How to Read an ABG in 3 Steps (With Random Gases You Can Practise On)

Reading an arterial blood gas is three decisions made in a fixed order. Every method you have been shown — ROME, tic-tac-toe, the arrows chart — is a way of remembering those three decisions. Here they are with the normal values, the trap at each step, and a simulator that deals you random gases so you can make the calls until they are automatic.

What are the normal ABG values?

• pH 7.35–7.45 — below is acidosis, above is alkalosis.
• PaCO₂ 35–45 mmHg — the respiratory acid. High means acid, low means base.
• HCO₃⁻ 22–26 mEq/L — the metabolic base. High means base, low means acid.
• PaO₂ 80–100 mmHg — oxygenation. It matters for the patient, but it does not decide the acid–base read.

Your lab’s ranges may differ slightly. The exam uses these.

Step 1 — Is the pH acidotic, normal or alkalotic?

Below 7.35, acidosis. Above 7.45, alkalosis. Between them, normal — and here is the first trap: a normal pH does not mean a normal gas. A fully compensated disorder has a normal pH with two abnormal values. So does a mixed disorder where two problems pull in opposite directions. Read the pH, then keep going.

If the pH is normal but you need to decide which side the primary problem is on, use 7.40 as the line: a pH of 7.36 is normal but on the acidotic side, so the primary problem is an acidosis that has been compensated; a pH of 7.44 leans alkalotic. Compensation brings pH toward normal but never overshoots to the other side.

Step 2 — Which value explains the pH?

Look at PaCO₂ and HCO₃⁻ and ask which one is pointing the same way the pH is telling you.

• CO₂ is an acid, so it moves opposite to pH when it is the cause. Low pH with a high CO₂ = respiratory acidosis. High pH with a low CO₂ = respiratory alkalosis.
• Bicarbonate is a base, so it moves with pH when it is the cause. Low pH with a low HCO₃⁻ = metabolic acidosis. High pH with a high HCO₃⁻ = metabolic alkalosis.

That is ROME — Respiratory Opposite, Metabolic Equal. The tic-tac-toe method is the same decision drawn as a grid: put pH, CO₂ and HCO₃⁻ into acid, normal or base columns and whichever value shares the pH’s column is the primary. Both work. The trap is when both values point the same way as pH — a low pH with a high CO₂ and a low HCO₃⁻. That is not a tie to resolve; it is two acidoses at once, and the patient is very sick.

Step 3 — Did the other system compensate?

Whichever value caused the problem, look at the other one. It is either still normal (uncompensated), moved in the helping direction with pH still abnormal (partially compensated), or moved far enough to bring pH back into range (fully compensated). The full rules, with the time it takes each organ, are in uncompensated vs partially vs fully compensated.

There is a fourth possibility that the basic methods skip: the other value moved the wrong amount. Compensation is predictable — in metabolic acidosis the CO₂ should land near 1.5 × HCO₃⁻ + 8. If it is far outside that band, a second disorder is present. That is the mixed-disorder check, and it is what separates a student who can label a gas from a nurse who can read one.

Now make the calls. Below is a live gas. Press Generate a gas and read it in three calls — the simulator withholds the explanation until each call is right, and it tells you which step you got wrong.

Worked examples

1. pH 7.27, PaCO₂ 56, HCO₃⁻ 25. Acidosis. CO₂ is high (opposite to pH) → respiratory. HCO₃⁻ is still normal → uncompensated respiratory acidosis. A fresh hypoventilation — think sedation, opioids, a postoperative patient breathing seven times a minute. Load this patient.
2. pH 7.36, PaCO₂ 60, HCO₃⁻ 33. Normal pH, but on the acid side of 7.40. CO₂ high and HCO₃⁻ high — they point opposite ways, so one is the cause and one is the answer. The pH leans acid and the acid is CO₂ → chronic respiratory acidosis, fully compensated by the kidneys. A COPD patient at baseline. Load this patient.
3. pH 7.26, PaCO₂ 23, HCO₃⁻ 10. Acidosis. HCO₃⁻ is low (with pH) → metabolic. CO₂ has fallen in the helping direction and pH is still low → partially compensated metabolic acidosis. Winter’s check: 1.5 × 10 + 8 = 23, and the CO₂ is 23 — right on target, so nothing else is going on. DKA with Kussmaul breathing. Load this patient.
4. pH 7.50, PaCO₂ 49, HCO₃⁻ 36. Alkalosis. HCO₃⁻ is high (with pH) → metabolic. CO₂ has risen to help → partially compensated metabolic alkalosis. Days of vomiting; the fix is chloride and potassium, not bicarbonate. Load this patient.

The mistakes students make most

• Calling a normal pH “normal gas.” Check both drivers every time.
• Reading PaO₂ as part of the acid–base decision. It tells you about oxygen, not pH.
• Forgetting that the kidneys are slow. A big bicarbonate change in a respiratory problem means the problem is days old — or that something else is wrong.
• Skipping the size check. Compensation of the wrong size is a second disorder. Here is how to catch it.

If the four disorders themselves are still fuzzy — what causes each one, what it looks like at the bedside, what the nurse does — start with respiratory vs metabolic acidosis and alkalosis and come back to the drill.