NCLEX-RN® Lab Values · Acid–Base

How to Interpret an ABG for the NCLEX-RN®: 4 Steps, ROME and Compensation

To interpret an arterial blood gas (ABG), read 4 values in the same order every time: the pH (below 7.35 is acidosis, above 7.45 is alkalosis), the PaCO2 (35–45 mmHg, the lung partner), the HCO3 (22–26 mEq/L, the kidney partner), then the PaO2. The ROME method names the cause, and the partner value tells you whether the body has started to compensate.

Mariano Alfred Acevedo, PhD, RN Founder and Curriculum Director · Jade NursingPrep™ Updated September 2026

The normal ranges you read against

ABG values: normal band and critical value.
Value	Normal	Critical
pH	7.35–7.45	below 7.25 · above 7.60
PaCO2	35–45 mmHg	below 25 · above 60 mmHg
HCO3	22–26 mEq/L	below 10 · above 40 mEq/L
PaO2	80–100 mmHg	below 60 mmHg
SaO2	95–100%	below 88%

Approximate teaching values. Reference ranges vary by laboratory, assay, age, sex, and clinical context — always read your facility's printed range. On the exam, NCSBN® states that items with a numeric lab value include the normal reference range. Read the full answer →

The 4 steps to read an ABG, in order

pH. Below 7.35 is acidosis. Above 7.45 is alkalosis.
PaCO2 — the lung partner. Out of range in the opposite direction to the pH means the cause is respiratory.
HCO3 — the kidney partner. Out of range in the same direction as the pH means the cause is metabolic.
PaO2 — the oxygen. Low oxygen is confirmed on the gas, not the probe.

The ROME method

ROME — a standard nursing memory aid (not Jade's)

Respiratory Opposite: the PaCO2 moves opposite to the pH.

Metabolic Equal: the HCO3 moves the same way as the pH.

Low pH with a high PaCO2 is respiratory acidosis. Low pH with a low HCO3 is metabolic acidosis. The same rule reads the two alkaloses.

The 4 acid–base disorders at a glance

Each disorder with one uncompensated example gas (Jade teaching gases, not client data).
Disorder	The rule	pH	PaCO2	HCO3
Respiratory acidosis	pH below 7.35 with PaCO2 above 45	7.26	55	24
Respiratory alkalosis	pH above 7.45 with PaCO2 below 35	7.53	30	24
Metabolic acidosis	pH below 7.35 with HCO3 below 22	7.28	40	18
Metabolic alkalosis	pH above 7.45 with HCO3 above 26	7.53	40	32

Compensated, partially compensated, or uncompensated?

Uncompensated: the pH is out of range and the partner value is still normal. Compensation has not started.
Partially compensated: the pH is still out of range, and the partner value has moved out of range to push it back.
Fully compensated: the pH is back in range and both partners are out of range. Name the cause from the history and the trend, such as COPD.

Timing is part of the answer: the lungs compensate for metabolic problems in minutes to hours; the kidneys take days to compensate for a breathing problem.

Worked example — respiratory acidosis (Jade teaching gases, not client data).
State	pH	PaCO2	HCO3	How to read it
Uncompensated	7.26	55	24	Low pH, high PaCO2, HCO3 still normal: compensation has not started.
Partially compensated	7.33	55	28	pH still low, HCO3 now above 26: the kidneys have started.
Fully compensated	7.38	60	34	pH back in range, both partners high. Often a COPD baseline; compare prior gases.

3 traps the exam uses

Do not accept a normal oxygen saturation as an answer to an acid-base question.

Do not call a normal pH resolved without checking the PaCO2 and the HCO3.

Do not expect the HCO3 to rise within hours. Renal compensation takes days.

Read these next: Potassium — ranges, ECG changes and the first action · All NCLEX-RN® lab value guides

Frequently asked questions

What are the steps to interpret an ABG?

pH, then PaCO2, then HCO3, then PaO2 — in that order every time.

What is the ROME method?

Respiratory Opposite, Metabolic Equal: in a respiratory problem the PaCO2 moves opposite to the pH; in a metabolic problem the HCO3 moves the same way.

What are normal ABG values?

pH 7.35–7.45, PaCO2 35–45 mmHg, HCO3 22–26 mEq/L, PaO2 80–100 mmHg, and SaO2 95–100%. These are approximate teaching values; always read your facility's printed range.

What does a high PaCO2 mean on an ABG?

A PaCO2 above 45 with a pH below 7.35 means respiratory acidosis: the lungs are keeping the acid. Oxygen alone does not remove CO2 — the named treatment is improving ventilation.

How do you know if an ABG is compensated?

If the pH is back in range while both the PaCO2 and the HCO3 are out of range, it is fully compensated. If the pH is still out of range but the partner value has moved, it is partially compensated.