Updated July 2026 · 10 min read

ABG Interpretation: A Fast, Reliable Method for ICU Nurses

Part of the ICU Emergencies Hub — browse every related guide in one place.

An arterial blood gas looks intimidating until you have a fixed order to read it in. Work the same five steps every single time and the numbers stop being a puzzle and start telling you a story: is the patient acidemic or alkalemic, is the lungs or the kidneys driving it, and is the body compensating or failing to.

The normal values worth memorizing

Hold onto one core idea: CO2 is an acid, HCO3 is a base. When CO2 rises, pH falls. When HCO3 rises, pH climbs. Everything downstream is just applying that relationship.

Step 1 — Is the pH acidemic or alkalemic?

Below 7.35 is acidemia; above 7.45 is alkalemia. If the pH sits inside 7.35–7.45 but the CO2 and HCO3 are both abnormal, you're looking at a fully compensated picture — use which side of 7.40 the pH falls on to identify the primary process.

Step 2 — Which value matches the pH? (find the primary problem)

Now compare the direction of CO2 and HCO3 against the pH:

A memory hook many nurses use is ROME: Respiratory Opposite, Metabolic Equal. In a respiratory problem the pH and CO2 move in opposite directions (CO2 up, pH down). In a metabolic problem the pH and HCO3 move in the same direction (HCO3 down, pH down).

Step 3 — Is there compensation?

The body doesn't sit still. If the lungs cause the problem, the kidneys try to fix it (and vice versa). Look at the value that didn't cause the primary disorder:

Timing matters. The lungs compensate for a metabolic problem within minutes to hours (breathe faster to blow off CO2). The kidneys compensate for a respiratory problem over hours to days (retain or dump bicarbonate). So a patient with an acute respiratory acidosis won't yet have a high HCO3 — that renal help takes a couple of days to arrive.

Step 4 — Calculate the anion gap (if there's a metabolic acidosis)

When the primary problem is a metabolic acidosis, the anion gap tells you why. Anion gap = Na − (Cl + HCO3); normal is roughly 8–12 mEq/L.

In the ICU, a rising lactate with a widening gap is one of the earliest hard signals of worsening perfusion — often before the blood pressure gives it away.

Step 5 — Read the oxygenation separately

Acid–base and oxygenation are two different questions on the same slip of paper. After you've classified the acid–base picture, look at PaO2 and SaO2 on their own. A PaO2 under 60 mmHg (roughly SaO2 <90%) is significant hypoxemia. Interpret it against the FiO2: a "normal" PaO2 of 95 on 100% oxygen is actually a failing lung. The P/F ratio (PaO2 divided by FiO2) is how the team grades ARDS severity.

The patterns you'll actually see

Bottom line

Never freelance an ABG. Run the same five steps — pH, primary problem, compensation, anion gap, then oxygenation — and the gas will hand you the diagnosis every time. The number that matters most isn't any single value; it's the trend. A single gas is a snapshot; the repeat gas after you've intervened tells you whether you're winning.

This article is general educational information for licensed clinicians and students, not medical advice or a substitute for your institution's protocols or a provider's orders. Always follow facility policy.