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. 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 P_aCO₂ 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

(Castro et al. 2022)

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 2009)

HCO₃^-: Respiratory or Metabolic?

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

If the cause is respiratory in nature, the P_aCO₂ will be out of the normal range, whereas for metabolic problems, the HCO₃^- will be abnormal. Low P_aCO₂ points to respiratory alkalosis, and high HCO₃^- can indicate metabolic alkalosis.

(Kaufman 2009)

P_aCO₂: 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 whether the body is compensating, we’d look at what the P_aCO₂ 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 full compensation has occurred.

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

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

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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		High
Compensated	Respiratory	Acidosis	Normal	High
Compensated	Respiratory	Alkalosis	Normal	Low
Compensated	Metabolic	Acidosis	Normal		Low
Compensated	Metabolic	Alkalosis	Normal		High

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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.