How is the anion gap calculated?

How is the anion gap calculated?

• A. (Na + K) - (Cl + HCO3)
• B. Na - (Cl + HCO3)
• C. (Na + H) - (Cl + HCO3)
• D. (Na + K + Ca) - (Cl + HCO3)

Answer: (A)

Anion gap reflects unmeasured anions by comparing the sum of measured cations to the sum of measured anions. The major measured cations are sodium and, sometimes, potassium; the major measured anions are chloride and bicarbonate. So the calculation is (Na+ + K+) − (Cl− + HCO3−). Including potassium shifts the gap a bit, which is why this form is used here. Omitting potassium (Na+ − [Cl− + HCO3−]) is a common alternative in some contexts, but not the one shown. Replacing potassium with hydrogen or adding calcium doesn’t reflect the standard set of measured ions, so those formulations aren’t correct.

Anion gap reflects unmeasured anions by comparing the sum of measured cations to the sum of measured anions. The major measured cations are sodium and, sometimes, potassium; the major measured anions are chloride and bicarbonate. So the calculation is (Na+ + K+) − (Cl− + HCO3−). Including potassium shifts the gap a bit, which is why this form is used here. Omitting potassium (Na+ − [Cl− + HCO3−]) is a common alternative in some contexts, but not the one shown. Replacing potassium with hydrogen or adding calcium doesn’t reflect the standard set of measured ions, so those formulations aren’t correct.