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How the Data Are Presented

While the laboratory will always label each value in the arterial blood gas results, it is not uncommon for residents, fellows and attending physicians to either write or state the results without labeling each value. For example, rather than stating: "the pH is 7.40, the PCO2 is 40, the PO2 is 85 and the HCO3- is 24" they may simply state or write: "7.4/40/85/24."

If ABG results are presented in this manner, by convention, they will be written or spoken in the following order:

pH arrow right PCO2 arrow right PO2 arrow right HCO3-

Before you get started…. Make Sure the Numbers Are Consistent

Before you do you acid-base interpretation, it is important to do a little troubleshooting and make sure there are no measurement errors with your blood gas results.

There are two things you should do.

First, make sure it is an arterial sample and not a venous sample. The best way to do this is to observe how the blood comes back into the blood gas syringe as the sample is drawn. Pulsatile flow is seen with an arterial sample but would be lacking with a venous sample. Similarly, arterial samples usually fill the syringe quickly, while venous samples move much more slowly into the syringe. You cannot always rely on the color of the blood to tell you it is arterial because a very hypoxemic patient will have dark, "venous-appearing" blood. If you did not see the sample as it was drawn into the syringe, you can use the PO2 as a guide. If the patient was not very hypoxemic when the blood gas was drawn but you get a very low PO2 with the results (30s-40s), it is likely that you have a venous sample. This tactic is a bit harder to use when the patient is very hypoxemic when the sample is drawn.

Second, you should make sure there are no measurement errors. A simple way to do this is to compare the bicarbonate value from the blood gas (a calculated value) with the bicarbonate from the chemistry panel (a measured value). They are not always exactly the same but they should be close to each other. This only works, however, if your chemistry panel and blood gas were measured at roughly the same time. You cannot do this if the samples were drawn many hours apart.

A more thorough approach is to see if there is consistency between the blood gas and the chemistry panel using the Henderson-Hasselbach equation. The equation is used to calculate the pH you would expect based on the measured PCO2 and HCO3-. This pH is then compared to the measured pH. If the values are similar, your sample is valid. If the values are far apart, there may be a measurement error.

Because no one can easily remember the full Henderson-Hasselbach equation, there is a modified process that can be used instead. This modified process is as follows:

  • Calculate the hydrogen ion concentration using a modified Henderson-Hasselbach equation: [H+] = 24 x PCO2 / HCO3-
  • Use the calculated [H+] to determine what the pH should be. pH is a function of the hydrogen ion concentration (pH = -log [H+]) but rather than doing the calculation you can refer to the following table:

    If the [H+] is… Then the Calculated pH is…
    100 7.00
    79 7.10
    63 7.20
    50 7.30
    45 7.35
    42 7.38
    41 7.39
    40 7.40
    39 7.41
    38 7.42
    35 7.45
    32 7.50
    25 7.60

  • Compare the calculated pH to the measured pH. If they are similar, your sample is valid. If the values are far apart, there may be a measurement error.

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