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Two or three simultaneous disorders can be present in a mixed acid-base disorder, but there can never be two primary respiratory disorders. Uncovering a mixed acid-base disorder is clinically important and requires a methodical approach to acid-base analysis (see box Step-by-Step Analysis of Acid-Base Status). Once the primary disturbance has been determined, the clinician should assess whether the compensatory response is appropriate (Table 21–11). An inadequate or an exaggerated response indicates the presence of another primary acid-base disturbance.


  • Step 1: Determine the primary (or main) disorder—whether it is metabolic or respiratory—from blood pH, HCO3, and PCO2 values.

  • Step 2: Determine the presence of mixed acid-base disorders by calculating the range of compensatory responses (see Table 21–11).

  • Step 3: Calculate the anion gap (see Table 21–12).

  • Step 4: Calculate the corrected HCO3 concentration if the anion gap is increased.

  • Step 5: Examine the patient to determine whether the clinical signs are compatible with the acid-base analysis.

Table 21–11.Primary acid-base disorders and expected compensation.
Table 21–12.Anion gap in metabolic acidosis.1

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