FT - ABG Analysis 8 Step Guide
ABG Analysis Using Tic-Tac-Toe Method

In this guide, we will teach you the eight (8) steps on how to interpret and analyze arterial blood gas (ABG) results using the tic-tac-toe method. This tic-tac-toe technique involves a few charts to help you visualize ABGs. Once you’ve mastered this technique, you’ll be doing actual ABG interpretation in the NCLEX with fun and excitement!

Arterial Blood Gases (ABGs) are measured in a laboratory test to determine the extent of compensation by the buffer system. It measures the acidity (pH) and the levels of oxygen and carbon dioxide in arterial blood. Blood for an ABG test is taken from an artery whereas most other blood tests are done on a sample of blood taken from a vein. This test is done to monitor several conditions that can cause serious health complications, especially to critically ill individuals.

The normal range for ABGs is used as a guide, and determination of disorders is often based on the blood pH. If the blood is basic, the HCO3 level is considered because the kidneys regulated bicarbonate ion levels. If the blood is acidic, the PaCO2 or partial pressure of carbon dioxide in arterial blood is assessed because the lungs regulate the majority of acid.

  • Respiratory acidosis occurs when breathing is inadequate and PaCO2 or respiratory acid builds up. The extra CO2 combines with water to form carbonic acid, causing a state of acidosis — a common occurrence in emphysema.
  • Respiratory alkalosis can occur as a result of hyperventilation.
  • Metabolic acidosis is when there is a decrease in bicarbonates and a buildup of lactic acid occurs. This happens in diarrhea, ketosis, and kidney disorders.
  • Metabolic alkalosis occurs when bicarbonate ion concentration increases, causing an elevation in blood pH. This can occur in excessive vomiting, dehydration, or endocrine disorders.

Every day, a lot of nursing and medical students assigned in acute areas encounter ABG results which they may not necessarily be able to interpret because of its complicated nature. Most students struggle over the interpretation of its measurements, but they are not especially complex nor difficult if you understand the basic physiology and have a step by step process to analyze and interpret them.

Goals of ABG analysis

Goals of ABG Analysis using Tic-Tac-Toe Method: To simplify this technique even further, keep these goals in mind.
To simplify this technique even further, keep these goals in mind.

For the purpose of this guide, we have set three (3) goals that we need to accomplish when interpreting arterial blood gases. The goals are as follows:

  • #1. Based on the given ABG values, determine if values interpret ACIDOSIS or ALKALOSIS.
  • #2. Second, we need to determine if values define METABOLIC or RESPIRATORY.
  • #3. Lastly, we need to determine the compensation if it is: FULLY COMPENSATED, PARTIALLY COMPENSATED, or UNCOMPENSATED.

We need to keep these goals in mind as they’ll come up later in the steps for the technique.

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Steps in ABG analysis using the tic-tac-toe method

There are eight (8) steps simple steps you need to know if you want to interpret ABG results using the tic-tac-toe technique.

1. Memorize the normal values. 

The first step is you need to familiarize yourself with the normal and abnormal ABG values when you review the lab results. They are easy to remember:

  • For  pH, the normal range is 7.35 to 7.45
  • For PaCO2, the normal range is 35 to 45
  • For HCO3, the normal range is 22 to 26
Step 1 - ABG Analysis Tic-Tac-Toe Method
Normal Blood pH Scale Diagram for the Tic-Tac-Toe Method for ABG Analysis

The recommended way of memorizing it is by drawing the diagram of normal values above. Write it down together with the arrows indicating ACIDOSIS or ALKALOSIS. Note that PaCO2 is intentionally inverted for the purpose of the Tic-Tac-Toe method.

2. Create your tic-tac-toe grid. 

Step 2 - The Tic-Tac-Toe Grid - ABG Analysis Tic-Tac-Toe Method
Make a 3×3 grid and label it as follows.

Once you’ve memorized the normal values and the diagram, create a blank your tic-tac-toe grid and label the top row as ACIDOSIS, NORMAL, and ALKALOSIS. Based on their values, we need to determine in which column we’ll place pH, PaCO2, and HCO3 in the grid.

3. Determine if pH is under NORMAL, ACIDOSIS, or ALKALOSIS. 

The third step of this technique is to determine the acidity or alkalinity of the blood with the given value of the pH as our determining factor. Remember in step #1 that the normal pH range is from 7.35 to 7.45.

  • If the blood pH is between 7.35 to 7.39, the interpretation is NORMAL but SLIGHTLY ACIDOSIS, place it under the NORMAL column.
  • If the blood pH is between 7.41 to 7.45, interpretation is NORMAL but SLIGHTLY ALKALOSIS, place it under the NORMAL column.
  • Any blood pH below 7.35 (7.34, 7.33, 7.32, and so on…) is ACIDOSIS, place it under the ACIDOSIS column.
  • Any blood pH above 7.45 (7.46, 7.47, 7.48, and so on…) is ALKALOSIS, place it under the ALKALOSIS column.

Please use the diagram below to help you visualize whether the normal value is ACIDOSIS or ALKALOSIS.

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Step 3 - ABG Analysis Tic-Tac-Toe Method
Now we need to determine where we’ll plot pH in the tic-tac-toe grid.

Once you’ve determined whether the pH is under the ACIDOSIS or ALKALOSIS, plot it on your tic-tac-toe grid under the appropriate column.

4. Determine if PaCO2 is under NORMAL, ACIDOSIS, or ALKALOSIS. 

Step 4 - ABG Analysis Tic-Tac-Toe Method
Do the same for the PaCO2. (Click to enlarge)

For this step, we need to interpret if the value of PaCO2 is within the NORMAL range, ACIDIC, or BASIC and plot it on the grid under the appropriate column. Remember that the normal range for PaCO2 is from 35 to 45:

  • If PaCO2 is below 35, place it under the ALKALOSIS column.
  • If PaCO2 is above 45, place it under the ACIDOSIS column.
  • If PaCO2 is within its normal range, place it under the NORMAL column.

5. Determine if HCO3 is under NORMAL, ACIDOSIS, or ALKALOSIS. 

Step 5 - ABG Analysis Tic-Tac-Toe Method: In the fifth step, we need to know where HCO3 is placed in the ABG tic-tac-toe grid.
In the fifth step, we need to know where HCO3 is placed in the ABG tic-tac-toe grid.

Next, we need to interpret if the value of HCO3 is within the NORMAL range, ACIDIC, or BASIC and plot it under the appropriate column in the tic-tac-toe grid. Remember that the normal range for HCO3 is from 22 to 26:

  • If HCO3 is below 22, place it under the ACIDOSIS column.
  • If HCO3 is above 26, place it under the ALKALOSIS column.
  • If HCO3 is within its normal range, place it under the NORMAL column.

6. Solve for goal #1: ACIDOSIS or ALKALOSIS. 

Step 6 - ABG Analysis Tic-Tac-Toe Method - Solving for goal #1. Determining if set of ABG values are ACIDOSIS or ALKALOSIS.
Solving for goal #1. Determining if the set of ABG values interpret as ACIDOSIS or ALKALOSIS.

Now, we will start solving for our goals. Looking at the tic-tac-toe grid, determine whether in what column the pH is placed and interpret the results:

  • If pH is under the ACIDOSIS column, it is ACIDOSIS.
  • If pH is under the ALKALOSIS column, it is ALKALOSIS.
  • If pH is under the NORMAL column, determine whether the value is leaning towards ACIDOSIS or ALKALOSIS and interpret accordingly.

In this step, we can accomplish goal #1 of determining ACIDOSIS or ALKALOSIS.

7. Solve for goal #2: METABOLIC or RESPIRATORY. 

Step 7 - ABG Analysis Tic-Tac-Toe Method
Solving for goal #2, we analyze where pH lines up with. If it lines up with PaCO2, it’s RESPIRATORY. If it lines up with HCO3, it’s METABOLIC.

Looking back again on the tic-tac-toe grid, determine if pH is under the same column as PaCO2 or HCO3 so we can accomplish our goal #2 of determining if the ABG is RESPIRATORY or METABOLIC. Interpret the results as follows:

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  • If pH is under the same column as PaCO2, it is RESPIRATORY.
  • If pH is under the same column as HCO3, it is METABOLIC.
  • If pH is under the NORMAL column, determine whether the value is leaning towards ACIDOSIS or ALKALOSIS and interpret accordingly.

8. Solve for goal #3: COMPENSATION. 

Step 8 - ABG Analysis Tic-Tac-Toe Method
Solving for goal #3 where we determine the compensation of the ABG result.

Lastly, we need to determine the compensation to accomplish our goal #3. Interpret the results as follows:

  • It is FULLY COMPENSATED if pH is normal.
  • It is PARTIALLY COMPENSATED if all three (3) values are abnormal.
  • It is UNCOMPENSATED if PaCO2 or HCO3 is normal and the other is abnormal.

Application and Examples

Let’s solve for the ABG interpretation with the examples below:


Practice Problem #1:
pH=7.26 | PaCO2=32 | HCO3=18

  1. Remember the normal values.
  2. Make your tic-tac-toe grid.
  3. pH of 7.26 ABNORMAL and under ACIDOSIS, so we place pH under ACIDOSIS.
  4. PaCO2 of 32 is ABNORMAL and under ALKALOSIS, so we place PaCO2 under ALKALOSIS.
  5. HCO3 of 18 is ABNORMAL and under ACIDOSIS, so we place HCO3 under ACIDOSIS.
  6. pH is under ACIDOSIS, therefore solving for goal #1, we have ACIDOSIS.
  7. pH is on the same column as HCO3, therefore solving for goal #2, we have METABOLIC.
  8. All three values are ABNORMAL, therefore solving for goal #3, we have a PARTIALLY COMPENSATED ABG.

The answer to Practice Problem #1:
Metabolic Acidosis, Partially Compensated


Practice Problem #2:
pH=7.44 | PaCO2=30 | HCO3=21

  1. Remember the normal values.
  2. Make your tic-tac-toe grid.
  3. pH of 7.44 is NORMAL but slightly leaning towards ALKALOSIS, so we place pH under the NORMAL column with an arrow pointing towards the ALKALOSIS column.
  4. PaCO2 of 30 is ABNORMAL and ALKALOSIS, so we place PaCO2 under the ALKALOSIS column.
  5. HCO3 of 21 is ABNORMAL and ACIDOSIS, so we place HCO3 under the ACIDOSIS column.
  6. pH of 7.44 is NORMAL but leaning towards ALKALOSIS, therefore solving for goal #1, we have ALKALOSIS.
  7. pH is NORMAL but is leaning towards ALKALOSIS, therefore under the same column as PaCO2. Solving for goal #2, we have RESPIRATORY.
  8. pH is NORMAL, therefore solving for goal #3, we have a FULLY COMPENSATED ABG.

The answer to Practice Problem #2:
Respiratory Alkalosis, Fully Compensated

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Practice Problem #3:
pH=7.1 | PaCO2=40 | HCO3=18

  1. Remember the normal values.
  2. Make your tic-tac-toe grid.
  3. pH of 7.1 is ABNORMAL and ACIDOSIS, therefore, we place pH under the ACIDOSIS column in the tic-tac-toe grid.
  4. PaCO2 of 40 is NORMAL, therefore, place it under the NORMAL column.
  5. HCO3 of 18 is ABNORMAL and ACIDOSIS, so we place HCO3 under the ACIDOSIS column.
  6. pH of 7.1 is ACIDOSIS, therefore, solving for goal #1, we have ACIDOSIS.
  7. pH is under the same column as HCO3, therefore, solving for goal #2, we have determined that it is METABOLIC.
  8. pH is ABNORMAL so as HCO3, but PaCO3 is under the NORMAL column. Solving for goal #3, we can interpret it as UNCOMPENSATED.

The answer to Practice Problem #3:
Metabolic Acidosis, Uncompensated


More Arterial Blood Gas Quizzes

Polish your way around the technique with even more examples. Check out these three (3) Arterial Blood Gas Analysis NCLEX Practice Exams:

  1. ABG Analysis NCLEX Exam 1 (10 Items)
  2. ABG Analysis NCLEX Exam 2 (10 Items)
  3. ABG Analysis NCLEX Exam 3 (10 Items)

This post was first published on January 21, 2015, and updated on January 16, 2019.

IG - ABG Analysis 8 Step Guide
ABG Analysis 8 Step Guide
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22 COMMENTS

  1. In the second example with these values: pH 7.1, PACO2 40, HCO3 18

    You mention that PaCO2 was abnormal, but I think you meant to say that HCO3 was abnormal. Everything else is right. pH is acidic and so is HCO3, therefore metabolic acidosis. Since pH is abnormal, PCO2 is WNL, and HCO3 is abnormal, it would uncompensated metabolic acidosis.

  2. Hi, I am preparing for the Board exam this year and found out your sites a big help for review, and I like the ABG analysis easy to comprehend very useful at work specially for the critical pt. keep us updated :))
    Thank you nurseslabs.com May God blessed the people behind this.

  3. Thank you for taking the time to share this. The type of work you do as a nurse does not really designate what kind of nurse you are, that comes from the heart. You have been blessed with the gift of clarity, and you share this with others as you further develop your nursing career through your educational sharing. Keep on, keeping on!

  4. what if all Three ( pH, HCO3, and PaCO2) are on the ACIDIC side. What then ? how will I determine if its respiratory or metabolic? I’m assuming it would be uncompensated since nothing is in the Metabolic side and pH is not normal

    • THAT is awesome! Glad to know you’ve found the technique useful and wasn’t it fun answering them using the tic-tac-toe method?
      Thanks for visiting the site, D!

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