Blood Gases: Acid-Base Balance Made Easy

Blood Gas Title

We all do struggle solving Acid-Base Balance problems. I, myself, hated Clinical Chemistry because of this. Haha. Not until I discovered that Acid-Base Balance problems can be just as easy as ABC. Nah, I mean, Tic-Tac-Toe.

Before we start, let’s have a little review first.

Metabolic Acidosis – decreased bicarbonate; compensated by hyperventilation

Metabolic Alkalosis – increased bicarbonate; compensated by hypoventilation

Respiratory Acidosis – increased carbonic acid; compensated by increased renal absorption  of bicarbonate

Respiratory Alkalosis – decreased carbonic acid; compensated by decreased renal absorption of bicarbonate



Here are a few things we need to remember:

  • Always refer to the normal values. Left side-Acidic; Right side-Basic
    • 7.35                     pH            7.45
    • 45mmHg          pCO2         35mmHG
    • 22mmol/L         HCO3        26mmol/L
  • If pH and HCO3 go together – metabolic
  • If pH and pCO2 go together- respiratory
  • Uncompensated – if pCO2 or HCO3 is normal
  • Partially compensated – if pCO2 and HCO3 are on opposite sides
  • Fully compensated – if pH is within normal range



pH is 7.26

pCO2 is 42

HCO3 is 17


1. Start by drawing Tic-Tac-Toe boxes.


2. Distribute the values on the boxes. Always refer to the normal values.


3. Interpret.

Since pH and HCO2 go together under the Acid column, we can interpret this as Metabolic Acidosis. pCo2 is within normal range which means no compensation is happening. Therefore, the final answer is UNCOMPENSATED METABOLIC ACIDOSIS.



pH is 7.44

pCo2 is 30

HCO3 is 21


We will refer again to this table of normal values. You will notice that pCO2 values are inverted (45 to 35) to match with its acidic and basic characteristics.



And now, we will distribute the values to our Tic-Tac-Toe boxes.


You will notice that the pH value is within the normal range but since it is nearest the basic value (7.45) we will interpret it as Respiratory Alkalosis. (Note that pCO2 is under the basic column too).

And since pCO2 and HCO3 fall on the opposite sides as pH is normal, we can conclude that there is full compensation. Therefore, the answer is FULLY COMPENSATED RESPIRATORY ALKALOSIS.

Easy right? Now, try it for yourself.

And oh, here’s a bonus!

Based on this formula:     pH = HCO3/H2CO3

R O M E – Respiratory, Opposite; Metabolic, Equal

pH = HCO3/H2CO3 -opposite

pH = HCO3/H2CO3 – equal


Enjoy Clinical Chemistry!

Wishing you all the best,




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