Few things make a junior doctor freeze quite like being handed a blood gas and a expectant look. The numbers seem to come at you all at once, and it is tempting to reach for the machine's own interpretation and hope. Resist that. An ABG is not hard once you have a fixed order to read it in, and a structured answer you build yourself is both safer and far more convincing on a ward round. Here is a four-step method that works every time, including for the mixed pictures that catch people out.
Before the four steps: look at the patient
A gas is a snapshot, and it means nothing without context. Note whether the sample is arterial or venous, and crucially how much oxygen the patient is on — a "normal" PaO2 on 60% oxygen is not reassuring at all. Then start the sequence.
Step 1 — Are they hypoxic?
Look at the PaO2 first and always, before you get lost in the acid-base weeds, because oxygenation is what kills people quickly. Interpret it against the inspired oxygen. On room air a PaO2 below roughly 10 kPa suggests impaired gas exchange; on supplemental oxygen you would expect the PaO2 to sit several kPa below whatever percentage they are breathing. A patient on high-flow oxygen with a PaO2 that is merely normal is failing to oxygenate, and that matters more than anything the pH is doing.
Step 2 — What is the pH doing?
Now anchor everything to the pH. Normal is 7.35 to 7.45.
- Below 7.35 — the patient is acidaemic.
- Above 7.45 — the patient is alkalaemic.
- Within range — either genuinely normal, or a fully compensated picture hiding two opposing problems.
The pH tells you the direction of the primary disturbance, and everything in the next two steps is about explaining why the pH is where it is.
Step 3 — Is it respiratory or metabolic?
Now look at the CO2 and the bicarbonate together, and ask which of them fits the pH.
- CO2 is the respiratory component. It is an acid, so a high CO2 pushes the pH down, a low CO2 pushes it up.
- Bicarbonate (and base excess) is the metabolic component. It is a base, so a low bicarbonate pushes the pH down, a high bicarbonate pushes it up.
Match the derangement to the pH. If the patient is acidaemic and the CO2 is high, that is a respiratory acidosis — think of the drowsy COPD patient who is not shifting enough air. If the patient is acidaemic and the bicarbonate is low, that is a metabolic acidosis — think DKA, sepsis, renal failure. The same logic, mirrored, gives you the two alkaloses.
The whole trick is this one question: does the CO2 or the bicarbonate explain the pH? Whichever one fits is your primary problem. The other is either normal or compensating.
Step 4 — Is there compensation, and does it all add up?
The body tries to drag the pH back towards normal. In a metabolic acidosis the lungs blow off CO2 to compensate; in a respiratory acidosis the kidneys retain bicarbonate over hours to days. Compensation moves the pH towards normal but, in a pure single disturbance, essentially never fully corrects it or overshoots. So:
- If the compensating value is moving the right way but the pH is still abnormal, you have partial compensation.
- If the pH has crept back into range with both CO2 and bicarbonate deranged, that is full compensation.
- If both values are pushing the pH the same way, you are not looking at compensation at all — you have a mixed picture, two problems at once, and those patients are usually the sickest.
Do not forget the anion gap
Whenever you meet a metabolic acidosis, calculate the anion gap — it splits the differential in half and is expected of you. A raised anion gap points to added acid: lactate from sepsis or ischaemia, ketones in DKA, toxins, or uraemia. A normal anion gap points to bicarbonate loss, classically from diarrhoea or renal tubular acidosis. The mnemonics are less important than the habit of always asking the question.
Putting it together at the bedside
A worked example makes the sequence concrete. A young woman is breathless and drowsy; her gas shows pH 7.18, PaO2 14 on room air, CO2 2.9, bicarbonate 9, with a raised anion gap. Step one: oxygenation is fine. Step two: she is acidaemic. Step three: the bicarbonate is low and fits the pH, so this is a metabolic acidosis, not a respiratory one. Step four: her CO2 is low, so her lungs are compensating by blowing it off — that fast breathing is appropriate, not a separate problem — and the raised anion gap in a thirsty, drowsy young patient points straight at DKA. Four steps, a coherent story, and a clear next move.
That is the real payoff. The method does not just label the gas; it hands you the differential and the urgency along with it. Run the same four steps on every gas you see — the dull ones on the ward as much as the frightening ones in resus — and within a few weeks the wall of numbers resolves, almost without effort, into a two-minute structured answer you can say out loud with confidence.