Arterial Blood Gases: Acid–Base Interpretation in 5 Steps

Arterial blood gases (ABGs) appear in almost every medicine exam and on every acute ward. They look complicated, but if you follow the same five steps every time, you can interpret nearly any ABG with confidence. This guide uses the normal values found in standard textbooks; always check your own laboratory's reference ranges.

Normal values to remember

• pH: 7.35–7.45
• PaCO₂: 4.7–6.0 kPa (35–45 mmHg)
• HCO₃⁻: 22–26 mmol/L
• PaO₂ on room air: above about 10.6 kPa (80 mmHg)
• Anion gap: roughly 8–12 mmol/L when calculated as Na⁺ − (Cl⁻ + HCO₃⁻)

Step 1: Look at the patient and the oxygen

Before the acid–base numbers, check the PaO₂ and how much oxygen the patient is breathing. Low PaO₂ means hypoxaemia. If the PaCO₂ is also high, think of type 2 (hypercapnic) respiratory failure; if PaCO₂ is normal or low, think of type 1 respiratory failure. The clinical picture always comes first.

Step 2: Is there acidaemia or alkalaemia?

Look at the pH. Below 7.35 is acidaemia; above 7.45 is alkalaemia. A normal pH does not rule out a disorder — two opposite processes can cancel each other out.

Step 3: Respiratory or metabolic?

Ask which value explains the pH:

• Acidaemia with a high PaCO₂ → respiratory acidosis. With a low HCO₃⁻ → metabolic acidosis.
• Alkalaemia with a low PaCO₂ → respiratory alkalosis. With a high HCO₃⁻ → metabolic alkalosis.

A simple memory aid is "ROME": in Respiratory disorders the pH and PaCO₂ move in Opposite directions; in Metabolic disorders the pH and HCO₃⁻ move in the same direction (Equal).

Step 4: Is there compensation?

The body tries to bring the pH back towards normal. In a metabolic acidosis, breathing increases and PaCO₂ falls. In a chronic respiratory acidosis (for example in COPD), the kidneys retain bicarbonate. Compensation rarely brings the pH fully back to normal. If the second value moves the "wrong" way, or far more than expected, suspect a mixed disorder.

For metabolic acidosis, Winters' formula estimates the expected PaCO₂: expected PaCO₂ (mmHg) = 1.5 × HCO₃⁻ + 8 (± 2). A measured value higher than this suggests an additional respiratory acidosis; lower suggests an additional respiratory alkalosis.

Step 5: In metabolic acidosis, calculate the anion gap

The anion gap helps you find the cause:

• Raised anion gap — extra acid has been added. Common causes include lactic acidosis, diabetic and alcoholic ketoacidosis, kidney failure, and poisoning (for example methanol, ethylene glycol or salicylates).
• Normal anion gap (hyperchloraemic) — bicarbonate has been lost, for example in diarrhoea, renal tubular acidosis, or after large volumes of 0.9% saline.

Albumin is a major unmeasured anion, so a low albumin lowers the anion gap and can hide a raised-gap acidosis.

A worked example

A patient with type 1 diabetes is vomiting and breathing deeply. ABG: pH 7.12, PaCO₂ 2.8 kPa (21 mmHg), HCO₃⁻ 8 mmol/L. Na⁺ 136, Cl⁻ 100.

• pH 7.12 → acidaemia.
• HCO₃⁻ low → metabolic acidosis.
• Expected PaCO₂ = 1.5 × 8 + 8 = 20 (± 2) mmHg; measured 21 mmHg → appropriate respiratory compensation.
• Anion gap = 136 − (100 + 8) = 28 → raised anion gap.

Conclusion: a raised anion gap metabolic acidosis with appropriate respiratory compensation, in keeping with diabetic ketoacidosis in this clinical setting.

Common mistakes

• Ignoring the oxygen and the patient.
• Assuming a normal pH means a normal gas.
• Forgetting to check whether compensation is appropriate.
• Not calculating the anion gap in every metabolic acidosis.

For much more on fluids, electrolytes and acid–base, see Part 5 of our Medicine Companion.

Test yourself

5 exam-style single-best-answer questions. Choose your answer, then tap “Show answer”.

1. ABG: pH 7.30, PaCO2 7.5 kPa, HCO3 26 mmol/L. What is the primary disorder?

   1. Metabolic acidosis
   2. Metabolic alkalosis
   3. Respiratory alkalosis
   4. Normal gas
   5. Respiratory acidosis
   Show answer

   Answer: E. Respiratory acidosis
   Acidaemia with a raised PaCO2 (normal 4.7–6.0 kPa) is a respiratory acidosis. Bicarbonate is not yet raised, suggesting an acute process.

2. Na 140, Cl 104, HCO3 12 mmol/L. What is the anion gap?

   1. 28
   2. 36
   3. 24
   4. 18
   5. 12
   Show answer

   Answer: C. 24
   Anion gap = Na − (Cl + HCO3) = 140 − (104 + 12) = 24, which is raised.

3. Which of these causes a normal anion gap metabolic acidosis?

   1. Methanol poisoning
   2. Severe diarrhoea
   3. Lactic acidosis
   4. Diabetic ketoacidosis
   5. Kidney failure with uraemia
   Show answer

   Answer: B. Severe diarrhoea
   Diarrhoea causes bicarbonate loss, giving a hyperchloraemic, normal anion gap acidosis. The others raise the anion gap.

4. In a metabolic acidosis with HCO3 10 mmol/L, Winters' formula predicts an expected PaCO2 of about:

   1. 23 mmHg (±2)
   2. 13 mmHg
   3. 35 mmHg
   4. 40 mmHg
   5. 50 mmHg
   Show answer

   Answer: A. 23 mmHg (±2)
   Expected PaCO2 = 1.5 × 10 + 8 = 23 ± 2 mmHg. A higher value suggests an additional respiratory acidosis.

5. Why can a low serum albumin mislead anion gap interpretation?

   1. It lowers the bicarbonate
   2. It changes the pH directly
   3. It lowers the anion gap and can hide a raised-gap acidosis
   4. It raises the anion gap falsely
   5. It has no effect
   Show answer

   Answer: C. It lowers the anion gap and can hide a raised-gap acidosis
   Albumin is a major unmeasured anion; hypoalbuminaemia lowers the calculated anion gap.

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Disclaimer

For education and exam preparation only. This is not medical advice and must not be used to diagnose or treat patients. Always follow current clinical guidelines and the advice of qualified senior colleagues.