- Respiratory failure: type 1 or 2, response to oxygen or NIV.
- Acid–base: DKA, sepsis, kidney failure, poisoning, vomiting.
- Shock and lactate (a venous gas is often enough for pH, lactate, potassium, glucose).
- Carbon monoxide and methaemoglobin (co-oximetry).
A venous gas gives pH, bicarbonate, lactate, potassium and glucose accurately enough for most decisions. Use an arterial gas when you need the oxygen and CO₂ (respiratory failure, NIV decisions).
Taking the sample
- PreparePre-heparinised ABG syringe, alcohol wipe, gauze, lidocaine, sharps bin, gloves.
- Check the handModified Allen test if in doubt: occlude both arteries, release the ulnar; colour should return within 5–10 seconds.
- PositionWrist extended over a rolled towel, palm up.
- Local anaesthetic0.5 mL lidocaine 1% subcutaneously, wait 1 minute.
- PunctureFeel the pulse with two fingers, insert the needle between them at 30–45°, bevel up. Advance slowly until blood flashes and fills the syringe.
- AfterWithdraw, press firmly for 5 minutes (longer if anticoagulated). Expel air, cap, label, analyse within 10–15 minutes (or on ice if delayed).
Complications: pain, haematoma, arterial spasm, rarely thrombosis or ischaemia of the hand, infection, nerve injury. If the radial fails, the femoral artery can be used by experienced staff (avoid in anticoagulated patients).
Reading the gas in five steps
- 1. OxygenationPaO₂ compared with the oxygen given. Below 60 mmHg (8 kPa) is respiratory failure.
- 2. pHAcidosis (below 7.35) or alkalosis (above 7.45).
- 3. CO₂High CO₂ with acidosis: respiratory acidosis. Low CO₂ with alkalosis: respiratory alkalosis.
- 4. Bicarbonate and base excessLow bicarbonate with acidosis: metabolic acidosis. High with alkalosis: metabolic alkalosis.
- 5. Compensation and anion gapIs the other system compensating as expected? In metabolic acidosis, calculate the anion gap.
Common patterns
| pH | CO₂ | HCO₃ | Pattern | Examples |
|---|---|---|---|---|
| Low | High | Normal | Acute respiratory acidosis | Opioids, tiring asthmatic, neuromuscular weakness |
| Low | High | High | Acute-on-chronic respiratory acidosis | COPD exacerbation |
| Normal | High | High | Compensated chronic respiratory acidosis | Stable COPD |
| Low | Low | Low | Metabolic acidosis with respiratory compensation | DKA, sepsis, kidney failure, poisoning |
| High | Low | Normal or low | Respiratory alkalosis | Anxiety, PE, early sepsis, pain, pregnancy |
| High | High | High | Metabolic alkalosis | Vomiting, NG losses, diuretics |
Metabolic acidosis and the anion gap
Anion gap = Na − (Cl + HCO₃). Normal about 8–12 mmol/L.
- Lactic acidosis (sepsis, shock, ischaemia, metformin)
- Ketoacidosis (DKA, starvation, alcohol)
- Kidney failure
- Toxins (methanol, ethylene glycol, salicylates)
- Diarrhoea
- Large volumes of 0.9% saline
- Renal tubular acidosis
- Ureteric diversion
- Acetazolamide
Is it compensating as expected?
- Metabolic acidosis: expected PaCO₂ (mmHg) ≈ 1.5 × HCO₃ + 8 (±2). Higher than expected: a respiratory acidosis is also present (tiring patient). Lower: added respiratory alkalosis.
- Chronic CO₂ retention: bicarbonate rises over days; a high bicarbonate with high CO₂ and normal pH means chronic, compensated.
- Compensation never overcorrects the pH.
Common mistakes
- Not writing the FiO₂ on the request (a PaO₂ means nothing without it).
- Leaving air bubbles in the syringe.
- Letting the sample sit for an hour.
- Not pressing long enough in an anticoagulated patient.
- Missing a second disorder (a "normal" CO₂ in a DKA patient who should be blowing it off).
- Using an arterial stab when a venous gas would answer the question.
Frequently asked questions
How do you interpret an ABG?
Look at oxygenation, then pH, then CO₂, then bicarbonate, then check compensation and the anion gap.
What is the normal pH of arterial blood?
7.35–7.45.
What causes a high anion gap metabolic acidosis?
Lactic acidosis, ketoacidosis (DKA, alcohol, starvation), kidney failure and toxins such as methanol, ethylene glycol and salicylates.
When is a venous gas enough?
For pH, bicarbonate, lactate, potassium and glucose. Use an arterial gas when oxygen and CO₂ levels matter.
Test yourself
Exam-style questions on this guide, from our two question banks. Choose an answer to see why each option is right or wrong.
1. An ABG shows pH 7.29, PaCO₂ 70 mmHg and bicarbonate 33 mmol/L. What is the pattern?
Answer: Chronic CO₂ retention, acute rise. High CO₂ with acidosis and raised bicarbonate: chronic retention with an acute rise.
- Metabolic acidosis: Bicarbonate is high.
- Normal: No.
- Respiratory alkalosis: pH is low.
- Metabolic alkalosis: pH is low.
2. Which artery is used for most ABG samples?
Answer: Radial artery. Radial artery at the wrist, with the ulnar supply as backup.
- Ulnar artery: Not routinely.
- Brachial artery first: Used if radial fails.
- Femoral artery first: Emergencies only.
- Dorsalis pedis: Not routinely.
3. A DKA patient has pH 7.15, PaCO₂ 22 mmHg and bicarbonate 8. What does the low CO₂ show?
Answer: Respiratory compensation. Kussmaul breathing blows off CO₂ to compensate for metabolic acidosis.
- Lab error: No.
- Respiratory alkalosis as the main problem: pH is low.
- Respiratory failure: No.
- Metabolic alkalosis: No.
4. A man has pH 7.18, HCO₃ 10 mmol/L, Na 140, Cl 100. Which cause fits this acid–base picture?
Answer: Methanol poisoning. Anion gap = Na − (Cl + HCO₃) = 140 − 110 = 30 (raised; normal ~8–12). High anion gap metabolic acidosis (MUDPILES/GOLDMARK): methanol, uraemia, DKA, paraldehyde/propylene glycol, iron/isoniazid, lactic acidosis, ethylene glycol, salicylates. Normal anion gap (hyperchloraemic): diarrhoea, RTA, Addison’s, acetazolamide, ureteric diversion, saline excess.
- Prolonged diarrhoea: Diarrhoea causes a normal anion gap acidosis.
- Renal tubular acidosis: RTA causes a normal anion gap acidosis.
- Ureterosigmoidostomy: Ureterosigmoidostomy causes a normal anion gap acidosis.
5. Na⁺ 140, Cl⁻ 100, HCO₃⁻ 10 mmol/L, with pH 7.2. What is the anion gap and what does it suggest?
Answer: 30; high anion gap acidosis. Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻) = 140 − 110 = 30 (normal about 8–12). A high gap means unmeasured acids: MUDPILES (methanol, uraemia, diabetic ketoacidosis, propylene glycol/paraldehyde, iron/isoniazid, lactic acidosis, ethylene glycol, salicylates). Normal gap acidosis comes from diarrhoea or renal tubular acidosis.
- 10; normal anion gap acidosis: Calculation is wrong.
- 50; respiratory acidosis: Calculation is wrong; this is metabolic.
- 30; metabolic alkalosis: pH is low, not alkalosis.
- 20; normal: Calculation is wrong.
6. A man with acute severe asthma becomes drowsy. Arterial blood gas: pH 7.23, PaCO₂ 62 mmHg (8.3 kPa), HCO₃⁻ 25 mmol/L. What is the acid–base disturbance?
Answer: Acute respiratory acidosis. Low pH with a high PaCO₂ means a respiratory acidosis. The bicarbonate is still normal, so the kidneys have not yet compensated; this is acute. In asthma, a rising CO₂ is a sign of exhaustion and life-threatening disease.
- Metabolic acidosis: Would need a low bicarbonate.
- Chronic compensated respiratory acidosis: Chronic compensation would raise bicarbonate (over days).
- Respiratory alkalosis: Would show a high pH and low CO₂.
- Mixed metabolic and respiratory alkalosis: pH is low, so this cannot be an alkalosis.