Med-Surg is hard enough, you need an easy way to interpret ABG values! It’s difficult to remember all the different normal values, what they mean, and which direction they’re supposed to be going to when an imbalance is occurring. When you’re integrating so much information, it’s super easy to get mixed up and make a silly mistake on an exam, even when you really DO know how to interpret ABGs.

When I’m teaching, I usually prefer to focus on the “why”, but in this case it’s important to learn the “How to.” I strongly recommend that you take a few minutes to remind yourself of why we interpret ABG values in order to assess Respiratory Imbalances and Metabolic Imbalances.

(Article updated August 13, 2018, Originally published on July 24, 2012)

Easy Way to Interpret ABG Values in 7 Simple Steps

Overview of the 7 Simple Steps

We don’t only want to interpret ABG values for fun, they have a very important purpose! The goal of interpreting ABG values is so that we can decipher what is happening to our patient, so that we can implement a plan to help them improve.

Whenever you are working with a potential ABG imbalance, you will have three puzzle pieces to put together:

1. Is it acidosis, alkalosis or normal?
2. Is it uncompensated, partially compensated, or compensated?
3. Is it respiratory or metabolic?

Let’s get started.

Step 1: Write the Normal Values

This might seem counterintuitive, but let’s start with what we’ve memorized. I don’t recommend memorization as your primary study strategy, but it DOES have its limited uses and this is one of them! If you don’t already have the lab values for pH, PaCO₂ , and HCO₃^– firmly memorized so that you can write them down in less than 15 seconds, then you need to work on that first. It’s that important.

So across the top of our page, take about 15 seconds (or less!) to write down the normal values for the three most important ABG lab results: pH (7.35-7.45), PaCO₂ (35-45), and HCO₃^– (22-26). This is one of our major reference points so that we don’t get accidentally confused later.

Step 2: Remind Yourself of pH

Let’s give ourselves another reference point that will help to prevent confusion later.

Underneath our normal pH range, write “acidosis” and “alkalosis”. Then draw arrows in the appropriate direction to remind us of which direction is acidic (down), and which direction is basic (up).

This will remind us later that the smaller numbers on the pH scale are acidic (1-7), while the larger numbers are considered basic (7-14).

Step 3: Reminding Yourself of PaCO₂, and HCO₃^–

We’re almost ready to interpret ABG values, but first we need one last reference point!

Underneath PaCO₂, and HCO₃^–, draw arrows to remind you what abnormally high and low values would do to the body’s pH.

PaCO₂

For example, PaCO₂ remember the hydrogen will follow carbon dioxide. So if the body starts holding on to carbon dioxide (which makes PaCO₂ increase), then hydrogen will also increase. Hydrogen is acidic, so if hydrogen increases, then the body’s pH will become more acidic. That means the pH will decrease because acid is on the 0-7 part of the pH scale.

If the body starts getting rid of carbon dioxide, then hydrogen will also decrease. Again, since hydrogen is acidic, then removing hydrogen removes acidity. The result is that the body becomes more basic, and the pH will increase.

HCO₃^–

We follow the same logical process for HCO₃^–, but we pay attention to the different anatomy and physiology of how it functions in the body. HCO₃^– is also called bicarbonate, and it functions as a base in the body. So when we start losing HCO₃^–, we are losing base, which moves the pH of the body down towards the acidic side of the scale. If we start holding on to HCO₃^–, then pH will move up towards the basic side of the scale.

When you’re done , your page should look something like this:

Tips for Writing Your Arrows Correctly Every Time

You can choose to memorize these directional reference points, OR make sure that you understand the underlying mechanisms and can figure it out on the spot.

Personally, I prefer to understand the underlying mechanisms and “figure it out” every time I interpret ABG values. I don’t trust my rote memory in a testing situation because there is no way for me to double check if I am right or wrong.

But if I understand the basics about the underlying mechanisms, then I can go through the whole process in my mind and feel confident that every arrow is pointing in the correct direction. If I have practiced this ahead of time, then it only takes me a few seconds, and I feel so much more confident that I’m remembering everything correctly.

Step 4: Assess the patient’s ABG values

Now we are finally ready to assess the patient’s ABG values. Start by looking at the pH and decide if the value is normal (7.35-7.45), high (>7.45), or low (<7.35). This should be super easy since you already wrote the reference values for pH at the top of your scratch paper.

Here’s where the decision making really begins.

• If the pH is abnormal, continue to step 5.
• If the pH is normal, check PaCO₂, and HCO₃^– 
  • If both are normal, then you patient does NOT have an ABG imbalance. You’re finished!
  • If one OR both of these values is abnormal, then continue to step 5.

Step 5: Identify Alkalosis or Acidosis.

At this point, you know that your patient has an imbalance, so we need to figure out what kind of imbalance.

• If the pH is abnormal, compare it to the reference arrows you wrote at the top of your paper and pick the imbalance that matches that direction (>7.45 is alkalosis, <7.35 is acidosis). Continue to step 6.
• If the pH is normal but at least one other value was abnormal, then choose the direction that the pH is “heading.” For example, 7.35-7.39 is toward acidosis, while 7.41-7.45 is toward alkalosis.  Continue to step 6.

You might be asking yourself, “What if pH is normal, one of the other values is abnormal, but the pH is 7.4? That’s exactly in the middle, how do I pick alkalosis or acidosis??” Great question! And my answer is: as a safe, effective, beginning nurse, you’re not going to be given a scenario like that.

While it’s true that in clinical practice something like that COULD happen, it would be a very complex case a therefore beyond the scope of practice for a nursing student. So don’t worry about seeing a scenario like that on NCLEX.

Step 6: Identify Uncompensated, Partially Compensated, or Compensated.

We’ve figured out our first puzzle piece, now let’s discover our second puzzle piece. We have 3 options this time.

• If pH is normal and PaCO₂ and HCO₃^– are BOTH abnormal, then the patient is compensated. Continue to step 7.
• If pH is abnormal and PaCO₂ and HCO₃^– are BOTH abnormal, then the patient is partially compensated. Continue to step 7.
• If pH is abnormal, and either PaCO₂ or HCO₃^– are abnormal, then the patient is uncompensated. Continue to step 7.

In nursing school, you’re most likely to be asked to interpret ABG values that turn out to be uncompensated. However, it’s good practice to be able to identify both partially compensated and fully compensated. And it’s not that hard, either, once you know how to do it!

Step 7: Identify Respiratory or Metabolic

It’s finally time to get our last puzzle piece! Let’s look at the directions of the PaCO₂ and HCO₃^– values.

• If only PaCO₂ is ABNORMAL and HCO₃^– is normal, then it is respiratory.
• If only PaCO₂ is normal and HCO₃^– is ABNORMAL, then it is metabolic.
• If PaCO₂ and HCO₃^– are BOTH abnormal, then you need to reference your arrows and compare to your pH.
  • If you identified that this patient has acidosis, then pick which of the following is also true:
    • and PaCO₂ is increasing (causing pH to decrease), then it is respiratory
    •  and HCO₃^– is decreasing (causing pH to decrease), then it is metabolic
  • If you identified that this patient has alkalosis, then pick which of the following is also true:
    • and PaCO₂ is decreasing (causing pH to increase), then it is respiratory
    •  and HCO₃^– is increasing (causing pH to increase), then it is metabolic

And You’re Finished!

Now that you have all the puzzle pieces you need, you can put them together to know exactly which acid-base imbalance your patient has, and (most importantly!) plan a strategy for addressing it.

Nicole Whitworth

Nicole is a Professional Nursing Tutor with over 15 years experience, and the founder of Your Nursing Tutor. She has a BSN, and an MA in Clinical Psychology. Nicole specializes in providing easy-to-follow, proven study methods (like the Silver Bullet Study System) that transform frustrated nursing students into calm, confident nurses! When she’s not helping students through her Live Tutoring Membership, Nicole loves spending time with her husband, homeschooling their 6 kids, and staring at sunflowers.