What are the normal ABG values?

pH 7.35 to 7.45, PaCO2 35 to 45 mmHg and HCO3 22 to 26 mEq/L. PaO2 is usually 80 to 100 mmHg. Some labs use slightly different ranges, so check your facility's values.

What does ROME stand for?

Respiratory Opposite, Metabolic Equal. In a respiratory disorder, the pH and PaCO2 move in opposite directions. In a metabolic disorder, the pH and HCO3 move in the same direction.

How do you know if an ABG is compensated?

Look at the value that did not cause the disorder. If it is normal, the result is uncompensated. If it has moved the opposite way and the pH is still abnormal, it is partially compensated. If the pH is back in range, it is fully compensated.

What is the tic-tac-toe method for ABGs?

Draw three columns: Acid, Normal and Base. Place the pH, PaCO2 and HCO3 in their columns. The value that shares a column with the pH names the disorder. The value left over tells you the compensation.

Can the pH be normal when the ABG is abnormal?

Yes. In full compensation, the pH is back in range but PaCO2 and HCO3 are both abnormal. Use 7.40 as the midpoint: a pH of 7.35 to 7.39 leans to acidosis, and 7.41 to 7.45 leans to alkalosis.

Which compensates faster, the lungs or the kidneys?

The lungs. They change the breathing rate within minutes. The kidneys change how much bicarbonate they keep, and that takes several days.

← All tools

ABG interpretation calculator

Enter pH, PaCO2 and HCO3. Get the acid-base disorder and its compensation, solved step by step with ROME or tic-tac-toe, then practise with NCLEX-style questions.

Enter the ABG values

7.35–7.45

PaCO2

mmHg · 35–45

HCO3

mEq/L · 22–26

Try an example

Interpretation

Respiratory acidosis

Partially compensated

Solve it with

Step by step

Read the pH. Normal is 7.35 to 7.45.pH 7.31 < 7.35 → acidosis
Read the PaCO2. CO2 is the acid that the lungs control. Normal is 35 to 45 mmHg.PaCO2 60 > 45 → high → acid
Read the HCO3. Bicarbonate is the base that the kidneys control. Normal is 22 to 26 mEq/L.HCO3 30 > 26 → high → base
Apply ROME: Respiratory Opposite, Metabolic Equal. Compare the direction of each value with the pH.pH ↓, PaCO2 ↑, HCO3 ↑ → pH and PaCO2 move in opposite directions → Respiratory
Check the other value, HCO3. It has moved the opposite way to fight the change, but the pH is still abnormal.HCO3 30 → opposite side, pH still abnormal → partially compensated respiratory acidosis

What the body does

The lungs are not removing enough CO2. The kidneys respond. They keep more bicarbonate and excrete more acid. This takes several days.

Common causes

COPD
Opioid or sedative overdose
Pneumonia or severe asthma
Airway obstruction
Neuromuscular weakness, such as Guillain-Barré syndrome
Chest wall injury

For exam preparation only. This tool follows the nursing school method. It does not use expected-compensation formulas, the anion gap or oxygenation. Do not use it to make care decisions for a real client.

How to interpret an ABG, step by step

Nursing programs teach two methods: ROME and tic-tac-toe. They give the same answer. Pick one to solve and use the other to check. Both start with the same three values.

Normal ABG values

Value	Normal	Acid side	Base side
pH	7.35 – 7.45	Below 7.35	Above 7.45
PaCO2	35 – 45 mmHg	Above 45	Below 35
HCO3	22 – 26 mEq/L	Below 22	Above 26

PaCO2 is the only value that runs backwards. CO2 is an acid, so more CO2 means more acid. Canadian labs report HCO3 in mmol/L. The number is the same.

The ROME method

Respiratory  Opposite  (pH ↓ PaCO2 ↑  or  pH ↑ PaCO2 ↓)
Metabolic    Equal     (pH ↓ HCO3 ↓   or  pH ↑ HCO3 ↑)

Read the pH first. Below 7.35 is acidosis and above 7.45 is alkalosis.

Then compare the arrows. If the PaCO2 moves opposite to the pH, the cause is respiratory. If the HCO3 moves with the pH, the cause is metabolic.

Example: pH 7.30 ↓, PaCO2 54 ↑, HCO3 25 normal. pH and PaCO2 are opposite, so this is respiratory acidosis. HCO3 is normal, so it is uncompensated.

The tic-tac-toe method

         Acid     Normal    Base
pH       7.30
PaCO2    54
HCO3              25

Draw three columns: Acid, Normal and Base. Put each value in its column. Remember that a high PaCO2 goes in Acid.

The value that shares a column with the pH names the disorder. The value left over shows the compensation.

Here the pH and PaCO2 share the Acid column, so this is respiratory acidosis. HCO3 sits in Normal, so it is uncompensated.

Uncompensated, partially or fully compensated

Uncompensated: the pH is abnormal and the other value is still normal. The body has not responded yet.

Partially compensated: the pH is abnormal, and the other value has moved the opposite way. The body is responding but has not finished.

Fully compensated: the pH is back in 7.35 to 7.45, and both PaCO2 and HCO3 are abnormal. Use 7.40 as the midpoint to find the cause. The value on the same side as the pH caused the disorder.

Common causes of each disorder

Exam questions usually give you the client's story first. Link the story to the disorder before you read the numbers.

respiratory acidosis

COPD
Opioid or sedative overdose
Pneumonia or severe asthma
Airway obstruction
Neuromuscular weakness, such as Guillain-Barré syndrome
Chest wall injury

respiratory alkalosis

Anxiety or panic attack
Pain
Fever or early sepsis
Pulmonary embolism
High altitude
A ventilator rate that is set too high

metabolic acidosis

Diabetic ketoacidosis
Lactic acidosis from shock or sepsis
Kidney failure
Severe diarrhea
Aspirin (salicylate) overdose
Starvation

metabolic alkalosis

Vomiting
Nasogastric suction
Loop or thiazide diuretics
Low potassium
Too much antacid or sodium bicarbonate

ABG practice questions

Try these in the format of the NCLEX and REx-PN. Each answer explains every option.

Question 1
A client is 6 hours after abdominal surgery and has a morphine PCA pump. The client is hard to wake, and the respiratory rate is 8/min. ABG: pH 7.30, PaCO2 54 mmHg, HCO3 25 mEq/L. How should the nurse interpret this result?
Question 2
A client is breathing fast during a panic attack and says the fingers are tingling. ABG: pH 7.50, PaCO2 29 mmHg, HCO3 23 mEq/L. Which interpretation is correct?
Question 3
A client with type 1 diabetes has a blood glucose of 30 mmol/L (540 mg/dL). Breathing is deep and rapid. ABG: pH 7.26, PaCO2 28 mmHg, HCO3 13 mEq/L. How should the nurse interpret this result?
Question 4
A client has had nasogastric suction for 3 days. ABG: pH 7.49, PaCO2 48 mmHg, HCO3 35 mEq/L. Which interpretation is correct?
Question 5
A client has had COPD for many years. Today's ABG: pH 7.36, PaCO2 58 mmHg, HCO3 32 mEq/L. How should the nurse interpret this result?
Question 6Select all that apply
Which conditions can cause metabolic acidosis?

Questions about ABG interpretation

What are the normal ABG values?: pH 7.35 to 7.45, PaCO2 35 to 45 mmHg and HCO3 22 to 26 mEq/L. PaO2 is usually 80 to 100 mmHg. Some labs use slightly different ranges, so check your facility's values.
What does ROME stand for?: Respiratory Opposite, Metabolic Equal. In a respiratory disorder, the pH and PaCO2 move in opposite directions. In a metabolic disorder, the pH and HCO3 move in the same direction.
How do you know if an ABG is compensated?: Look at the value that did not cause the disorder. If it is normal, the result is uncompensated. If it has moved the opposite way and the pH is still abnormal, it is partially compensated. If the pH is back in range, it is fully compensated.
What is the tic-tac-toe method for ABGs?: Draw three columns: Acid, Normal and Base. Place the pH, PaCO2 and HCO3 in their columns. The value that shares a column with the pH names the disorder. The value left over tells you the compensation.
Can the pH be normal when the ABG is abnormal?: Yes. In full compensation, the pH is back in range but PaCO2 and HCO3 are both abnormal. Use 7.40 as the midpoint: a pH of 7.35 to 7.39 leans to acidosis, and 7.41 to 7.45 leans to alkalosis.
Which compensates faster, the lungs or the kidneys?: The lungs. They change the breathing rate within minutes. The kidneys change how much bicarbonate they keep, and that takes several days.

Practise acid-base every day

Medibara is a free study app for NCLEX-RN, NCLEX-PN and REx-PN. Short lessons teach the topic, then questions in your exam's format test it. When an ABG question does not make sense, ask Cappy, the tutor, to walk you through it. Start learning on Medibara.