Decreased unmeasured cations, such as in hypocalcemia, would have what effect on the anion gap?

• A. Decrease
• B. Increase
• C. No effect
• D. Variable

Answer: (A)

An understanding of the anion gap centers on the balance of charges in the plasma and the way we estimate unmeasured ions. The anion gap is a surrogate for unmeasured anions and is calculated from the measured cation and anions: roughly, a difference between the main measured cation (sodium) and the sum of measured anions (chloride and bicarbonate). Changes in ions not normally measured—unmeasured cations like calcium—still influence the overall charge balance in the body.

If unmeasured cations decrease, there is less positive charge coming from those unmeasured ions. To preserve electroneutrality, the amount of unmeasured anions that balance that charge also decreases. Since the anion gap reflects the amount of unmeasured ions that are not accounted for in the basic panel, a fall in unmeasured cations leads to a smaller pool of unmeasured anions and thus a smaller gap.

So, the decrease in unmeasured cations would cause the anion gap to decrease. The other options don’t fit because the gap would not reliably increase or stay unchanged with loss of unmeasured cations.