Stepwise approach to interpreting the arterial blood gas.

1. H&P.  The most clinical useful information comes from the clinical description of the patient by the history and physical examination. The H&P usually gives an idea of what acid base disorder might be present even before collecting the ABG sample

2. Look at the pH. Is there an acid base disorder present?
        - If pH < 7.35, then acidemia
        -  if pH > 7.45, then alkalemia
        -  If pH within normal range, then acid base disorder not likely present.
        -   pH may be normal in the presence of a mixed acid base disorder,  particularly if other parameters of the ABG are abnormal.

3. Look at PCO2, HCO3-.  What is the acid base process (alkalosis vs acidosis) leading to the abnormal pH?  Are both values normal or abnormal?
     -  In simple acid base disorders, both values are abnormal and direction of the abnormal change is the same for both parameters.
     -  One abnormal value will be the initial change and the other will be the compensatory response.

3a. Distinguish the initial change from the compensatory response.
        - The initial change will be the abnormal value that correlates with the abnormal pH.
        - If Alkalosis, then PCO2 low or HCO3- high
       -  If Acidosis, then PCO2 high or HCO3-  low.

 Once the initial change is identified, then the other abnormal parameter is the compensatory response if the direction of the change is the same. If not, suspect a mixed disorder.

3b. Once the initial chemical change and the compensatory response is distinguished, then identify the specific disorder. See table below.
       - If PCO2 is the initial chemical change, then process is respiratory.
      -  if HCO3- is the initial chemical change, then process is metabolic.

Acid Base Disorder

Initial Chemical Change

Compensatory Response

Respiratory Acidosis

↑ PCO2


Respiratory Alkalosis

↓ PCO2

↓ HCO3-

Metabolic Acidosis

↓ HCO3-

↓ PCO2

Metabolic Alkalosis

↑ HCO3-

↑ PCO2

4. If respiratory process, is it acute or chronic?
          - An acute respiratory process will produce a compensatory response that is due primarily to rapid intracellular buffering.
         - A chronic respiratory process will produce a more significant compensatory response that is due primarily to renal adaptation, which takes a longer time to develop.
         - To assess if acute or chronic, determine the extent of compensation. See table.

5. If metabolic acidosis, then look at the Anion Gap.
        - If elevated (> than 16), then acidosis due to KULT. (Ketoacidosis, Uremia, Lactic acidosis, Toxins). See table.
        - If anion gap is normal, then acidosis likely due to diarrhea, RTA.

6. If metabolic process, is degree of compensation adequate?
        - Calculate the estimated PCO2, this will help to determine if a seperate respiratory disorder is present. See table.

7.  If anion gap is elevated, then calculate the Delta-Ratio (∆/∆) to assess for other simultaneous disorders.
      - ∆/∆ compares the change in the anion gap to the change in bicarbonate.
      - If ratio between 1 and 2, then pure elevated anion gap acidosis
      -  If < 1, then there is a simultaneous normal anion gap acidosis present.
       - if > 2, then there is a simultaneous metabolic alkalosis present or a compensated chronic respiratory acidosis.

8.  If normal anion gap and cause is unknown, then calculate the Urine Anion Gap (UAG).  This will help to differentiate RTAs from other causes of non elevated anion gap acidosis.
      - In RTA, UAG is positive.
      -  In diarrhea and other causes of metabolic acidosis, the UAG is negative. (neGUTive in diarrhea)