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A Step-Wise Approach to Acid-Base Status Interpretation

As with reading an electrocardiogram or a chest x-ray, it is important to use a system when reading arterial blood gases. Adhering to a system will allow you to identify the primary and compensatory process and any additional disorders that may be present.

A suggested step-wise approach for reading an arterial blood gas is as follows:

  • Examine the pH and comparing it to the normal range
  • Identify the primary process that led to the change in pH
  • Calculate the serum anion gap
  • Identify the compensatory process (if one is present)
  • Identify if any other disorders are present or there is a mixed acid-base process

Each of these steps is described below in greater detail. After working through these steps, you should be able to give a one- or two-sentence synopsis of the patient’s acid-base status such as "This patient has a primary respiratory acidosis with a compensatory metabolic alkalosis."

As you go through this process, try not to lose track of the clinical scenario that led to the blood gas being drawn in the first place. You will use the results and their interpretation to help you figure out what is going on with the patient. In addition, you should always ask if the results make sense in light of what you know about the patient’s case. If the results do not make sense, either your interpretation was wrong or there may be some additional processes at work that were not recognized on the initial analysis.

With that in mind, the main steps in interpreting an arterial blood gas in greater detail are presented below.

Step 1: Examine the pH and compare it to the normal range.

As noted above, if the pH is low, the patient has an acidemia. If the pH is above this range, the patient has an alkalemia. Be aware that patients can have mixed metabolic disorders (e.g., concurrent metabolic acidosis and alkalosis) that can give them a pH in the normal range. This will be discussed further below.

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Step 2: Determine the primary process that led to the change in the pH:

For a patient with a low pH (acidemia)

  • If the PCO2 is elevated (> 44), the primary process is a respiratory acidosis
  • If the HCO3- is low (< 22), the primary process is a metabolic acidosis

For a patient with a high pH (alkalemia)

  • If the PCO2 is low (< 36), the primary process is a respiratory alkalosis
  • If the HCO3- is high (> 26), the primary process is a metabolic alkalosis.
    This framework is depicted in Figure 1.

Figure 1: identifying Primary Process

Some examples of how to work through this first step in the process›

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Step 3: Calculate the serum anion gap (SAG)

Serum Anion Gap (SAG) = Na+ - (Cl- + HCO3- )

You should use the bicarbonate from the chemistry panel for this calculation. If this value is elevated (> 12), the person is deemed to have an "elevated anion gap." This implies that the patient has a primary elevated serum anion gap metabolic acidosis regardless of what other abnormalities you identify or what else is happening with the pH and bicarbonate.  There must be an additional disorder because the body does not generate an anion gap in order to compensate for a primary respiratory disorder.

Be aware, however, that an elevated anion gap acidosis may not be the only primary process. For example, patients with salicylate intoxication may have a primary respiratory alkalosis and a concurrent primary elevated anion gap acidosis at the same time.

An example of how to work through this situation›

You should also note that the normal anion gap is affected by the patient's serum albumin level. As a general rule of thumb, the normal anion gap is roughly three times the albumin value. By way of example, for a patient with an albumin of 4.0, the normal anion gap would be 12. For a patient with chronic liver disease and an albumin of 2.0, the upper limit of normal for the anion gap would be 6. Other people propose that the ceiling value for a normal anion gap is reduced by 2.5 for every 1g/dL reduction in the plasma albumin concentration.

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< Previous: How the Data Are Presented | Next: Step 4: Identify the compensatory process (if one is present)

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