Arterial blood gases look harder than they are. Four numbers, four possible disorders, and one sequence that sorts them every time.

The reason candidates struggle is not the chemistry. It is that most of them try to reason from first principles under time pressure instead of running a fixed sequence.

This guide gives you that sequence, explains compensation properly, covers the causes behind each disorder, and works through five examples. Written for internationally educated nurses (IENs) and repeat test-takers.

Quick answer: ABG interpretation on the NCLEX takes four steps. Check the pH to decide acidosis or alkalosis. Check the PaCO2, and if it moves opposite to the pH the cause is respiratory. Check the HCO3, and if it moves with the pH the cause is metabolic. Then check compensation by seeing whether the other value has shifted to correct the imbalance. Oxygenation is assessed separately from acid-base.

ABG Interpretation: The One Thing Worth Memorizing

The exam supplies reference ranges. NCSBN states that items containing a numeric laboratory value include the corresponding normal reference range, so you will be told what normal looks like.

What it will not supply is the interpretation sequence. That is why ABG interpretation is one of the few genuine memorization tasks left in NCLEX preparation, and it is a sequence rather than a list.

For familiarity, these are the commonly cited adult values you will see alongside the results.

One framing that saves confusion. PaCO2 is an acid. When it rises, pH falls. HCO3 is a base. When it rises, pH rises. That single fact drives the whole method.

The Four-Step ABG Interpretation Method

Run these ABG interpretation steps in order, every time, without improvising.

Step 1: Look at the pH

Below 7.35 is acidosis. Above 7.45 is alkalosis. Between the two is normal, which does not automatically mean the ABG is normal, as step four explains.

This step tells you which direction the body has gone. It does not yet tell you why.

Step 2: Look at the PaCO2

If the PaCO2 has moved in the opposite direction to the pH, the cause is respiratory.

A low pH with a high PaCO2 is respiratory acidosis. A high pH with a low PaCO2 is respiratory alkalosis.

This makes sense once you remember that carbon dioxide is an acid. Retaining it makes the blood more acidic; blowing it off makes the blood more alkaline.

Step 3: Look at the HCO3

If the HCO3 has moved in the same direction as the pH, the cause is metabolic.

A low pH with a low HCO3 is metabolic acidosis. A high pH with a high HCO3 is metabolic alkalosis.

Bicarbonate is a base, so losing it makes the blood more acidic and gaining it makes the blood more alkaline.

The mnemonic that holds this together is ROME. Respiratory Opposite, Metabolic Equal. Opposite directions mean respiratory; the same direction means metabolic.

Step 4: Check Compensation

The body tries to correct an imbalance using the system that did not cause it. If the problem is respiratory, the kidneys adjust bicarbonate. If the problem is metabolic, the lungs adjust breathing rate to change carbon dioxide.

The rule for fully compensated ABGs. When the pH sits inside the normal range but both other values are abnormal, use 7.40 as the dividing line. A pH of 7.35 to 7.39 means the original problem was acidosis. A pH of 7.41 to 7.45 means it was alkalosis.

That single rule resolves the ABG interpretation questions candidates most often get wrong.

Oxygenation Is a Separate Question

This is the most common source of confusion in ABG interpretation, and it costs points on otherwise correct answers.

PaO2 and SaO2 tell you about oxygenation. They are not part of the acid-base interpretation at all.

A client can have a perfectly normal acid-base picture and be dangerously hypoxic. Another can have a severe acidosis with adequate oxygenation.

Read them as two separate questions:

1. What is the acid-base disorder, and is it compensated?
2. Is this client oxygenating adequately?

On the exam, the second question often drives the priority action, because hypoxia threatens the client faster than most acid-base imbalances.

The Four Disorders and What Causes Them

Recognizing the pattern is half of ABG interpretation on an item. The other half is knowing what produces it, because the exam usually gives you a client rather than a set of numbers alone.

Respiratory Acidosis

The picture: low pH, high PaCO2. The client is not blowing off carbon dioxide.

Common causes: hypoventilation of any kind. Chronic obstructive pulmonary disease, opioid or sedative overdose, atelectasis, severe pneumonia, airway obstruction, neuromuscular weakness affecting the respiratory muscles, and chest wall injury.

What the nurse is thinking: ventilation. Assess airway and breathing, position to support chest expansion, and escalate.

Respiratory Alkalosis

The picture: high pH, low PaCO2. The client is blowing off too much carbon dioxide.

Common causes: hyperventilation. Anxiety and panic, pain, fever, early sepsis, high altitude, and pulmonary embolism.

What the nurse is thinking: why is this client hyperventilating? Anxiety is the familiar answer, but pain, hypoxia, and pulmonary embolism are the ones that matter clinically, so rule those out before reaching for reassurance.

Metabolic Acidosis

The picture: low pH, low HCO3. Acid is accumulating or base is being lost.

Common causes: diabetic ketoacidosis, kidney failure, prolonged diarrhoea, shock and lactic acidosis, and salicylate overdose.

What the nurse is thinking: what is producing the acid, or where is the bicarbonate going? Expect fluid and electrolyte involvement, particularly potassium.

Metabolic Alkalosis

The picture: high pH, high HCO3. Acid is being lost or base gained.

Common causes: prolonged vomiting, nasogastric suction, excessive antacid intake, and diuretic therapy. Hypokalaemia is frequently involved.

What the nurse is thinking: where is the acid going? Vomiting and suction remove stomach acid, which is why these clients tilt alkaline.

An ABG Interpretation Table You Can Rebuild From Memory

If you can redraw this table from memory, your ABG interpretation is solid.

Two patterns are worth noticing in that table.

The primary value always matches the disorder's name. In respiratory disorders the PaCO2 is the abnormal one first. In metabolic disorders it is the HCO3.

The compensating value always moves the same way as the primary one. In respiratory acidosis both PaCO2 and HCO3 end up high. In metabolic alkalosis both end up high too. That is why you cannot identify the disorder from direction alone once compensation has started, and why step one is always the pH.

Five Worked ABG Interpretation Examples

Run the four ABG interpretation steps on each before reading the answer.

Example 1

pH 7.28, PaCO2 58, HCO3 24.

Step 1: pH is low, so acidosis. Step 2: PaCO2 is high, moving opposite to pH, so respiratory. Step 3: HCO3 is normal, so the kidneys have not responded yet. Step 4: uncompensated.

Answer: uncompensated respiratory acidosis. A client who received opioids and is now drowsy with shallow breathing fits this exactly.

Example 2

pH 7.50, PaCO2 28, HCO3 24.

Step 1: pH is high, so alkalosis. Step 2: PaCO2 is low, moving opposite, so respiratory. Step 3: HCO3 normal. Step 4: uncompensated.

Answer: uncompensated respiratory alkalosis. Consistent with hyperventilation, but check for pain, fever, and hypoxia before assuming anxiety.

Example 3

pH 7.25, PaCO2 38, HCO3 16.

Step 1: pH is low, so acidosis. Step 2: PaCO2 is normal, so not respiratory. Step 3: HCO3 is low, moving with the pH, so metabolic. Step 4: uncompensated.

Answer: uncompensated metabolic acidosis. Diabetic ketoacidosis is the classic scenario.

Example 4

pH 7.52, PaCO2 44, HCO3 32.

Step 1: pH is high, so alkalosis. Step 2: PaCO2 normal. Step 3: HCO3 high, moving with the pH, so metabolic. Step 4: uncompensated.

Answer: uncompensated metabolic alkalosis. Prolonged vomiting or nasogastric suction fits.

Example 5

pH 7.37, PaCO2 58, HCO3 34.

Step 1: pH is inside the normal range, which is where candidates stop and call it normal. Step 2 and 3: both PaCO2 and HCO3 are abnormal, which means compensation has occurred. Step 4: apply the 7.40 rule. A pH of 7.37 sits on the acidic side, so the original problem was acidosis, and since the PaCO2 is high the cause was respiratory.

Answer: fully compensated respiratory acidosis. This is the classic picture in long-standing chronic obstructive pulmonary disease, where the kidneys have retained bicarbonate over time.

Linking the Gas to the Client

The strongest ABG interpretation answers connect the numbers to what you would expect to see at the bedside. That connection is what turns a naming exercise into a priority decision.

Two of these deserve special attention.

Deep rapid breathing in metabolic acidosis is compensation, not the problem. A client in diabetic ketoacidosis breathing heavily is trying to blow off carbon dioxide to raise the pH. Treating that as a respiratory issue and slowing the breathing would make things worse.

Tingling and twitching point alkaline. Both respiratory and metabolic alkalosis produce neuromuscular irritability, which is why those symptoms appear so often in items where the gas tilts high.

When an item gives you the client picture and asks for the likely disorder, work backwards through this table. When it gives you the gas and asks what to assess, work forwards.

What the NCLEX Actually Asks About ABGs

This is where most ABG interpretation resources stop being useful. The exam rarely asks you to name the disorder and nothing else.

More often it embeds the ABG in a clinical judgment item and asks what you do about it.

Common item patterns:

• Which finding requires immediate follow-up, with an ABG among several results
• Which condition is the client most likely experiencing, with the ABG as one cue among many
• Which action should the nurse take first
• Whether the client's condition is improving, using two sets of gases across time

That last pattern is a trend item. A PaCO2 falling from 62 to 54 after an intervention is improvement even though it remains abnormal. Our guide to NGN trend questions covers the format.

The practical consequence. Naming the disorder is step one of your answer, not the answer itself. Always finish the thought: what does this mean for this client, and what does the nurse do next?

Where ABG Interpretation Appears on the Exam

ABG interpretation sits mainly within Physiological Adaptation, which carries roughly 14% of the RN exam, and within Reduction of Risk Potential at 12%.

The 2026 NCLEX-RN Test Plan lists managing care of clients with impaired ventilation or oxygenation, caring for clients on a ventilator, and monitoring diagnostic test results among its activity statements.

In practice, ABG interpretation items tend to cluster around respiratory clients, post-operative clients, and clients with diabetic or renal complications.

Inside case studies, gases appear in the laboratory results tab alongside vital signs and nurses' notes, which means you will usually be comparing them against a respiratory rate, an oxygen saturation, and a mental status description. Our guide to the three unfolding case studies explains the structure.

Six Mistakes With ABG Interpretation

1. Calling a normal pH a normal ABG. Example 5 above is the trap. Check the other two values before concluding anything.

2. Mixing oxygenation into the acid-base answer. PaO2 and SaO2 are a separate question. Answer both, but do not blend them.

3. Skipping the ABG interpretation sequence under pressure. Candidates who reason from scratch each time are slower and less accurate than those running four fixed steps.

4. Stopping at the label. The exam wants the action. Naming respiratory acidosis is not an answer to "what should the nurse do first."

5. Assuming anxiety whenever the picture is respiratory alkalosis. Pain, fever, hypoxia, and pulmonary embolism all hyperventilate a client, and some of those are emergencies.

6. Forgetting potassium. Acid-base shifts and potassium move together, which is why metabolic disorders so often appear in items that are really about electrolytes. Our guide to NCLEX lab values covers those pairings.

How to Practise ABG Interpretation

Drill the ABG interpretation sequence, not the answers. Twenty gases run through the four steps beats a hundred you pattern-match. If you can produce the reasoning aloud, it will survive a differently worded item.

Cover the answer before you look. Write your interpretation down before checking, so you rehearse retrieval rather than recognition.

Always add the action. For every set, finish with what the nurse would do first. That converts a naming exercise into exam practice.

Practise compensated examples deliberately. These are where candidates lose points, and they are underrepresented in most question banks. Write out several with a normal pH and two abnormal values, and apply the 7.40 rule.

Time yourself. The target is ten to fifteen seconds for the interpretation, leaving your thinking time for the clinical decision.

Our guide to NCLEX remediation covers how to tag misses, which matters here because an ABG miss can be a knowledge gap, a sequence error, or a priority error, and each needs a different fix.

A Note for Internationally Educated Nurses

Three points about ABG interpretation are worth flagging if you trained outside North America.

Units may look unfamiliar. PaCO2 and PaO2 are commonly reported in mmHg on the NCLEX, where you may have trained with kPa. Because the reference range is supplied in the same unit as the value, no conversion is needed. Practise with US-unit question banks so the format stops being a distraction.

The action is jurisdictional. Interpreting the gas is universal; what the nurse does next is not. When to notify the provider, when to call a rapid response team, and what falls within nursing scope all follow North American practice. An experienced nurse can interpret perfectly and still select an action that was correct in the system where they trained.

Terminology varies. Bicarbonate may appear as HCO3 or as a calculated value on a metabolic panel. Recognize the value rather than the label.

One-on-one tutoring can help you tell interpretation problems apart from practice framework problems, which look identical from the inside.

Frequently Asked Questions

What is the easiest way to interpret ABGs for the NCLEX?

ABG interpretation runs in four steps: check the pH for acidosis or alkalosis, check whether the PaCO2 moves opposite to the pH, which means respiratory, check whether the HCO3 moves with the pH, which means metabolic, then check compensation.

What does ROME mean in ABG interpretation?

ROME is the ABG interpretation mnemonic: Respiratory Opposite, Metabolic Equal. If the PaCO2 moves in the opposite direction to the pH, the disorder is respiratory. If the HCO3 moves in the same direction as the pH, it is metabolic.

How do you know if an ABG is fully compensated?

The pH is back inside the normal range while both the PaCO2 and HCO3 remain abnormal. Use 7.40 as the dividing line: a pH of 7.35 to 7.39 means the original problem was acidosis, and 7.41 to 7.45 means it was alkalosis.

Are ABG values given on the NCLEX?

Yes. NCSBN states that items containing a numeric laboratory value include the corresponding normal reference range. What is not supplied is the interpretation sequence, which is why it is worth memorizing.

Do I need to interpret oxygenation with the ABG?

Assess it separately from ABG interpretation. PaO2 and SaO2 describe oxygenation rather than acid-base balance, and a client can be hypoxic with a normal acid-base picture.

Practise the ABG Interpretation Sequence, Then the Decision

ABG interpretation is one of the few areas where a memorized method genuinely wins, because the sequence never changes. What the exam adds on top is the clinical decision, and that is where most points are lost.

If you have tested before, your Candidate Performance Report shows whether your gap is content knowledge or clinical judgment, which tells you whether ABG practice is even where your time should go. Our guide to reading your NCLEX Candidate Performance Report walks through every line.

Submit your CPR for analysis on the NCLEX High Yield website, or text us at 725-444-7551 to speak with our team.

We will tell you honestly what your report shows, including when more content study is not the answer.

You can also join our free weekly Zoom sessions, explore our NCLEX programs, or build a schedule with the NCLEX High Yield Study Planner.

CPRs must be dated within the last 12 months. Reviews are professional guidance based on experience and are not affiliated with NCSBN or a prediction of your exam result. Exam details are from NCSBN's published material and were checked in September 2026.

Reference ranges shown are commonly cited adult values for study familiarity only. Ranges vary between laboratories, so always use the range supplied in the item and follow your own institution's values in practice.