Arterial blood gases (ABGs) appear in almost every medical exam – MRCP, PLAB, the SCEs and Middle East licensing exams – and they are a daily part of acute medicine. A simple step-by-step method makes them quick and reliable to interpret.

Typical reference ranges

  • pH: 7.35–7.45
  • PaCO2: 4.7–6.0 kPa
  • PaO2: above 10 kPa on room air
  • Bicarbonate (HCO3−): 22–26 mmol/L
  • Base excess: −2 to +2 mmol/L

Ranges vary slightly between laboratories – always use the ones provided in the question.

A five-step method

1. Look at the patient and the oxygen

Is the patient hypoxic? Compare the PaO2 with the inspired oxygen – a "normal" PaO2 on high-flow oxygen can still mean a significant gas-exchange problem.

2. Is there an acidaemia or alkalaemia?

pH below 7.35 is acidaemia; above 7.45 is alkalaemia.

3. Is the primary problem respiratory or metabolic?

  • Acidaemia with a high PaCO2 – respiratory acidosis.
  • Acidaemia with a low bicarbonate – metabolic acidosis.
  • Alkalaemia with a low PaCO2 – respiratory alkalosis.
  • Alkalaemia with a high bicarbonate – metabolic alkalosis.

4. Is there compensation – or a second disorder?

The body compensates in the opposite system, but rarely fully. In metabolic acidosis, Winter's formula estimates the expected PaCO2 (in mmHg) as 1.5 × HCO3− + 8 (± 2). A PaCO2 higher than expected suggests a coexisting respiratory acidosis; lower than expected suggests a coexisting respiratory alkalosis.

5. In metabolic acidosis, calculate the anion gap

Anion gap = Na+ − (Cl− + HCO3−). A normal value is roughly 8–12 mmol/L, depending on the laboratory.

  • Raised anion gap: lactic acidosis, ketoacidosis, kidney failure and toxins such as methanol, ethylene glycol and salicylates.
  • Normal anion gap: diarrhoea, renal tubular acidosis and large volumes of 0.9% saline.

Worked example

A patient with diabetic ketoacidosis has pH 7.18, PaCO2 3.1 kPa, HCO3− 10 mmol/L, Na+ 136, Cl− 100. The pH shows acidaemia; the low bicarbonate shows a metabolic acidosis; the anion gap is 136 − (100 + 10) = 26, which is raised; and the low PaCO2 reflects respiratory compensation.

Master ABGs with License Medical

Our Master ABG course takes you through arterial blood gas interpretation in more depth, with practice cases for exams and clinical work.