An easy method for ARTERIAL BLOOD GAS INTERPRETATION is described with the help of a test case.

We require three values in this regard.


  • pH
  • PCO2
  • HCO3

Firstly, let us determine what the normal values are for these. These are rough estimates which hold true in most cases and one can let go of normal ranges especially from exam point of views and generally interpreting ABGs.

Remember these 3 values

7.4 / 40 / 24

The normal pH is taken as 7.4, the PCO2 as 40 and bicarbonate as 24, roughly.

Test case to understand the 3 step method

Let us take a test case of ABGs and interpret it accordingly to understand the three steps.

The following test case ABGs are given


Step 1

Acidosis or Alkalosis ?

The pH is less than 7.4, so we take it as an acidotic patient.

Step 2

Are the lungs functioning properly ?

As the patient is acidotic with a low pH, so the lungs should be removing CO2 by exhaling it out and lowering the PCO2 but we have a high PCO2, instead. So the respiratory component in maintaining pH neutrality is not compensating and is involved.

Remember that respiratory rate is inversely proportional to the PCO2. An increased respiratory rate would alter the PCO2, quickly, within minutes and subsequently affect the pH, accordingly. Bicarbonate is altered much more slowly , in a matter of hours , in kidneys.

Step 3

Are the kidneys working properly ?

In case of the acidotic stress, the kidneys in response should be holding on to the bicarbonate and reabsorbing them. In this case the bicarbonate is greater than 24 , so kidneys are working fine in this regard.



Respiratory Acidosis



  1. Acidic or basic in terms of pH
  2. Lungs in terms of PCO2
  3. Kidneys in terms of bicarbonate

These three can be used accordingly for interpreting further cases in the same pattern.