A base excess of +4 is most consistent with which acid-base disturbance?

• A. Metabolic alkalosis
• B. Respiratory acidosis
• C. Metabolic acidosis
• D. Bicarbonate toxicity

Answer: (A)

Base excess reflects how much buffering base, mainly bicarbonate, is present beyond normal in the blood. Normal base excess is roughly -2 to +2 mEq/L. A positive base excess means there is an excess of bicarbonate, pointing to a metabolic process rather than a primary change in respiration.

A base excess of +4 indicates a bicarbonate surplus, which fits metabolic alkalosis. This condition arises when there is a gain of base or loss of hydrogen ions through mechanisms like vomiting, diuretic use, or mineralocorticoid activity, all of which raise bicarbonate and push the base excess above normal.

In contrast, a primary respiratory disturbance alters CO2 and pH directly rather than increasing bicarbonate. Respiratory acidosis elevates CO2 with a lower pH, and metabolic acidosis lowers bicarbonate with a lower base excess. Bicarbonate toxicity isn’t a standard primary acid-base category, so it doesn’t describe a typical pattern seen with a positive base excess.