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Interpreting the Acid-Base Balance Using Tic Tac Toe (0s and Xs)

This information is taken from an excellent video by Radiometer, shown here:

Put Simply:

To interpret the acid-base blood gas results, you first need to know what normal levels are – and be careful, because there are international variations in what units we use, and you may also find slight variations in results. You also need to know what a high figure means vs a low figure – is this acidemia, or alkalemia? Let’s help you out:

Your normal levels are:

Ph = 7.35-7.45

Pc02 = 4.7-6.0 kPa

Hc03 = 22-26 mmol L

Which way is acid, and which way is alkaline?

You might notice that respiratory acidosis and respiratory alkalosis are in bold. That is to highlight the fact that these are opposite to the other parameters – a high pC02 = acidotic. A low pC02 – alkalosis. One simple way to remember this is to try to spell ‘opp’ (opposite) backwards, using the other parameters. To enable you to do this, you’re only allowed to swap one C for a P. Go ahead, try it.

For pH, you obviously can’t do this.

For HC03, you still can’t: Even if you change the C to a P, you still have an H in the way.

For pc02, you can swap the C for a P and you can spell: 0PP backwards… That’s the parameter that is opposite to the others!

How to Use Tic-Tac-Toe (0s and Xs)

First, draw a tic-tac-toe table like so.

Next, put your pH into the acidosis, normal, or alkalosis column:

pH 7.12  

Next, put your HC03 or pC02 into the corresponding column. In this case, it’s the pC02:

pH 7.12  
pC02 13.9  

It only takes 3 in a row for tic-tac-toe, and that includes the title, so you have an acidosis here. Because we are looking at the respiratory component (pC02), this is a respiratory acidosis.

But we keep looking because we want to know if the body is trying to compensate. If it is, the opposite component – in this case, the metabolic component, HC03 – will be going in the opposite direction to the general trend. In this case, the general trend is respiratory acidosis, so we’re looking to see if the metabolic component is alkalotic. If there is no compensation, it will be in the normal range.

pH 7.12  
pC02 12  
  HC03 33

And it’s above the reference range, so there is partial compensation here. But it’s only partial compensation because the pH isn’t normal.

 pH 7.36 
pC02 11  
  HC03 33

This is now fully compensated. We know it was probably respiratory acidosis before because a) we have the luxury of repeat blood gas results, and b) the pH is only just normal; in fact, it’s heading towards acidosis. We need to keep monitoring to see if this continues to normalise or if it heads in the wrong direction.

You can use tic-tac-toe to identify respiratory or metabolic alkalosis or acidosis, mixed alkaloses/acidoses, and partial and full compensation. Don’t forget to look at other parameters as well though – more on those soon.


For this post, the video above, and:

Thompson, D. A. 2007. Blood Gases Made Simple, Easy, and Quick. Lulu Press.

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