If serum pH is within normal range but data suggest an abnormal acid-base pattern, which should be considered?

• A. Simple metabolic acidosis
• B. Mixed acid-base disorder with an added primary respiratory abnormality
• C. Primary respiratory disorder only
• D. No disorder

Answer: (B)

When pH stays within the normal range but there are other acid-base indicators that don’t fit a single disorder, think of two processes happening at once. A normal pH can mask an underlying problem if one disturbance pushes toward acidosis and another toward alkalosis, balancing pH but leaving abnormal signals in the other values.

The best interpretation here is a mixed acid-base disorder with an added primary respiratory abnormality. That means there is a primary metabolic disturbance (altering bicarbonate) occurring together with an independent primary respiratory disturbance (altering CO2). The result can keep the pH in the normal range while the CO2 and bicarbonate levels show abnormal directions or magnitudes that don’t line up with a single process. For example, a metabolic acidosis with a concurrent respiratory alkalosis could normalize pH but still reveal an abnormal CO2 and HCO3- pattern. Conversely, a metabolic alkalosis with a respiratory acidosis would produce a similar masking effect.

If you only had a simple metabolic or simple respiratory disorder, the pH would typically show a clearer direction of change unless there’s exact compensation—something that usually leaves a consistent pattern rather than a mixed one. So the normal pH alongside abnormal acid-base data points to a mixed disorder with an added primary respiratory abnormality.