ABG Interpretation Made Simple: The Tic-Tac-Toe Method vs. ROME

ABG Interpretation Made Simple: The Tic-Tac-Toe Method vs. ROME (Complete NCLEX Guide)

Arterial Blood Gases (ABGs) are the Sudoku puzzles of the nursing world.

When you first see a slip of paper with:

• pH 7.32
• PaCO2 50
• HCO3 28

it looks like random data.

But ABGs are not random. They follow strict, unbreakable rules of chemistry and physiology.

If you are a nursing student — or a new grad stepping into the ICU — mastering ABGs is not optional. It is the difference between recognizing early respiratory failure and calling a Code Blue an hour later.

Under the current structure outlined in the 2026 NCLEX-RN and NCLEX-PN test plan breakdown, acid–base interpretation is no longer tested as isolated memorization. It is embedded into clinical judgment, prioritization, and evolving case studies.

Most textbooks teach one method and hope it sticks.

In this guide, you will master two:

• The ROME mnemonic — fast, bedside-friendly.
• The Tic-Tac-Toe method — the ultimate visual system for partially compensated and NGN-style questions.

By the end of this guide, you won’t just memorize the numbers. You’ll understand the story your patient’s lungs and kidneys are telling.

Quick Self-Test (Do Not Scroll)

pH: 7.30
PaCO2: 52
HCO3: 24

1. Acid or base?
2. Respiratory or metabolic?
3. Compensated or uncompensated?

(Answer revealed in Practice Scenarios.)

Table of Contents

• Part 1: The Three Players (Normal Ranges)
• Part 2: Method 1 — The ROME Mnemonic
• Part 3: Method 2 — The Tic-Tac-Toe Method
• Part 4: The Four Acid–Base Imbalances (Patho + Interventions)
• Part 5: The Boss Level — Compensation
• Part 6: Interactive Practice Scenarios
• AI ABG Simulator
• Further Reading

Part 1: The Three Players (Normal Ranges)

Before you can interpret pathology, you must memorize physiology.

If lab values are not automatic yet, review the complete Normal Lab Values for Nurses – Ultimate Cheat Sheet (2025) first. Acid–base questions often integrate potassium shifts, renal markers, and lactate.

There are three key players in the ABG game.

1. The Boss: pH (Potential Hydrogen)

Normal Range: 7.35 – 7.45
Perfect pH: 7.40

Rule:

• < 7.35 = Acidosis

• 7.45 = Alkalosis

Think:

• “Sliding down” into acid
• “Kicking up” into base

The pH tells you the overall direction of the problem.

Always start here.

2. The Lungs: PaCO2 (Respiratory Acid)

Normal Range: 35 – 45 mmHg

PaCO2 represents carbon dioxide — an acid.

Rule:

• 45 = Respiratory Acidosis (retaining acid / hypoventilating)

• < 35 = Respiratory Alkalosis (blowing off acid / hyperventilating)

Visual Hack: The numbers 35–45 match the pH decimals 7.35–7.45.

If CO2 is high, ask: What is preventing ventilation?

If CO2 is low, ask: What is driving hyperventilation?

3. The Kidneys: HCO3 (Bicarbonate)

Normal Range: 22 – 26 mEq/L

Bicarbonate is base.

Rule:

• < 22 = Metabolic Acidosis

• 26 = Metabolic Alkalosis

Visual Hack: You buy bicarbonate (baking soda) to bake a cake for a 22 to 26 year old.

Part 2: Method 1 — The ROME Mnemonic

ROME stands for:

Respiratory
Opposite
Metabolic
Equal

How to Use ROME:

Step 1: Look at the pH. Is it high or low?

Step 2: Compare pH to PaCO2.

If they move in opposite directions → Respiratory.

Example:

• pH ↓ (acid)
• CO2 ↑ (acid)

Opposite arrows → Respiratory Acidosis.

Step 3: Compare pH to HCO3.

If they move in the same direction → Metabolic.

Example:

• pH ↓
• HCO3 ↓

Equal arrows → Metabolic Acidosis.

The Limitation of ROME

ROME works great for simple, uncompensated problems.

However, when the body begins to compensate, all three values can become abnormal — and that’s where students get confused.

NGN case stems frequently test compensation inside layered scenarios. If you struggle with multi-step stems, review How to Read NGN Case Stems (Clinical Judgment Strategy Guide) to strengthen interpretation skills.

This is where the Tic-Tac-Toe method becomes superior.

Part 3: Method 2 — The Tic-Tac-Toe Method

This method is ideal for:

• Partially compensated disorders
• Fully compensated disorders
• Chronic COPD patterns
• NGN-style prioritization items

Step 1: Draw the Grid

Step 2: Place Each Value

Example Patient:

pH 7.25
PaCO2 60
HCO3 24

pH 7.25 → ACID
PaCO2 60 → ACID
HCO3 24 → NORMAL

Step 3: Find the Vertical Match

The column where pH aligns with another value gives you the diagnosis.

Here: pH + CO2 under ACID → Respiratory Acidosis.

This method eliminates arrow confusion and works beautifully for compensation.

For extended case-based acid–base practice, work through ABG Interpretation Practice Questions (2025: 15 Cases with Answers & Rationales).

Part 4: The Four Acid–Base Imbalances (Patho + Interventions)

Interpretation is useless if you don’t know how to treat it.

1. Respiratory Acidosis — “The Hypoventilator”

Problem: The lungs are holding onto too much CO2.

Common Causes:

• Opioid overdose
• Benzodiazepines
• Anesthesia
• COPD
• Severe asthma
• Pneumonia
• Sleep apnea

Nursing Interventions:

• High Fowler’s positioning
• Encourage deep breathing
• Suction airway if indicated
• Administer reversal agents (e.g., naloxone)
• Escalate to BiPAP or intubation if worsening

Respiratory acidosis frequently appears in med-surg prioritization. See full clinical integration in NGN Med-Surg Physiological Adaptation Case Studies (2025).

2. Respiratory Alkalosis — “The Hyperventilator”

Problem: The lungs are blowing off too much CO2.

Common Causes:

• Panic attacks
• Pain
• Fever
• Early aspirin toxicity

Nursing Interventions:

• Calm the patient
• Address pain or fever
• Evaluate ventilator settings if applicable

3. Metabolic Acidosis — “The Acid Builder”

Problem: Loss of bicarbonate or excessive acid production.

Common Causes:

• Diabetic ketoacidosis
• Renal failure
• Shock (lactic acidosis)
• Severe diarrhea

Nursing Interventions:

• IV fluids
• Insulin for DKA
• Treat shock aggressively
• Dialysis for severe renal failure

DKA + acid–base frequently integrates pharmacology and prioritization. Practice with NGN Pharmacology Case Studies with Full Rationales (2026).

4. Metabolic Alkalosis — “The Vomiter”

Problem: Loss of stomach acid or excess base intake.

Common Causes:

• Vomiting
• NG suction
• Excess antacids

Nursing Interventions:

• Stop the loss (antiemetics)
• Replace potassium and chloride
• Monitor for dysrhythmias

Part 5: The Boss Level — Compensation

The body is smart.

If the lungs fail, the kidneys attempt to compensate. If the kidneys fail, the lungs attempt to compensate.

There are three stages.

1. Uncompensated

• pH abnormal
• One system abnormal
• Other system normal

2. Partially Compensated

• All three values abnormal
• pH still abnormal

3. Fully Compensated

• pH normal
• Both PaCO2 and HCO3 abnormal

Important Trick: Even if pH is normal, determine which side of 7.40 it leans toward.

This concept is reinforced in comprehensive prep strategies like the Ultimate NCLEX-RN Study Guide (2026).

Part 6: Interactive Practice Scenarios

Scenario 1

pH 7.28
PaCO2 55
HCO3 24

Answer: Uncompensated Respiratory Acidosis.

Scenario 2

pH 7.50
PaCO2 40
HCO3 30

Answer: Uncompensated Metabolic Alkalosis.

Scenario 3 (Hard One)

pH 7.32
PaCO2 60
HCO3 30

All three abnormal. pH matches CO2.

Answer: Partially Compensated Respiratory Acidosis.

Scenario 4 (The Trick)

pH 7.36
PaCO2 60
HCO3 35

pH is technically normal but on the acid side.

Answer: Fully Compensated Respiratory Acidosis.

Opening Quiz Answer

pH 7.30
PaCO2 52
HCO3 24

Uncompensated Respiratory Acidosis.

AI ABG Simulator

Want unlimited practice?

Use structured prompts from the AI Prompt Library for Nursing Students (2025) to generate endless acid–base scenarios and grade yourself.

Further Reading

• ABG Interpretation Practice Questions (15 Case-Based Scenarios)
• Normal Lab Values for Nurses (Complete 2025 Guide)
• NGN Med-Surg Physiological Adaptation Case Studies
• NGN Pharmacology Case Studies with Full Rationales
• Ultimate NCLEX-RN Study Guide (2025)