ABG interpretation is one of the most challenging things you’ll learn in nursing school, but the good news is…it DOES get much easier with practice.

In this article, I’m going to talk you through ABG interpretation using the tic-tac-toe method, which was originally shared in the article “Turn ABGs into child’s play” by Doreen Mays and Eileen O’Connor [Mays, D. A. (1995, January). Turn ABGs into child’s play. RN, 58(1), 36+.]

The first thing to solidify are the normal values:

pH: 7.35 – 7.45 (lower than 7.35 is acidic, higher than 7.45 is basic)
CO2:  35-45 (lower than 35 is basic, higher than 45 is acidic)
Bicarb: 22-26 (lower than 22 is acidic, higher than 26 is basic)

Metabolic or Respiratory?

The next thing you need to understand is that CO2 has to do with the lungs, and Bicarb has to do with the kidneys…so CO2 is respiratory related and Bicarb is metabolic related. Doing okay so far?

Let’s practice!

In this scenario, you have a patient with pneumonia who is looking worse. He’s needing more oxygen, has lower O2 saturation levels, and is complaining of more shortness of breath.  You are definitely worried about this patient, so you get an ABG. Your results show the following:

pH = 7.16; Bicarb = 24; CO2 = 59; O2 = 45.

Right from the start, you can probably surmise this is a pretty bad ABG. To determine exactly what’s going on with your patient, the first thing you’re going to do is draw a tic-tac-toe grid, like this:

Step 1

Step 1

So, looking back at his ABG results, we’re going to plug them into our grid. Start with the pH…it’s 7.16. Is that acidic, normal, or alkalotic? It’s acidic, so write “pH” under the appropriate column:

Step 2

Step 2

The next value we’re going to look at is the Bicarb, which is 24. Is it acidic, normal or alkalotic? That bicarb value is within the normal range, so we’re going to write “Bicarb” (or HCO3) in the middle column like this:

Step 3

Step 3

Actually, you can look at Bicarb or CO2 after your pH, it doesn’t really matter, just make sure you work in a systematic manner.

Now let’s look at your CO2. It’s 59. Is this acidic, normal or or basic/alkalotic? Hmmm….high CO2 is acidotic, you say? Yes, you’re right! So, let’s write CO2 in the acid column just like this:

Step 4

Step 4

And then you step back and admire your tic-tac-toe board because guess what? You’ve got three-in-a-row…you win!



Because your three-in-a-row is in the acidic column, you know you’re dealing with an ACIDOSIS. Now, is it respiratory or is it metabolic? This one is easy. Respiratory acid/base is determined by the CO2, while metabolic acid/base is determined by the Bicarb. So which one is in your column? The CO2, right? That means, this is a RESPIRATORY ACIDOSIS.

Let’s talk compensation

The next question you’re going to ask yourself is, “Is this compensated, partially compensated or uncompensated respiratory acidosis?” To determine compensation, we look at the pH first. Is it abnormal? Yes…so this are definitely not compensated.

Next we look at the Bicarb…the only number we have left (except for O2, which we’ll get to in a minute). The Bicarb is normal, so what does this mean? This means the kidneys have not yet had a chance to kick in and start buffering that acid. If they had, the Bicarb would be high. But in this case, it’s within the normal range, which means the renal system has not yet had a chance to balance out the acidic pH. So, we have UNCOMPENSATED RESPIRATORY ACIDOSIS.

What about the oxygen level?

Now we’re going to take note of his O2 and his clinical presentation. This patient most likely needs to be intubated RIGHT AWAY, or at the very least needs some aggressive oxygen support. Let’s try another one.

Patient scenario #2

You’ve got a patient who’s on a vent, sedated, and breathing at a set rate of 16. He’s suspected of taking an overdose of antacids, thinking they were candy.  His ABG results are such:

pH = 7.56; Bicarb = 32; CO2 = 37; O2 = 90

So, first draw your tic-tac-toe grid. I’ll wait. Now, let’s look at our pH. Is it acidotic, normal, or alkalotic? It’s alkalotic, so let’s write it in:

So far so good!

So far so good!

Your next value is the Bicarb of 32. Where does that go? Yep! You got it…his Bicarb is high so he’s alkalotic.

It's getting easier, huh?

It’s getting easier, huh?

Even though you have tic-tac-toe already, you still need to analyze the CO2 to see if we’re compensated or not. So, his CO2 is 37. Where’s that go? Yes, it’s normal!

You're rocking it!

You’re rocking it!

So, what do you think? You have a tic-tac-toe with the Bicarb matching up with the pH…so you have a METABOLIC ALKALOSIS. Is it compensated or not? Have the lungs adjusted CO2 to balance things out? Nope…the CO2 is normal and the pH is abnormal,  so this tells us we have an UNCOMPENSATED METABOLIC ALKALOSIS.

Since he’s sedated, his body isn’t going to breathe at a rate to naturally compensate for the alkalosis. In this case, we’ll either lighten the sedation or turn the rate on the ventilator down a bit. We’ll do another ABG in about 30 minutes to see how he’s doing.

In the next lesson, we’ll tackle compensated and partially compensated acid/base disorders. See you there!


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