ABG Interpretation Using ROME: The 5-Step NCLEX Method

ABG interpretation is one of the most-feared topics in nursing school and one of the most-tested on the NCLEX. The good news is that you do not need to be a respiratory therapist to nail these questions. You need ROME, a five-step system, and 20 minutes of focused practice.

By the time you finish this guide, you will be able to interpret any arterial blood gas in under 30 seconds.

The ROME method explained

ROME stands for Respiratory Opposite, Metabolic Equal. It tells you whether a pH problem is respiratory or metabolic in origin by comparing the direction of the pH to the direction of CO2 or HCO3.

The normal ABG values to memorize

• pH: 7.35 to 7.45
• PaCO2: 35 to 45 mmHg
• HCO3: 22 to 26 mEq/L
• PaO2: 80 to 100 mmHg
• O2 saturation: 95% to 100%

The 5-step ROME process

Step 1: Look at the pH

Below 7.35 is acidosis. Above 7.45 is alkalosis. Normal is 7.35 to 7.45. If pH is exactly normal you may have compensation — we will get to that.

Step 2: Look at the CO2

If CO2 is high, it is respiratory acidosis (CO2 is acid). If CO2 is low, it is respiratory alkalosis.

Step 3: Look at the HCO3

If HCO3 is low, it is metabolic acidosis. If HCO3 is high, it is metabolic alkalosis.

Step 4: Apply ROME

If pH and CO2 are opposite (one up, one down) it is respiratory. If pH and HCO3 are equal (both up or both down) it is metabolic.

Step 5: Check for compensation

If the unaffected value (the one that does not match the imbalance) is shifting in the opposite direction to compensate, the body is partially or fully compensating.

Practice problems with answers

Problem 1

pH 7.30, CO2 50, HCO3 24. What is the imbalance?

Answer: pH is low (acidosis). CO2 is high. pH and CO2 are opposite (rule R = O). HCO3 is normal. This is respiratory acidosis, uncompensated.

Problem 2

pH 7.50, CO2 28, HCO3 24.

Answer: pH is high (alkalosis). CO2 is low. pH and CO2 opposite. Respiratory alkalosis, uncompensated.

Problem 3

pH 7.28, CO2 38, HCO3 16.

Answer: pH low (acidosis). HCO3 low. Both down (rule M = E). Metabolic acidosis, uncompensated.

Problem 4

pH 7.32, CO2 50, HCO3 30.

Answer: pH low. CO2 high (opposite = respiratory acidosis). HCO3 high (shifting up to compensate). Respiratory acidosis, partially compensated.

Causes you must memorize

Respiratory acidosis

COPD, sleep apnea, drug overdose, opioids, neuromuscular disease. Hypoventilation = CO2 retention.

Respiratory alkalosis

Anxiety, pain, fever, hyperventilation, high altitude. Blowing off CO2.

Metabolic acidosis

DKA, lactic acidosis, kidney failure, diarrhea, shock. Acid buildup or bicarb loss.

Metabolic alkalosis

Vomiting, NG suction, antacid overuse, diuretics. Acid loss or bicarb gain.

Kussmaul respirations — the NCLEX trap

Deep, rapid breathing in a patient with DKA is the body's attempt to blow off CO2 and compensate for metabolic acidosis. The NCLEX will ask you what is causing the breathing pattern. Answer: compensation for metabolic acidosis.

Frequently Asked Questions

What does ROME stand for in ABG?

ROME stands for Respiratory Opposite, Metabolic Equal. It helps you decide whether a pH abnormality is respiratory or metabolic by comparing pH direction to CO2 and HCO3 direction.

What is a normal pH range?

Normal arterial pH is 7.35 to 7.45.

How do you know if an ABG is compensated?

Look at the value not driving the imbalance. If it has shifted in the direction that pulls pH back toward normal, the body is compensating. Fully compensated means the pH is back within the 7.35 to 7.45 range.

Is Kussmaul breathing acidosis or alkalosis?

Kussmaul respirations are deep, rapid breathing that compensates for metabolic acidosis, most often seen in DKA.