8-Step Guide to ABG Analysis: Tic-Tac-Toe Method

The easiest method to interpret ABGs!

An arterial blood gas (ABG) is a blood test that measures the acidity (pH) and the levels of oxygen and carbon dioxide in the 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.

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 with its knotty aspect. They struggle over the interpretation of its measurements, but they are not especially complicated nor difficult if you understand the basic physiology and have a step by step process to analyze and interpret them.

There may be various tips and strategies to guide you, from mnemonics to charts, to lectures, to practice, but this article will tell you how to interpret ABGs in the easiest possible way. And once you have finished reading this, you’ll be doing actual ABG analysis in the NCLEX with fun and excitement! Here are the steps:

1. Know the normal values

Know the normal and abnormal ABG values when you review the lab reports. They’re fairly easy to remember: for pH, the normal value is 7.35 to 7.45; 35-45 for paCO2; and 22-26 for HCO3. Remember also this diagram and note that paCO2 is intentionally inverted for the purpose of this method.


2. Determine if pH is under acidosis or alkalosis

Next thing to do is to determine the acidity or alkalinity of the blood through the value of pH. The pH level of a healthy human should be between 7.35 to 7.45. The human body is constantly striving to keep pH in balance.

pH level below 7.35 is acidosis
pH level above 7.45 is alkalosis

3. Determine if acid-base is respiratory or metabolic

Next thing you need to determine is whether the acid base is Respiratory or Metabolic.

paCO2 = Respiratory
HCO3 = Metabolic

4. Remember ROME

Still, it all boils down to mnemonics. The mnemonic RO-ME.

Respiratory Opposite

When pH is up, PaCO2 is down = Alkalosis
When pH is down, PaCO2 is up = Acidosis

Metabolic Equal

When pH is up, HCO3 is up = Alkalosis
When pH is down, HCO3 is down = Acidosis

5. Tic-Tac-Toe

And yes, ABG problems can be solved by using the tic-tac-toe method. All you have to do is make a blank chart similar to this:


6. Mark the Chart

Using the lab result values, mark them on your tic-tac-toe. Let’s begin with this sample problem:

pH: 7.26, paCO2: 32, HCO3: 18

Using the normal values reference chart in the first step, determine where the values should be under in the tic-tac-toe. In the given example, the solution is as follows:

pH of 7.26 is LOW = ACID so place pH under Acid
paCO2 of 32 is LOW = BASE so place paCO2 under Base
HCO3 of 18 is LOW = ACID so place HCO3 under Acid

Your chart should look like this:


7. Match it up

In this step, determine at which column matches up with the pH. In the given example, HCO3 goes with pH. HCO3 is considered Metabolic (shown in step 3), and both are under Acid, so this example implies Metabolic Acidosis.


8. Determine compensation

The last step is to determine if the ABG is Compensated, Partially Compensated, or Uncompensated. Here’s the trick:

If pH is NORMAL, PaCO2 and HCO3 are both ABNORMAL = Compensated
If pH is ABNORMAL, PaCO2 and HCO3 are both ABNORMAL = Partially Compensated
If pH is ABNORMAL, PaCO2 or HCO3 is ABNORMAL = Uncompensated


By applying the steps above, interpret the following ABGs:

pH:7.44, PaCO2: 30, HCO3: 21

pH is NORMAL = NORMAL so place pH under Normal
PaCO2 is LOW = BASE so place PaCO2 under Base
HCO3 is LOW = ACID so place HCO3 under Acid

*Since the acidity of the blood is determined by the value of the pH, determine whether the normal pH is SLIGHTLY ACIDIC or SLIGHTLY BASIC. In this example, pH is NORMAL but SLIGHTLY BASIC therefore it is ALKALOSIS.

In this case PaCO2 goes with pH. PaCO2 is considered Respiratory (shown in step 3), and both are under Basic, so this example implies Respiratory Alkalosis. The HCO3 is also abnormal. When pH is NORMAL and PaCO2 and HCO3 are both ABNORMAL, it indicates FULL COMPENSATION.



Try this problem next:

pH 7.1, PaCO2 40, HCO3 18

pH is LOW = ACID so place pH under Acid
PaCO2 is NORMAL = NORMAL so place PaCO2 under Normal
HCO3 is LOW = ACID so place HCO3 under Acid

In this case, HCO3 goes with pH. HCO3 is considered Metabolic (shown in step 3), and both are under Acidic, so this example implies Metabolic Acidosis. The PaCO2 is normal. When pH is ABNORMAL, and when either one of PaCO2 or HCO3 is ABNORMAL, it indicates UNCOMPENSATION.



Try it out with these quizzes

Want to try out the Tic-Tac-Toe method? Here are some sample ABG Analysis quizzes:

  • 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.

  • 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.

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