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Interpreting ABGs (Arterial Blood Gases) Made Easy

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General Practice ( GP ) / Family Medicine
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6 minutes

Arterial blood gasses, or ABGs, are among the more complex assessments performed by clinical healthcare professionals.

Fortunately, there are some easy ways to remember how to decipher these important results.


What is Normal?

When interpreting ABG results, it is essential to know what ABG values are considered ‘normal’. From this baseline, you can then begin to recognise significant variations in a patient’s results, which could indicate clinical deterioration.

The first value is the pH, which measures how many hydrogen ions (H+) are in the sample. This determines if the blood is acidotic or alkalotic. Normal values for pH range from 7.35 - 7.45.

The next value is the carbon dioxide level, and this will tell you if the problem is respiratory in origin, as CO2 is regulated by the lungs (Berman et al. 2017). The normal range for PaCO2 is 35 to 45 mmHg.

Finally, bicarbonate ions, or HCO3-, will tell you if the problem is related to metabolic changes in your patient and refers to the renal system (Berman et al. 2017). Normal is considered to be from 22 to 26 mmol/L.


Normal ABG Levels pH PaCO2 HCO3-
Hydrogen 7.35 - 7.45
Carbon dioxide 35 - 45 mmHg
Bicarbonate 22 - 26 mmol/L

(Castro & Keenaghan 2020)


Put simply, when the numbers in an ABG result fall outside of these ranges, you can then determine what type of problem the patient is experiencing.


pH: Acidic or Alkalotic?

If the ABG results reveal pH numbers are not within the normal range, the patient’s pH level is either acidotic or alkalotic.

The lower the number, the more acidotic the patient is. For instance, a pH of 3 is severely acidotic and requires emergency intervention.



Alkalosis is the opposite. The higher the pH, the more base is in the blood sample, which can disrupt the normal functioning of the body.

Once you’ve determined whether there is too much acid or too much base, you can move on to determine the cause of it.

(Kaufman 2020)


HCO3-: Respiratory or Metabolic?

After you’ve determined whether the sample is acidic or alkaline, you need to work out if it’s due to respiratory or metabolic causes.

If the cause is respiratory in nature, the PaCO2 will be out of the normal range, whereas for metabolic problems the HCO3- will be abnormal. Low PaCO2 points to respiratory alkalosis, and high HCO3- can indicate metabolic alkalosis.

(Kaufman 2020)


PaCO2: Compensated or Uncompensated?

Compensation can be thought of as the body’s attempt at correcting an imbalance: Is one system in the body trying to compensate for an abnormality in another system? We can investigate this by looking at the opposing component of the problem.

For example, in an acidosis, we’d look at the level of HCO3-. Whereas, in an alkalosis, to determine if the body is compensating, we’d look at what the PaCO2 is doing.

If the other level (or component) is within normal ranges, then the problem is non-compensated or uncompensated. Ultimately, the body is yet to fix the problem or has been unable to fix the problem.


However, if the other component has gone outside its normal reference ranges, we can think of it as compensation occurring (the body is trying to fix the problem). To assess how well it has been able to do this, we need to refer back to the pH. If the pH is not within or close to the normal ranges, then a partial-compensation exists. If the pH is back within normal ranges then a full-compensation has occurred.

A non-compensated or uncompensated abnormality usually represents an acute change occurring in the body.

And note - The term partial or fully-compensated is used to describe the level of compensation and does not necessarily mean the patient’s ABG is normal or that they are healthy.


To Simplify... Compensated or
Uncompensated? Respiratory or
Metabolic? Acidic or
Alkalotic? pH PaCO2 HCO3-
  Respiratory Acidosis Low High  
  Respiratory Alkalosis High Low  
  Metabolic Acidosis Low   Low
  Metabolic Alkalosis High   Low
Compensated Respiratory Acidosis Normal High  
Compensated Respiratory Alkalosis Normal Low  
Compensated Metabolic Acidosis Normal   Low
Compensated Metabolic Alkalosis Normal  




Case Study 1

Consider the following:

pH = 7.50

PaCO2 = 47

HCO3- = 32

Q1) Is it an acidosis or an alkalosis?

The pH is 7.50. This is higher than normal, so we have an alkalosis.

Q2) Is the problem of a respiratory or metabolic nature?

Where else is there an alkalosis? The HCO3- is 32, which is high. So we have metabolic alkalosis.

Q3) Is there any compensation occurring? Has the body tried to fix the problem?

We need to look at the other component, in this case, what is the CO2? The CO2 is outside its normal ranges. It’s 47, which is high. So the body is trying to fix the problem. However, the pH is not yet back within normal ranges so a partial compensation exists.


This ABG is an example of a partially compensated metabolic alkalosis.



Case Study 2

Consider the following:

pH = 7.30

PaCO2 = 50

HCO3- = 30

Q1) Is it an acidosis or an alkalosis?

The pH is 7.30. This is lower than normal, so we have an acidosis.

Q2) Is the problem of a respiratory or metabolic nature?

What else is acidotic? The CO2 is 50, which is high. So we have respiratory acidosis.

Q3) Is there any compensation occurring? Has the body tried to fix the problem?

We need to look at the other component, being HCO3- in this case. Is the HCO3- outside its normal ranges? Yes, normal HCO3- is between 22-28. So the body is trying to fix this problem. Has the body done a good job at fixing the problem? Is the pH back within normal ranges? No, the pH is not within normal ranges, so there is partial compensation occurring.


This ABG is an example of a partially compensated respiratory acidosis.


Note: ABGs should be thought of as a snapshot of how the body is interacting with its environment at a particular time. They should always be interpreted as part of a wider assessment of a patient’s respiratory function and in line with your organisation’s policies.

  • Anderson, AH, Bednarek, S, Brady, CL, Burns, SN, Haynes, N, Kennedy, LD, Lopez, C, Madara, M, Ralph, J & York, DR 2012, Pathophysiology Made Incredibly Easy 2012, 5th edn, Lippincott Williams & Wilkins: Ambler, PA, USA.


  • Chang, E, Daly, J & Elliot, D 2006, Pathophysiology Applied to Nursing Practice, 2nd edn, Mosby Australia, Port Melbourne, VIC, Australia.
  • Gaines, K 2020, ‘Know Your ABHs - Arterial Blood Gases Explained’, Nurse.org, viewed 20 April 2020, https://nurse.org/articles/arterial-blood-gas-test/.
  • Healthline 2019, Blood Gas Test, viewed 20 April 2020, https://www.healthline.com/health/blood-gases.
  • Kaufman, DA 2020, Interpretation of ABGs, American Thoracic Society, viewed 20 April 2020, https://www.thoracic.org/professionals/clinical-resources/critical-care/clinical-education/abgs.php.
  • Medlineplus 2018, Blood Gases, viewed 20 April 2020, https://medlineplus.gov/ency/article/003855.htm.




Lynda Lampert

Lynda is a registered nurse with three years experience on a busy surgical floor in a city hospital. She graduated with an Associates degree in Nursing from Mercyhurst College Northeast in 2007 and lives in Erie, Pennsylvania in the United States. In her work, she took care of patients post operatively from open heart surgery, immediately post-operatively from gastric bypass, gastric banding surgery and post abdominal surgery. She also dealt with patient populations that experienced active chest pain, congestive heart failure, end stage renal disease, uncontrolled diabetes and a variety of other chronic, mental and surgical conditions. See Educator Profile


Ausmed Editorial Team

Ausmed’s Editorial team is committed to providing high-quality and thoroughly researched content to our readers, free of any commercial bias or conflict of interest. All articles are developed in consultation with healthcare professionals and peer reviewed where necessary, undergoing a yearly review to ensure all healthcare information is kept up to date. See Educator Profile


Interpreting ABGs (Arterial Blood Gases) Made Easy
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